Merge branch 'master' into to-merge-release-2.19

CHANGELOG.md

@@ -10,6 +10,9 @@
 * [BUGFIX] React UI: Don't null out data when clicking on the current tab. #7243
 * [BUGFIX] PromQL: Correctly track number of samples for a query. #7307
 * [BUGFIX] PromQL: Return NaN when histogram buckets have 0 observations. #7318
+
+## 2.18.2 / 2020-06-09
+
 * [BUGFIX] TSDB: Fix incorrect query results when using Prometheus with remote reads configured #7361
 
 ## 2.18.1 / 2020-05-07

pkg/labels/labels_test.go

@@ -531,3 +531,84 @@ func TestLabels_Copy(t *testing.T) {
 func TestLabels_Map(t *testing.T) {
 	testutil.Equals(t, map[string]string{"aaa": "111", "bbb": "222"}, Labels{{"aaa", "111"}, {"bbb", "222"}}.Map())
 }
+
+func TestLabels_WithLabels(t *testing.T) {
+	testutil.Equals(t, Labels{{"aaa", "111"}, {"bbb", "222"}}, Labels{{"aaa", "111"}, {"bbb", "222"}, {"ccc", "333"}}.WithLabels("aaa", "bbb"))
+}
+
+func TestLabels_WithoutLabels(t *testing.T) {
+	testutil.Equals(t, Labels{{"aaa", "111"}}, Labels{{"aaa", "111"}, {"bbb", "222"}, {"ccc", "333"}}.WithoutLabels("bbb", "ccc"))
+	testutil.Equals(t, Labels{{"aaa", "111"}}, Labels{{"aaa", "111"}, {"bbb", "222"}, {MetricName, "333"}}.WithoutLabels("bbb"))
+}
+
+func TestLabels_FromStrings(t *testing.T) {
+	testutil.Equals(t, Labels{{"aaa", "111"}, {"bbb", "222"}}, FromStrings("aaa", "111", "bbb", "222"))
+}
+
+func TestBulider_NewBulider(t *testing.T) {
+	testutil.Equals(
+		t,
+		&Builder{
+			base: Labels{{"aaa", "111"}},
+			del:  []string{},
+			add:  []Label{},
+		},
+		NewBuilder(Labels{{"aaa", "111"}}),
+	)
+}
+
+func TestBuilder_Del(t *testing.T) {
+	testutil.Equals(
+		t,
+		&Builder{
+			del: []string{"bbb"},
+			add: []Label{{"aaa", "111"}, {"ccc", "333"}},
+		},
+		(&Builder{
+			del: []string{},
+			add: []Label{{"aaa", "111"}, {"bbb", "222"}, {"ccc", "333"}},
+		}).Del("bbb"),
+	)
+}
+
+func TestBuilder_Set(t *testing.T) {
+	testutil.Equals(
+		t,
+		&Builder{
+			base: Labels{{"aaa", "111"}},
+			del:  []string{},
+			add:  []Label{{"bbb", "222"}},
+		},
+		(&Builder{
+			base: Labels{{"aaa", "111"}},
+			del:  []string{},
+			add:  []Label{},
+		}).Set("bbb", "222"),
+	)
+
+	testutil.Equals(
+		t,
+		&Builder{
+			base: Labels{{"aaa", "111"}},
+			del:  []string{},
+			add:  []Label{{"bbb", "333"}},
+		},
+		(&Builder{
+			base: Labels{{"aaa", "111"}},
+			del:  []string{},
+			add:  []Label{{"bbb", "222"}},
+		}).Set("bbb", "333"),
+	)
+}
+
+func TestBuilder_Labels(t *testing.T) {
+	testutil.Equals(
+		t,
+		Labels{{"aaa", "111"}, {"ccc", "333"}, {"ddd", "444"}},
+		(&Builder{
+			base: Labels{{"aaa", "111"}, {"bbb", "222"}, {"ccc", "333"}},
+			del:  []string{"bbb"},
+			add:  []Label{{"ddd", "444"}},
+		}).Labels(),
+	)
+}

promql/engine.go

@@ -29,7 +29,7 @@ import (
 
 	"github.com/go-kit/kit/log"
 	"github.com/go-kit/kit/log/level"
-	opentracing "github.com/opentracing/opentracing-go"
+	"github.com/opentracing/opentracing-go"
 	"github.com/pkg/errors"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/common/model"
@@ -418,7 +418,7 @@ func (ng *Engine) newTestQuery(f func(context.Context) error) Query {
 //
 // At this point per query only one EvalStmt is evaluated. Alert and record
 // statements are not handled by the Engine.
-func (ng *Engine) exec(ctx context.Context, q *query) (v parser.Value, w storage.Warnings, err error) {
+func (ng *Engine) exec(ctx context.Context, q *query) (v parser.Value, ws storage.Warnings, err error) {
 	ng.metrics.currentQueries.Inc()
 	defer ng.metrics.currentQueries.Dec()
 
@@ -517,13 +517,9 @@ func (ng *Engine) execEvalStmt(ctx context.Context, query *query, s *parser.Eval
 	}
 	defer querier.Close()
 
-	warnings, err := ng.populateSeries(ctxPrepare, querier, s)
+	ng.populateSeries(querier, s)
 	prepareSpanTimer.Finish()
 
-	if err != nil {
-		return nil, warnings, err
-	}
-
 	evalSpanTimer, ctxInnerEval := query.stats.GetSpanTimer(ctx, stats.InnerEvalTime, ng.metrics.queryInnerEval)
 	// Instant evaluation. This is executed as a range evaluation with one step.
 	if s.Start == s.End && s.Interval == 0 {
@@ -539,7 +535,7 @@ func (ng *Engine) execEvalStmt(ctx context.Context, query *query, s *parser.Eval
 			lookbackDelta:       ng.lookbackDelta,
 		}
 
-		val, err := evaluator.Eval(s.Expr)
+		val, warnings, err := evaluator.Eval(s.Expr)
 		if err != nil {
 			return nil, warnings, err
 		}
@@ -588,7 +584,7 @@ func (ng *Engine) execEvalStmt(ctx context.Context, query *query, s *parser.Eval
 		logger:              ng.logger,
 		lookbackDelta:       ng.lookbackDelta,
 	}
-	val, err := evaluator.Eval(s.Expr)
+	val, warnings, err := evaluator.Eval(s.Expr)
 	if err != nil {
 		return nil, warnings, err
 	}
@@ -649,35 +645,29 @@ func (ng *Engine) findMinTime(s *parser.EvalStmt) time.Time {
 	return s.Start.Add(-maxOffset)
 }
 
-func (ng *Engine) populateSeries(ctx context.Context, querier storage.Querier, s *parser.EvalStmt) (storage.Warnings, error) {
-	var (
-		// Whenever a MatrixSelector is evaluated, evalRange is set to the corresponding range.
-		// The evaluation of the VectorSelector inside then evaluates the given range and unsets
-		// the variable.
-		evalRange time.Duration
-		warnings  storage.Warnings
-		err       error
-	)
+func (ng *Engine) populateSeries(querier storage.Querier, s *parser.EvalStmt) {
+	// Whenever a MatrixSelector is evaluated, evalRange is set to the corresponding range.
+	// The evaluation of the VectorSelector inside then evaluates the given range and unsets
+	// the variable.
+	var evalRange time.Duration
 
 	parser.Inspect(s.Expr, func(node parser.Node, path []parser.Node) error {
-		var set storage.SeriesSet
-		var wrn storage.Warnings
-		hints := &storage.SelectHints{
-			Start: timestamp.FromTime(s.Start),
-			End:   timestamp.FromTime(s.End),
-			Step:  durationToInt64Millis(s.Interval),
-		}
-
-		// We need to make sure we select the timerange selected by the subquery.
-		// TODO(gouthamve): cumulativeSubqueryOffset gives the sum of range and the offset
-		// we can optimise it by separating out the range and offsets, and subtracting the offsets
-		// from end also.
-		subqOffset := ng.cumulativeSubqueryOffset(path)
-		offsetMilliseconds := durationMilliseconds(subqOffset)
-		hints.Start = hints.Start - offsetMilliseconds
-
 		switch n := node.(type) {
 		case *parser.VectorSelector:
+			hints := &storage.SelectHints{
+				Start: timestamp.FromTime(s.Start),
+				End:   timestamp.FromTime(s.End),
+				Step:  durationToInt64Millis(s.Interval),
+			}
+
+			// We need to make sure we select the timerange selected by the subquery.
+			// TODO(gouthamve): cumulativeSubqueryOffset gives the sum of range and the offset
+			// we can optimise it by separating out the range and offsets, and subtracting the offsets
+			// from end also.
+			subqOffset := ng.cumulativeSubqueryOffset(path)
+			offsetMilliseconds := durationMilliseconds(subqOffset)
+			hints.Start = hints.Start - offsetMilliseconds
+
 			if evalRange == 0 {
 				hints.Start = hints.Start - durationMilliseconds(ng.lookbackDelta)
 			} else {
@@ -696,20 +686,12 @@ func (ng *Engine) populateSeries(ctx context.Context, querier storage.Querier, s
 				hints.End = hints.End - offsetMilliseconds
 			}
 
-			set, wrn, err = querier.Select(false, hints, n.LabelMatchers...)
-			warnings = append(warnings, wrn...)
-			if err != nil {
-				level.Error(ng.logger).Log("msg", "error selecting series set", "err", err)
-				return err
-			}
-			n.UnexpandedSeriesSet = set
-
+			n.UnexpandedSeriesSet = querier.Select(false, hints, n.LabelMatchers...)
 		case *parser.MatrixSelector:
 			evalRange = n.Range
 		}
 		return nil
 	})
-	return warnings, err
 }
 
 // extractFuncFromPath walks up the path and searches for the first instance of
@@ -743,34 +725,40 @@ func extractGroupsFromPath(p []parser.Node) (bool, []string) {
 	return false, nil
 }
 
-func checkForSeriesSetExpansion(ctx context.Context, expr parser.Expr) {
+func checkAndExpandSeriesSet(ctx context.Context, expr parser.Expr) (storage.Warnings, error) {
 	switch e := expr.(type) {
 	case *parser.MatrixSelector:
-		checkForSeriesSetExpansion(ctx, e.VectorSelector)
+		return checkAndExpandSeriesSet(ctx, e.VectorSelector)
 	case *parser.VectorSelector:
-		if e.Series == nil {
-			series, err := expandSeriesSet(ctx, e.UnexpandedSeriesSet)
-			if err != nil {
-				panic(err)
-			} else {
-				e.Series = series
-			}
+		if e.Series != nil {
+			return nil, nil
 		}
+		series, ws, err := expandSeriesSet(ctx, e.UnexpandedSeriesSet)
+		e.Series = series
+		return ws, err
 	}
+	return nil, nil
 }
 
-func expandSeriesSet(ctx context.Context, it storage.SeriesSet) (res []storage.Series, err error) {
+func expandSeriesSet(ctx context.Context, it storage.SeriesSet) (res []storage.Series, ws storage.Warnings, err error) {
 	for it.Next() {
 		select {
 		case <-ctx.Done():
-			return nil, ctx.Err()
+			return nil, nil, ctx.Err()
 		default:
 		}
 		res = append(res, it.At())
 	}
-	return res, it.Err()
+	return res, it.Warnings(), it.Err()
 }
 
+type errWithWarnings struct {
+	err      error
+	warnings storage.Warnings
+}
+
+func (e errWithWarnings) Error() string { return e.err.Error() }
+
 // An evaluator evaluates given expressions over given fixed timestamps. It
 // is attached to an engine through which it connects to a querier and reports
 // errors. On timeout or cancellation of its context it terminates.
@@ -799,26 +787,33 @@ func (ev *evaluator) error(err error) {
 }
 
 // recover is the handler that turns panics into returns from the top level of evaluation.
-func (ev *evaluator) recover(errp *error) {
+func (ev *evaluator) recover(ws *storage.Warnings, errp *error) {
 	e := recover()
 	if e == nil {
 		return
 	}
-	if err, ok := e.(runtime.Error); ok {
+
+	switch err := e.(type) {
+	case runtime.Error:
 		// Print the stack trace but do not inhibit the running application.
 		buf := make([]byte, 64<<10)
 		buf = buf[:runtime.Stack(buf, false)]
 
 		level.Error(ev.logger).Log("msg", "runtime panic in parser", "err", e, "stacktrace", string(buf))
 		*errp = errors.Wrap(err, "unexpected error")
-	} else {
+	case errWithWarnings:
+		*errp = err.err
+		*ws = append(*ws, err.warnings...)
+	default:
 		*errp = e.(error)
 	}
 }
 
-func (ev *evaluator) Eval(expr parser.Expr) (v parser.Value, err error) {
-	defer ev.recover(&err)
-	return ev.eval(expr), nil
+func (ev *evaluator) Eval(expr parser.Expr) (v parser.Value, ws storage.Warnings, err error) {
+	defer ev.recover(&ws, &err)
+
+	v, ws = ev.eval(expr)
+	return v, ws, nil
 }
 
 // EvalNodeHelper stores extra information and caches for evaluating a single node across steps.
@@ -884,17 +879,20 @@ func (enh *EvalNodeHelper) signatureFunc(on bool, names ...string) func(labels.L
 // the given function with the values computed for each expression at that
 // step.  The return value is the combination into time series of all the
 // function call results.
-func (ev *evaluator) rangeEval(f func([]parser.Value, *EvalNodeHelper) Vector, exprs ...parser.Expr) Matrix {
+func (ev *evaluator) rangeEval(f func([]parser.Value, *EvalNodeHelper) (Vector, storage.Warnings), exprs ...parser.Expr) (Matrix, storage.Warnings) {
 	numSteps := int((ev.endTimestamp-ev.startTimestamp)/ev.interval) + 1
 	matrixes := make([]Matrix, len(exprs))
 	origMatrixes := make([]Matrix, len(exprs))
 	originalNumSamples := ev.currentSamples
 
+	var warnings storage.Warnings
 	for i, e := range exprs {
 		// Functions will take string arguments from the expressions, not the values.
 		if e != nil && e.Type() != parser.ValueTypeString {
 			// ev.currentSamples will be updated to the correct value within the ev.eval call.
-			matrixes[i] = ev.eval(e).(Matrix)
+			val, ws := ev.eval(e)
+			warnings = append(warnings, ws...)
+			matrixes[i] = val.(Matrix)
 
 			// Keep a copy of the original point slices so that they
 			// can be returned to the pool.
@@ -946,11 +944,12 @@ func (ev *evaluator) rangeEval(f func([]parser.Value, *EvalNodeHelper) Vector, e
 		}
 		// Make the function call.
 		enh.ts = ts
-		result := f(args, enh)
+		result, ws := f(args, enh)
 		if result.ContainsSameLabelset() {
 			ev.errorf("vector cannot contain metrics with the same labelset")
 		}
 		enh.out = result[:0] // Reuse result vector.
+		warnings = append(warnings, ws...)
 
 		ev.currentSamples += len(result)
 		// When we reset currentSamples to tempNumSamples during the next iteration of the loop it also
@@ -969,7 +968,7 @@ func (ev *evaluator) rangeEval(f func([]parser.Value, *EvalNodeHelper) Vector, e
 				mat[i] = Series{Metric: s.Metric, Points: []Point{s.Point}}
 			}
 			ev.currentSamples = originalNumSamples + mat.TotalSamples()
-			return mat
+			return mat, warnings
 		}
 
 		// Add samples in output vector to output series.
@@ -1001,29 +1000,30 @@ func (ev *evaluator) rangeEval(f func([]parser.Value, *EvalNodeHelper) Vector, e
 		mat = append(mat, ss)
 	}
 	ev.currentSamples = originalNumSamples + mat.TotalSamples()
-	return mat
+	return mat, warnings
 }
 
 // evalSubquery evaluates given SubqueryExpr and returns an equivalent
 // evaluated MatrixSelector in its place. Note that the Name and LabelMatchers are not set.
-func (ev *evaluator) evalSubquery(subq *parser.SubqueryExpr) *parser.MatrixSelector {
-	val := ev.eval(subq).(Matrix)
+func (ev *evaluator) evalSubquery(subq *parser.SubqueryExpr) (*parser.MatrixSelector, storage.Warnings) {
+	val, ws := ev.eval(subq)
+	mat := val.(Matrix)
 	vs := &parser.VectorSelector{
 		Offset: subq.Offset,
-		Series: make([]storage.Series, 0, len(val)),
+		Series: make([]storage.Series, 0, len(mat)),
 	}
 	ms := &parser.MatrixSelector{
 		Range:          subq.Range,
 		VectorSelector: vs,
 	}
-	for _, s := range val {
+	for _, s := range mat {
 		vs.Series = append(vs.Series, NewStorageSeries(s))
 	}
-	return ms
+	return ms, ws
 }
 
 // eval evaluates the given expression as the given AST expression node requires.
-func (ev *evaluator) eval(expr parser.Expr) parser.Value {
+func (ev *evaluator) eval(expr parser.Expr) (parser.Value, storage.Warnings) {
 	// This is the top-level evaluation method.
 	// Thus, we check for timeout/cancellation here.
 	if err := contextDone(ev.ctx, "expression evaluation"); err != nil {
@@ -1035,16 +1035,16 @@ func (ev *evaluator) eval(expr parser.Expr) parser.Value {
 	case *parser.AggregateExpr:
 		unwrapParenExpr(&e.Param)
 		if s, ok := e.Param.(*parser.StringLiteral); ok {
-			return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) Vector {
-				return ev.aggregation(e.Op, e.Grouping, e.Without, s.Val, v[0].(Vector), enh)
+			return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) (Vector, storage.Warnings) {
+				return ev.aggregation(e.Op, e.Grouping, e.Without, s.Val, v[0].(Vector), enh), nil
 			}, e.Expr)
 		}
-		return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) Vector {
+		return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) (Vector, storage.Warnings) {
 			var param float64
 			if e.Param != nil {
 				param = v[0].(Vector)[0].V
 			}
-			return ev.aggregation(e.Op, e.Grouping, e.Without, param, v[1].(Vector), enh)
+			return ev.aggregation(e.Op, e.Grouping, e.Without, param, v[1].(Vector), enh), nil
 		}, e.Param, e.Expr)
 
 	case *parser.Call:
@@ -1056,15 +1056,19 @@ func (ev *evaluator) eval(expr parser.Expr) parser.Value {
 			// a vector selector.
 			vs, ok := e.Args[0].(*parser.VectorSelector)
 			if ok {
-				return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) Vector {
-					return call([]parser.Value{ev.vectorSelector(vs, enh.ts)}, e.Args, enh)
+				return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) (Vector, storage.Warnings) {
+					val, ws := ev.vectorSelector(vs, enh.ts)
+					return call([]parser.Value{val}, e.Args, enh), ws
 				})
 			}
 		}
 
 		// Check if the function has a matrix argument.
-		var matrixArgIndex int
-		var matrixArg bool
+		var (
+			matrixArgIndex int
+			matrixArg      bool
+			warnings       storage.Warnings
+		)
 		for i := range e.Args {
 			unwrapParenExpr(&e.Args[i])
 			a := e.Args[i]
@@ -1078,14 +1082,16 @@ func (ev *evaluator) eval(expr parser.Expr) parser.Value {
 				matrixArgIndex = i
 				matrixArg = true
 				// Replacing parser.SubqueryExpr with parser.MatrixSelector.
-				e.Args[i] = ev.evalSubquery(subq)
+				val, ws := ev.evalSubquery(subq)
+				e.Args[i] = val
+				warnings = append(warnings, ws...)
 				break
 			}
 		}
 		if !matrixArg {
 			// Does not have a matrix argument.
-			return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) Vector {
-				return call(v, e.Args, enh)
+			return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) (Vector, storage.Warnings) {
+				return call(v, e.Args, enh), warnings
 			}, e.Args...)
 		}
 
@@ -1095,16 +1101,22 @@ func (ev *evaluator) eval(expr parser.Expr) parser.Value {
 		otherInArgs := make([]Vector, len(e.Args))
 		for i, e := range e.Args {
 			if i != matrixArgIndex {
-				otherArgs[i] = ev.eval(e).(Matrix)
+				val, ws := ev.eval(e)
+				otherArgs[i] = val.(Matrix)
 				otherInArgs[i] = Vector{Sample{}}
 				inArgs[i] = otherInArgs[i]
+				warnings = append(warnings, ws...)
 			}
 		}
 
 		sel := e.Args[matrixArgIndex].(*parser.MatrixSelector)
 		selVS := sel.VectorSelector.(*parser.VectorSelector)
 
-		checkForSeriesSetExpansion(ev.ctx, sel)
+		ws, err := checkAndExpandSeriesSet(ev.ctx, sel)
+		warnings = append(warnings, ws...)
+		if err != nil {
+			ev.error(errWithWarnings{errors.Wrap(err, "expanding series"), warnings})
+		}
 		mat := make(Matrix, 0, len(selVS.Series)) // Output matrix.
 		offset := durationMilliseconds(selVS.Offset)
 		selRange := durationMilliseconds(sel.Range)
@@ -1182,7 +1194,7 @@ func (ev *evaluator) eval(expr parser.Expr) parser.Value {
 			// Iterate once to look for a complete series.
 			for _, s := range mat {
 				if len(s.Points) == steps {
-					return Matrix{}
+					return Matrix{}, warnings
 				}
 			}
 
@@ -1193,7 +1205,7 @@ func (ev *evaluator) eval(expr parser.Expr) parser.Value {
 					found[p.T] = struct{}{}
 				}
 				if i > 0 && len(found) == steps {
-					return Matrix{}
+					return Matrix{}, warnings
 				}
 			}
 
@@ -1209,20 +1221,21 @@ func (ev *evaluator) eval(expr parser.Expr) parser.Value {
 					Metric: createLabelsForAbsentFunction(e.Args[0]),
 					Points: newp,
 				},
-			}
+			}, warnings
 		}
 
 		if mat.ContainsSameLabelset() {
 			ev.errorf("vector cannot contain metrics with the same labelset")
 		}
 
-		return mat
+		return mat, warnings
 
 	case *parser.ParenExpr:
 		return ev.eval(e.Expr)
 
 	case *parser.UnaryExpr:
-		mat := ev.eval(e.Expr).(Matrix)
+		val, ws := ev.eval(e.Expr)
+		mat := val.(Matrix)
 		if e.Op == parser.SUB {
 			for i := range mat {
 				mat[i].Metric = dropMetricName(mat[i].Metric)
@@ -1234,53 +1247,56 @@ func (ev *evaluator) eval(expr parser.Expr) parser.Value {
 				ev.errorf("vector cannot contain metrics with the same labelset")
 			}
 		}
-		return mat
+		return mat, ws
 
 	case *parser.BinaryExpr:
 		switch lt, rt := e.LHS.Type(), e.RHS.Type(); {
 		case lt == parser.ValueTypeScalar && rt == parser.ValueTypeScalar:
-			return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) Vector {
+			return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) (Vector, storage.Warnings) {
 				val := scalarBinop(e.Op, v[0].(Vector)[0].Point.V, v[1].(Vector)[0].Point.V)
-				return append(enh.out, Sample{Point: Point{V: val}})
+				return append(enh.out, Sample{Point: Point{V: val}}), nil
 			}, e.LHS, e.RHS)
 		case lt == parser.ValueTypeVector && rt == parser.ValueTypeVector:
 			switch e.Op {
 			case parser.LAND:
-				return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) Vector {
-					return ev.VectorAnd(v[0].(Vector), v[1].(Vector), e.VectorMatching, enh)
+				return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) (Vector, storage.Warnings) {
+					return ev.VectorAnd(v[0].(Vector), v[1].(Vector), e.VectorMatching, enh), nil
 				}, e.LHS, e.RHS)
 			case parser.LOR:
-				return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) Vector {
-					return ev.VectorOr(v[0].(Vector), v[1].(Vector), e.VectorMatching, enh)
+				return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) (Vector, storage.Warnings) {
+					return ev.VectorOr(v[0].(Vector), v[1].(Vector), e.VectorMatching, enh), nil
 				}, e.LHS, e.RHS)
 			case parser.LUNLESS:
-				return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) Vector {
-					return ev.VectorUnless(v[0].(Vector), v[1].(Vector), e.VectorMatching, enh)
+				return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) (Vector, storage.Warnings) {
+					return ev.VectorUnless(v[0].(Vector), v[1].(Vector), e.VectorMatching, enh), nil
 				}, e.LHS, e.RHS)
 			default:
-				return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) Vector {
-					return ev.VectorBinop(e.Op, v[0].(Vector), v[1].(Vector), e.VectorMatching, e.ReturnBool, enh)
+				return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) (Vector, storage.Warnings) {
+					return ev.VectorBinop(e.Op, v[0].(Vector), v[1].(Vector), e.VectorMatching, e.ReturnBool, enh), nil
 				}, e.LHS, e.RHS)
 			}
 
 		case lt == parser.ValueTypeVector && rt == parser.ValueTypeScalar:
-			return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) Vector {
-				return ev.VectorscalarBinop(e.Op, v[0].(Vector), Scalar{V: v[1].(Vector)[0].Point.V}, false, e.ReturnBool, enh)
+			return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) (Vector, storage.Warnings) {
+				return ev.VectorscalarBinop(e.Op, v[0].(Vector), Scalar{V: v[1].(Vector)[0].Point.V}, false, e.ReturnBool, enh), nil
 			}, e.LHS, e.RHS)
 
 		case lt == parser.ValueTypeScalar && rt == parser.ValueTypeVector:
-			return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) Vector {
-				return ev.VectorscalarBinop(e.Op, v[1].(Vector), Scalar{V: v[0].(Vector)[0].Point.V}, true, e.ReturnBool, enh)
+			return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) (Vector, storage.Warnings) {
+				return ev.VectorscalarBinop(e.Op, v[1].(Vector), Scalar{V: v[0].(Vector)[0].Point.V}, true, e.ReturnBool, enh), nil
 			}, e.LHS, e.RHS)
 		}
 
 	case *parser.NumberLiteral:
-		return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) Vector {
-			return append(enh.out, Sample{Point: Point{V: e.Val}})
+		return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) (Vector, storage.Warnings) {
+			return append(enh.out, Sample{Point: Point{V: e.Val}}), nil
 		})
 
 	case *parser.VectorSelector:
-		checkForSeriesSetExpansion(ev.ctx, e)
+		ws, err := checkAndExpandSeriesSet(ev.ctx, e)
+		if err != nil {
+			ev.error(errWithWarnings{errors.Wrap(err, "expanding series"), ws})
+		}
 		mat := make(Matrix, 0, len(e.Series))
 		it := storage.NewBuffer(durationMilliseconds(ev.lookbackDelta))
 		for i, s := range e.Series {
@@ -1307,9 +1323,8 @@ func (ev *evaluator) eval(expr parser.Expr) parser.Value {
 			} else {
 				putPointSlice(ss.Points)
 			}
-
 		}
-		return mat
+		return mat, ws
 
 	case *parser.MatrixSelector:
 		if ev.startTimestamp != ev.endTimestamp {
@@ -1342,11 +1357,11 @@ func (ev *evaluator) eval(expr parser.Expr) parser.Value {
 			newEv.startTimestamp += newEv.interval
 		}
 
-		res := newEv.eval(e.Expr)
+		res, ws := newEv.eval(e.Expr)
 		ev.currentSamples = newEv.currentSamples
-		return res
+		return res, ws
 	case *parser.StringLiteral:
-		return String{V: e.Val, T: ev.startTimestamp}
+		return String{V: e.Val, T: ev.startTimestamp}, nil
 	}
 
 	panic(errors.Errorf("unhandled expression of type: %T", expr))
@@ -1357,13 +1372,12 @@ func durationToInt64Millis(d time.Duration) int64 {
 }
 
 // vectorSelector evaluates a *parser.VectorSelector expression.
-func (ev *evaluator) vectorSelector(node *parser.VectorSelector, ts int64) Vector {
-	checkForSeriesSetExpansion(ev.ctx, node)
-
-	var (
-		vec = make(Vector, 0, len(node.Series))
-	)
-
+func (ev *evaluator) vectorSelector(node *parser.VectorSelector, ts int64) (Vector, storage.Warnings) {
+	ws, err := checkAndExpandSeriesSet(ev.ctx, node)
+	if err != nil {
+		ev.error(errWithWarnings{errors.Wrap(err, "expanding series"), ws})
+	}
+	vec := make(Vector, 0, len(node.Series))
 	it := storage.NewBuffer(durationMilliseconds(ev.lookbackDelta))
 	for i, s := range node.Series {
 		it.Reset(s.Iterator())
@@ -1381,7 +1395,7 @@ func (ev *evaluator) vectorSelector(node *parser.VectorSelector, ts int64) Vecto
 			ev.error(ErrTooManySamples(env))
 		}
 	}
-	return vec
+	return vec, ws
 }
 
 // vectorSelectorSingle evaluates a instant vector for the iterator of one time series.
@@ -1429,21 +1443,23 @@ func putPointSlice(p []Point) {
 }
 
 // matrixSelector evaluates a *parser.MatrixSelector expression.
-func (ev *evaluator) matrixSelector(node *parser.MatrixSelector) Matrix {
-	checkForSeriesSetExpansion(ev.ctx, node)
-
-	vs := node.VectorSelector.(*parser.VectorSelector)
-
+func (ev *evaluator) matrixSelector(node *parser.MatrixSelector) (Matrix, storage.Warnings) {
 	var (
+		vs = node.VectorSelector.(*parser.VectorSelector)
+
 		offset = durationMilliseconds(vs.Offset)
 		maxt   = ev.startTimestamp - offset
 		mint   = maxt - durationMilliseconds(node.Range)
 		matrix = make(Matrix, 0, len(vs.Series))
+
+		it = storage.NewBuffer(durationMilliseconds(node.Range))
 	)
+	ws, err := checkAndExpandSeriesSet(ev.ctx, node)
+	if err != nil {
+		ev.error(errWithWarnings{errors.Wrap(err, "expanding series"), ws})
+	}
 
-	it := storage.NewBuffer(durationMilliseconds(node.Range))
 	series := vs.Series
-
 	for i, s := range series {
 		if err := contextDone(ev.ctx, "expression evaluation"); err != nil {
 			ev.error(err)
@@ -1461,7 +1477,7 @@ func (ev *evaluator) matrixSelector(node *parser.MatrixSelector) Matrix {
 			putPointSlice(ss.Points)
 		}
 	}
-	return matrix
+	return matrix, ws
 }
 
 // matrixIterSlice populates a matrix vector covering the requested range for a

promql/engine_test.go

@@ -19,7 +19,6 @@ import (
 	"io/ioutil"
 	"os"
 	"sort"
-	"strings"
 	"testing"
 	"time"
 
@@ -117,9 +116,7 @@ func TestQueryTimeout(t *testing.T) {
 	testutil.NotOk(t, res.Err, "expected timeout error but got none")
 
 	var e ErrQueryTimeout
-	// TODO: when circleci-windows moves to go 1.13:
-	// testutil.Assert(t, errors.As(res.Err, &e), "expected timeout error but got: %s", res.Err)
-	testutil.Assert(t, strings.HasPrefix(res.Err.Error(), e.Error()), "expected timeout error but got: %s", res.Err)
+	testutil.Assert(t, errors.As(res.Err, &e), "expected timeout error but got: %s", res.Err)
 }
 
 const errQueryCanceled = ErrQueryCanceled("test statement execution")
@@ -175,8 +172,8 @@ type errQuerier struct {
 	err error
 }
 
-func (q *errQuerier) Select(bool, *storage.SelectHints, ...*labels.Matcher) (storage.SeriesSet, storage.Warnings, error) {
-	return errSeriesSet{err: q.err}, nil, q.err
+func (q *errQuerier) Select(bool, *storage.SelectHints, ...*labels.Matcher) storage.SeriesSet {
+	return errSeriesSet{err: q.err}
 }
 func (*errQuerier) LabelValues(string) ([]string, storage.Warnings, error) { return nil, nil, nil }
 func (*errQuerier) LabelNames() ([]string, storage.Warnings, error)        { return nil, nil, nil }
@@ -187,9 +184,10 @@ type errSeriesSet struct {
 	err error
 }
 
-func (errSeriesSet) Next() bool         { return false }
-func (errSeriesSet) At() storage.Series { return nil }
-func (e errSeriesSet) Err() error       { return e.err }
+func (errSeriesSet) Next() bool                   { return false }
+func (errSeriesSet) At() storage.Series           { return nil }
+func (e errSeriesSet) Err() error                 { return e.err }
+func (e errSeriesSet) Warnings() storage.Warnings { return nil }
 
 func TestQueryError(t *testing.T) {
 	opts := EngineOpts{
@@ -211,14 +209,14 @@ func TestQueryError(t *testing.T) {
 
 	res := vectorQuery.Exec(ctx)
 	testutil.NotOk(t, res.Err, "expected error on failed select but got none")
-	testutil.Equals(t, errStorage, res.Err)
+	testutil.Assert(t, errors.Is(res.Err, errStorage), "expected error doesn't match")
 
 	matrixQuery, err := engine.NewInstantQuery(queryable, "foo[1m]", time.Unix(1, 0))
 	testutil.Ok(t, err)
 
 	res = matrixQuery.Exec(ctx)
 	testutil.NotOk(t, res.Err, "expected error on failed select but got none")
-	testutil.Equals(t, errStorage, res.Err)
+	testutil.Assert(t, errors.Is(res.Err, errStorage), "expected error doesn't match")
 }
 
 // hintCheckerQuerier implements storage.Querier which checks the start and end times
@@ -234,7 +232,7 @@ type hintCheckerQuerier struct {
 	t *testing.T
 }
 
-func (q *hintCheckerQuerier) Select(_ bool, sp *storage.SelectHints, _ ...*labels.Matcher) (storage.SeriesSet, storage.Warnings, error) {
+func (q *hintCheckerQuerier) Select(_ bool, sp *storage.SelectHints, _ ...*labels.Matcher) storage.SeriesSet {
 	testutil.Equals(q.t, q.start, sp.Start)
 	testutil.Equals(q.t, q.end, sp.End)
 	testutil.Equals(q.t, q.grouping, sp.Grouping)
@@ -242,7 +240,7 @@ func (q *hintCheckerQuerier) Select(_ bool, sp *storage.SelectHints, _ ...*label
 	testutil.Equals(q.t, q.selRange, sp.Range)
 	testutil.Equals(q.t, q.function, sp.Func)
 
-	return errSeriesSet{err: nil}, nil, nil
+	return errSeriesSet{err: nil}
 }
 func (*hintCheckerQuerier) LabelValues(string) ([]string, storage.Warnings, error) {
 	return nil, nil, nil
@@ -499,9 +497,7 @@ func TestEngineShutdown(t *testing.T) {
 	testutil.NotOk(t, res2.Err, "expected error on querying with canceled context but got none")
 
 	var e ErrQueryCanceled
-	// TODO: when circleci-windows moves to go 1.13:
-	// testutil.Assert(t, errors.As(res2.Err, &e), "expected cancellation error but got: %s", res2.Err)
-	testutil.Assert(t, strings.HasPrefix(res2.Err.Error(), e.Error()), "expected cancellation error but got: %s", res2.Err)
+	testutil.Assert(t, errors.As(res2.Err, &e), "expected cancellation error but got: %s", res2.Err)
 }
 
 func TestEngineEvalStmtTimestamps(t *testing.T) {
@@ -599,9 +595,8 @@ load 10s
 		}
 
 		testutil.Ok(t, res.Err)
-		testutil.Equals(t, c.Result, res.Value)
+		testutil.Equals(t, c.Result, res.Value, "query %q failed", c.Query)
 	}
-
 }
 
 func TestMaxQuerySamples(t *testing.T) {
@@ -831,7 +826,7 @@ load 10s
 
 		res := qry.Exec(test.Context())
 		testutil.Equals(t, c.Result.Err, res.Err)
-		testutil.Equals(t, c.Result.Value, res.Value)
+		testutil.Equals(t, c.Result.Value, res.Value, "query %q failed", c.Query)
 	}
 }
 
@@ -839,7 +834,7 @@ func TestRecoverEvaluatorRuntime(t *testing.T) {
 	ev := &evaluator{logger: log.NewNopLogger()}
 
 	var err error
-	defer ev.recover(&err)
+	defer ev.recover(nil, &err)
 
 	// Cause a runtime panic.
 	var a []int
@@ -862,7 +857,31 @@ func TestRecoverEvaluatorError(t *testing.T) {
 			t.Fatalf("wrong error message: %q, expected %q", err, e)
 		}
 	}()
-	defer ev.recover(&err)
+	defer ev.recover(nil, &err)
+
+	panic(e)
+}
+
+func TestRecoverEvaluatorErrorWithWarnings(t *testing.T) {
+	ev := &evaluator{logger: log.NewNopLogger()}
+	var err error
+	var ws storage.Warnings
+
+	warnings := storage.Warnings{errors.New("custom warning")}
+	e := errWithWarnings{
+		err:      errors.New("custom error"),
+		warnings: warnings,
+	}
+
+	defer func() {
+		if err.Error() != e.Error() {
+			t.Fatalf("wrong error message: %q, expected %q", err, e)
+		}
+		if len(ws) != len(warnings) && ws[0] != warnings[0] {
+			t.Fatalf("wrong warning message: %q, expected %q", ws[0], warnings[0])
+		}
+	}()
+	defer ev.recover(&ws, &err)
 
 	panic(e)
 }

promql/test_test.go

@@ -133,8 +133,7 @@ func TestLazyLoader_WithSamplesTill(t *testing.T) {
 					}
 
 					// Get the series for the matcher.
-					ss, _, err := querier.Select(false, nil, matchers...)
-					testutil.Ok(t, err)
+					ss := querier.Select(false, nil, matchers...)
 					testutil.Assert(t, ss.Next(), "")
 					storageSeries := ss.At()
 					testutil.Assert(t, !ss.Next(), "Expecting only 1 series")

rules/manager.go

@@ -707,12 +707,7 @@ func (g *Group) RestoreForState(ts time.Time) {
 				matchers = append(matchers, mt)
 			}
 
-			sset, err, _ := q.Select(false, nil, matchers...)
-			if err != nil {
-				level.Error(g.logger).Log("msg", "Failed to restore 'for' state",
-					labels.AlertName, alertRule.Name(), "stage", "Select", "err", err)
-				return
-			}
+			sset := q.Select(false, nil, matchers...)
 
 			seriesFound := false
 			var s storage.Series
@@ -727,6 +722,17 @@ func (g *Group) RestoreForState(ts time.Time) {
 				}
 			}
 
+			if err := sset.Err(); err != nil {
+				// Querier Warnings are ignored. We do not care unless we have an error.
+				level.Error(g.logger).Log(
+					"msg", "Failed to restore 'for' state",
+					labels.AlertName, alertRule.Name(),
+					"stage", "Select",
+					"err", err,
+				)
+				return
+			}
+
 			if !seriesFound {
 				return
 			}

rules/manager_test.go

@@ -563,9 +563,7 @@ func TestStaleness(t *testing.T) {
 	matcher, err := labels.NewMatcher(labels.MatchEqual, model.MetricNameLabel, "a_plus_one")
 	testutil.Ok(t, err)
 
-	set, _, err := querier.Select(false, nil, matcher)
-	testutil.Ok(t, err)
-
+	set := querier.Select(false, nil, matcher)
 	samples, err := readSeriesSet(set)
 	testutil.Ok(t, err)
 
@@ -686,9 +684,7 @@ func TestDeletedRuleMarkedStale(t *testing.T) {
 	matcher, err := labels.NewMatcher(labels.MatchEqual, "l1", "v1")
 	testutil.Ok(t, err)
 
-	set, _, err := querier.Select(false, nil, matcher)
-	testutil.Ok(t, err)
-
+	set := querier.Select(false, nil, matcher)
 	samples, err := readSeriesSet(set)
 	testutil.Ok(t, err)
 
@@ -1107,9 +1103,7 @@ func countStaleNaN(t *testing.T, st storage.Storage) int {
 	matcher, err := labels.NewMatcher(labels.MatchEqual, model.MetricNameLabel, "test_2")
 	testutil.Ok(t, err)
 
-	set, _, err := querier.Select(false, nil, matcher)
-	testutil.Ok(t, err)
-
+	set := querier.Select(false, nil, matcher)
 	samples, err := readSeriesSet(set)
 	testutil.Ok(t, err)
 

scrape/scrape.go

@@ -1140,7 +1140,7 @@ loop:
 
 		if ok {
 			err = app.AddFast(ce.ref, t, v)
-			sampleAdded, err = sl.checkAddError(ce, met, tp, err, &sampleLimitErr, &appErrs)
+			_, err = sl.checkAddError(ce, met, tp, err, &sampleLimitErr, &appErrs)
 			// In theory this should never happen.
 			if err == storage.ErrNotFound {
 				ok = false
@@ -1187,10 +1187,10 @@ loop:
 			}
 		}
 
-		// Increment added even if there's a sampleLimitErr so we correctly report the number of samples scraped.
-		if sampleAdded || sampleLimitErr != nil {
-			added++
-		}
+		// Increment added even if there's an error so we correctly report the
+		// number of samples remaining after relabelling.
+		added++
+
 	}
 	if sampleLimitErr != nil {
 		if err == nil {
@@ -1275,7 +1275,7 @@ const (
 	scrapeSeriesAddedMetricName  = "scrape_series_added" + "\xff"
 )
 
-func (sl *scrapeLoop) report(start time.Time, duration time.Duration, scraped, appended, seriesAdded int, scrapeErr error) (err error) {
+func (sl *scrapeLoop) report(start time.Time, duration time.Duration, scraped, added, seriesAdded int, scrapeErr error) (err error) {
 	sl.scraper.Report(start, duration, scrapeErr)
 
 	ts := timestamp.FromTime(start)
@@ -1302,7 +1302,7 @@ func (sl *scrapeLoop) report(start time.Time, duration time.Duration, scraped, a
 	if err = sl.addReportSample(app, scrapeSamplesMetricName, ts, float64(scraped)); err != nil {
 		return
 	}
-	if err = sl.addReportSample(app, samplesPostRelabelMetricName, ts, float64(appended)); err != nil {
+	if err = sl.addReportSample(app, samplesPostRelabelMetricName, ts, float64(added)); err != nil {
 		return
 	}
 	if err = sl.addReportSample(app, scrapeSeriesAddedMetricName, ts, float64(seriesAdded)); err != nil {

scrape/scrape_test.go

@@ -1340,7 +1340,7 @@ func TestScrapeLoopAppendGracefullyIfAmendOrOutOfOrderOrOutOfBounds(t *testing.T
 	}
 	testutil.Equals(t, want, app.result, "Appended samples not as expected")
 	testutil.Equals(t, 4, total)
-	testutil.Equals(t, 1, added)
+	testutil.Equals(t, 4, added)
 	testutil.Equals(t, 1, seriesAdded)
 }
 
@@ -1365,7 +1365,7 @@ func TestScrapeLoopOutOfBoundsTimeError(t *testing.T) {
 	now := time.Now().Add(20 * time.Minute)
 	total, added, seriesAdded, err := sl.append([]byte("normal 1\n"), "", now)
 	testutil.Equals(t, 1, total)
-	testutil.Equals(t, 0, added)
+	testutil.Equals(t, 1, added)
 	testutil.Equals(t, 0, seriesAdded)
 
 	testutil.Ok(t, err)
@@ -1617,9 +1617,9 @@ func TestScrapeLoopDiscardDuplicateLabels(t *testing.T) {
 
 	q, err := s.Querier(ctx, time.Time{}.UnixNano(), 0)
 	testutil.Ok(t, err)
-	series, _, err := q.Select(false, nil, labels.MustNewMatcher(labels.MatchRegexp, "__name__", ".*"))
-	testutil.Ok(t, err)
+	series := q.Select(false, nil, labels.MustNewMatcher(labels.MatchRegexp, "__name__", ".*"))
 	testutil.Equals(t, false, series.Next(), "series found in tsdb")
+	testutil.Ok(t, series.Err())
 
 	// We add a good metric to check that it is recorded.
 	_, _, _, err = sl.append([]byte("test_metric{le=\"500\"} 1\n"), "", time.Time{})
@@ -1627,9 +1627,9 @@ func TestScrapeLoopDiscardDuplicateLabels(t *testing.T) {
 
 	q, err = s.Querier(ctx, time.Time{}.UnixNano(), 0)
 	testutil.Ok(t, err)
-	series, _, err = q.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "le", "500"))
-	testutil.Ok(t, err)
+	series = q.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "le", "500"))
 	testutil.Equals(t, true, series.Next(), "series not found in tsdb")
+	testutil.Ok(t, series.Err())
 	testutil.Equals(t, false, series.Next(), "more than one series found in tsdb")
 }
 
@@ -1663,9 +1663,9 @@ func TestScrapeLoopDiscardUnnamedMetrics(t *testing.T) {
 
 	q, err := s.Querier(ctx, time.Time{}.UnixNano(), 0)
 	testutil.Ok(t, err)
-	series, _, err := q.Select(false, nil, labels.MustNewMatcher(labels.MatchRegexp, "__name__", ".*"))
-	testutil.Ok(t, err)
+	series := q.Select(false, nil, labels.MustNewMatcher(labels.MatchRegexp, "__name__", ".*"))
 	testutil.Equals(t, false, series.Next(), "series found in tsdb")
+	testutil.Ok(t, series.Err())
 }
 
 func TestReusableConfig(t *testing.T) {

storage/fanout.go

@@ -16,14 +16,15 @@ package storage
 import (
 	"container/heap"
 	"context"
-	"reflect"
 	"sort"
 	"strings"
+	"sync"
 
 	"github.com/go-kit/kit/log"
 	"github.com/go-kit/kit/log/level"
 	"github.com/pkg/errors"
 	"github.com/prometheus/common/model"
+
 	"github.com/prometheus/prometheus/pkg/labels"
 	"github.com/prometheus/prometheus/tsdb/chunkenc"
 	"github.com/prometheus/prometheus/tsdb/chunks"
@@ -37,8 +38,15 @@ type fanout struct {
 	secondaries []Storage
 }
 
-// NewFanout returns a new fan-out Storage, which proxies reads and writes
+// NewFanout returns a new fanout Storage, which proxies reads and writes
 // through to multiple underlying storages.
+//
+// The difference between primary and secondary Storage is only for read (Querier) path and it goes as follows:
+// * If the primary querier returns an error, then any of the Querier operations will fail.
+// * If any secondary querier returns an error the result from that queries is discarded. The overall operation will succeed,
+// and the error from the secondary querier will be returned as a warning.
+//
+// NOTE: In the case of Prometheus, it treats all remote storages as secondary / best effort.
 func NewFanout(logger log.Logger, primary Storage, secondaries ...Storage) Storage {
 	return &fanout{
 		logger:      logger,
@@ -56,8 +64,8 @@ func (f *fanout) StartTime() (int64, error) {
 		return int64(model.Latest), err
 	}
 
-	for _, storage := range f.secondaries {
-		t, err := storage.StartTime()
+	for _, s := range f.secondaries {
+		t, err := s.StartTime()
 		if err != nil {
 			return int64(model.Latest), err
 		}
@@ -69,29 +77,27 @@ func (f *fanout) StartTime() (int64, error) {
 }
 
 func (f *fanout) Querier(ctx context.Context, mint, maxt int64) (Querier, error) {
-	queriers := make([]Querier, 0, 1+len(f.secondaries))
-
-	// Add primary querier.
-	primaryQuerier, err := f.primary.Querier(ctx, mint, maxt)
+	primary, err := f.primary.Querier(ctx, mint, maxt)
 	if err != nil {
 		return nil, err
 	}
-	queriers = append(queriers, primaryQuerier)
 
-	// Add secondary queriers.
+	secondaries := make([]Querier, 0, len(f.secondaries))
 	for _, storage := range f.secondaries {
 		querier, err := storage.Querier(ctx, mint, maxt)
 		if err != nil {
-			for _, q := range queriers {
-				// TODO(bwplotka): Log error.
-				_ = q.Close()
+			// Close already open Queriers, append potential errors to returned error.
+			errs := tsdb_errors.MultiError{err}
+			errs.Add(primary.Close())
+			for _, q := range secondaries {
+				errs.Add(q.Close())
 			}
-			return nil, err
+			return nil, errs.Err()
 		}
-		queriers = append(queriers, querier)
-	}
 
-	return NewMergeQuerier(primaryQuerier, queriers, ChainedSeriesMerge), nil
+		secondaries = append(secondaries, querier)
+	}
+	return NewMergeQuerier(primary, secondaries, ChainedSeriesMerge), nil
 }
 
 func (f *fanout) Appender() Appender {
@@ -109,18 +115,12 @@ func (f *fanout) Appender() Appender {
 
 // Close closes the storage and all its underlying resources.
 func (f *fanout) Close() error {
-	if err := f.primary.Close(); err != nil {
-		return err
+	errs := tsdb_errors.MultiError{}
+	errs.Add(f.primary.Close())
+	for _, s := range f.secondaries {
+		errs.Add(s.Close())
 	}
-
-	// TODO return multiple errors?
-	var lastErr error
-	for _, storage := range f.secondaries {
-		if err := storage.Close(); err != nil {
-			lastErr = err
-		}
-	}
-	return lastErr
+	return errs.Err()
 }
 
 // fanoutAppender implements Appender.
@@ -188,153 +188,138 @@ func (f *fanoutAppender) Rollback() (err error) {
 }
 
 type mergeGenericQuerier struct {
-	mergeFunc genericSeriesMergeFunc
-
-	primaryQuerier genericQuerier
-	queriers       []genericQuerier
-	failedQueriers map[genericQuerier]struct{}
-	setQuerierMap  map[genericSeriesSet]genericQuerier
-}
-
-// NewMergeQuerier returns a new Querier that merges results of chkQuerierSeries queriers.
-// NewMergeQuerier will return NoopQuerier if no queriers are passed to it
-// and will filter NoopQueriers from its arguments, in order to reduce overhead
-// when only one querier is passed.
-// The difference between primary and secondary is as follows: f the primaryQuerier returns an error, query fails.
-// For secondaries it just return warnings.
-func NewMergeQuerier(primaryQuerier Querier, queriers []Querier, mergeFunc VerticalSeriesMergeFunc) Querier {
-	filtered := make([]genericQuerier, 0, len(queriers))
-	for _, querier := range queriers {
-		if _, ok := querier.(noopQuerier); !ok && querier != nil {
-			filtered = append(filtered, newGenericQuerierFrom(querier))
-		}
-	}
+	queriers []genericQuerier
 
-	if len(filtered) == 0 {
-		return primaryQuerier
-	}
+	// mergeFn is used when we see series from different queriers Selects with the same labels.
+	mergeFn genericSeriesMergeFunc
+}
 
-	if primaryQuerier == nil && len(filtered) == 1 {
-		return &querierAdapter{filtered[0]}
+// NewMergeQuerier returns a new Querier that merges results of given primary and slice of secondary queriers.
+// See NewFanout commentary to learn more about primary vs secondary differences.
+//
+// In case of overlaps between the data given by primary + secondaries Selects, merge function will be used.
+func NewMergeQuerier(primary Querier, secondaries []Querier, mergeFn VerticalSeriesMergeFunc) Querier {
+	queriers := make([]genericQuerier, 0, len(secondaries)+1)
+	if primary != nil {
+		queriers = append(queriers, newGenericQuerierFrom(primary))
+	}
+	for _, querier := range secondaries {
+		if _, ok := querier.(noopQuerier); !ok && querier != nil {
+			queriers = append(queriers, newSecondaryQuerierFrom(querier))
+		}
 	}
 
 	return &querierAdapter{&mergeGenericQuerier{
-		mergeFunc:      (&seriesMergerAdapter{VerticalSeriesMergeFunc: mergeFunc}).Merge,
-		primaryQuerier: newGenericQuerierFrom(primaryQuerier),
-		queriers:       filtered,
-		failedQueriers: make(map[genericQuerier]struct{}),
-		setQuerierMap:  make(map[genericSeriesSet]genericQuerier),
+		mergeFn:  (&seriesMergerAdapter{VerticalSeriesMergeFunc: mergeFn}).Merge,
+		queriers: queriers,
 	}}
 }
 
-// NewMergeChunkQuerier returns a new ChunkQuerier that merges results of chkQuerierSeries chunk queriers.
-// NewMergeChunkQuerier will return NoopChunkQuerier if no chunk queriers are passed to it,
-// and will filter NoopQuerieNoopChunkQuerierrs from its arguments, in order to reduce overhead
-// when only one chunk querier is passed.
-func NewMergeChunkQuerier(primaryQuerier ChunkQuerier, queriers []ChunkQuerier, merger VerticalChunkSeriesMergerFunc) ChunkQuerier {
-	filtered := make([]genericQuerier, 0, len(queriers))
-	for _, querier := range queriers {
+// NewMergeChunkQuerier returns a new ChunkQuerier that merges results of given primary and slice of secondary chunk queriers.
+// See NewFanout commentary to learn more about primary vs secondary differences.
+//
+// In case of overlaps between the data given by primary + secondaries Selects, merge function will be used.
+// TODO(bwplotka): Currently merge will compact overlapping chunks with bigger chunk, without limit. Split it: https://github.com/prometheus/tsdb/issues/670
+func NewMergeChunkQuerier(primary ChunkQuerier, secondaries []ChunkQuerier, mergeFn VerticalChunkSeriesMergerFunc) ChunkQuerier {
+	queriers := make([]genericQuerier, 0, len(secondaries)+1)
+	if primary != nil {
+		queriers = append(queriers, newGenericQuerierFromChunk(primary))
+	}
+	for _, querier := range secondaries {
 		if _, ok := querier.(noopChunkQuerier); !ok && querier != nil {
-			filtered = append(filtered, newGenericQuerierFromChunk(querier))
+			queriers = append(queriers, newSecondaryQuerierFromChunk(querier))
 		}
 	}
 
-	if len(filtered) == 0 {
-		return primaryQuerier
-	}
-
-	if primaryQuerier == nil && len(filtered) == 1 {
-		return &chunkQuerierAdapter{filtered[0]}
-	}
-
 	return &chunkQuerierAdapter{&mergeGenericQuerier{
-		mergeFunc:      (&chunkSeriesMergerAdapter{VerticalChunkSeriesMergerFunc: merger}).Merge,
-		primaryQuerier: newGenericQuerierFromChunk(primaryQuerier),
-		queriers:       filtered,
-		failedQueriers: make(map[genericQuerier]struct{}),
-		setQuerierMap:  make(map[genericSeriesSet]genericQuerier),
+		mergeFn:  (&chunkSeriesMergerAdapter{VerticalChunkSeriesMergerFunc: mergeFn}).Merge,
+		queriers: queriers,
 	}}
 }
 
 // Select returns a set of series that matches the given label matchers.
-func (q *mergeGenericQuerier) Select(sortSeries bool, hints *SelectHints, matchers ...*labels.Matcher) (genericSeriesSet, Warnings, error) {
+func (q *mergeGenericQuerier) Select(sortSeries bool, hints *SelectHints, matchers ...*labels.Matcher) genericSeriesSet {
 	if len(q.queriers) == 1 {
 		return q.queriers[0].Select(sortSeries, hints, matchers...)
 	}
 
 	var (
-		seriesSets = make([]genericSeriesSet, 0, len(q.queriers))
-		warnings   Warnings
-		priErr     error
+		seriesSets    = make([]genericSeriesSet, 0, len(q.queriers))
+		wg            sync.WaitGroup
+		seriesSetChan = make(chan genericSeriesSet)
 	)
-	type queryResult struct {
-		qr          genericQuerier
-		set         genericSeriesSet
-		wrn         Warnings
-		selectError error
-	}
-	queryResultChan := make(chan *queryResult)
 
+	// Schedule all Selects for all queriers we know about.
 	for _, querier := range q.queriers {
+		wg.Add(1)
 		go func(qr genericQuerier) {
+			defer wg.Done()
+
 			// We need to sort for NewMergeSeriesSet to work.
-			set, wrn, err := qr.Select(true, hints, matchers...)
-			queryResultChan <- &queryResult{qr: qr, set: set, wrn: wrn, selectError: err}
+			seriesSetChan <- qr.Select(true, hints, matchers...)
 		}(querier)
 	}
-	for i := 0; i < len(q.queriers); i++ {
-		qryResult := <-queryResultChan
-		q.setQuerierMap[qryResult.set] = qryResult.qr
-		if qryResult.wrn != nil {
-			warnings = append(warnings, qryResult.wrn...)
-		}
-		if qryResult.selectError != nil {
-			q.failedQueriers[qryResult.qr] = struct{}{}
-			// If the error source isn't the primary querier, return the error as a warning and continue.
-			if !reflect.DeepEqual(qryResult.qr, q.primaryQuerier) {
-				warnings = append(warnings, qryResult.selectError)
-			} else {
-				priErr = qryResult.selectError
+	go func() {
+		wg.Wait()
+		close(seriesSetChan)
+	}()
+
+	for r := range seriesSetChan {
+		seriesSets = append(seriesSets, r)
+	}
+	return &lazySeriesSet{create: create(seriesSets, q.mergeFn)}
+}
+
+func create(seriesSets []genericSeriesSet, mergeFn genericSeriesMergeFunc) func() (genericSeriesSet, bool) {
+	// Returned function gets called with the first call to Next().
+	return func() (genericSeriesSet, bool) {
+		if len(seriesSets) == 1 {
+			return seriesSets[0], seriesSets[0].Next()
+		}
+		var h genericSeriesSetHeap
+		for _, set := range seriesSets {
+			if set == nil {
+				continue
+			}
+			if set.Next() {
+				heap.Push(&h, set)
+				continue
+			}
+			// When primary fails ignore results from secondaries.
+			// Only the primary querier returns error.
+			if err := set.Err(); err != nil {
+				return errorOnlySeriesSet{err}, false
 			}
-			continue
 		}
-		seriesSets = append(seriesSets, qryResult.set)
-	}
-	if priErr != nil {
-		return nil, nil, priErr
+		set := &genericMergeSeriesSet{
+			mergeFn: mergeFn,
+			sets:    seriesSets,
+			heap:    h,
+		}
+		return set, set.Next()
 	}
-	return newGenericMergeSeriesSet(seriesSets, q, q.mergeFunc), warnings, nil
 }
 
 // LabelValues returns all potential values for a label name.
 func (q *mergeGenericQuerier) LabelValues(name string) ([]string, Warnings, error) {
-	var results [][]string
-	var warnings Warnings
+	var (
+		results  [][]string
+		warnings Warnings
+	)
 	for _, querier := range q.queriers {
 		values, wrn, err := querier.LabelValues(name)
 		if wrn != nil {
+			// TODO(bwplotka): We could potentially wrap warnings.
 			warnings = append(warnings, wrn...)
 		}
 		if err != nil {
-			q.failedQueriers[querier] = struct{}{}
-			// If the error source isn't the primary querier, return the error as a warning and continue.
-			if querier != q.primaryQuerier {
-				warnings = append(warnings, err)
-				continue
-			} else {
-				return nil, nil, err
-			}
+			return nil, nil, errors.Wrapf(err, "LabelValues() from Querier for label %s", name)
 		}
 		results = append(results, values)
 	}
 	return mergeStringSlices(results), warnings, nil
 }
 
-func (q *mergeGenericQuerier) IsFailedSet(set genericSeriesSet) bool {
-	_, isFailedQuerier := q.failedQueriers[q.setQuerierMap[set]]
-	return isFailedQuerier
-}
-
 func mergeStringSlices(ss [][]string) []string {
 	switch len(ss) {
 	case 0:
@@ -381,37 +366,31 @@ func (q *mergeGenericQuerier) LabelNames() ([]string, Warnings, error) {
 	for _, querier := range q.queriers {
 		names, wrn, err := querier.LabelNames()
 		if wrn != nil {
+			// TODO(bwplotka): We could potentially wrap warnings.
 			warnings = append(warnings, wrn...)
 		}
-
 		if err != nil {
-			q.failedQueriers[querier] = struct{}{}
-			// If the error source isn't the primaryQuerier querier, return the error as a warning and continue.
-			if querier != q.primaryQuerier {
-				warnings = append(warnings, err)
-				continue
-			} else {
-				return nil, nil, errors.Wrap(err, "LabelNames() from Querier")
-			}
+			return nil, nil, errors.Wrap(err, "LabelNames() from Querier")
 		}
-
 		for _, name := range names {
 			labelNamesMap[name] = struct{}{}
 		}
 	}
+	if len(labelNamesMap) == 0 {
+		return nil, warnings, nil
+	}
 
 	labelNames := make([]string, 0, len(labelNamesMap))
 	for name := range labelNamesMap {
 		labelNames = append(labelNames, name)
 	}
 	sort.Strings(labelNames)
-
 	return labelNames, warnings, nil
 }
 
 // Close releases the resources of the Querier.
 func (q *mergeGenericQuerier) Close() error {
-	var errs tsdb_errors.MultiError
+	errs := tsdb_errors.MultiError{}
 	for _, querier := range q.queriers {
 		if err := querier.Close(); err != nil {
 			errs.Add(err)
@@ -420,18 +399,6 @@ func (q *mergeGenericQuerier) Close() error {
 	return errs.Err()
 }
 
-// genericMergeSeriesSet implements genericSeriesSet
-type genericMergeSeriesSet struct {
-	currentLabels labels.Labels
-	mergeFunc     genericSeriesMergeFunc
-
-	heap genericSeriesSetHeap
-	sets []genericSeriesSet
-
-	currentSets []genericSeriesSet
-	querier     *mergeGenericQuerier
-}
-
 // VerticalSeriesMergeFunc returns merged series implementation that merges series with same labels together.
 // It has to handle time-overlapped series as well.
 type VerticalSeriesMergeFunc func(...Series) Series
@@ -447,7 +414,7 @@ func NewMergeSeriesSet(sets []SeriesSet, merger VerticalSeriesMergeFunc) SeriesS
 		genericSets = append(genericSets, &genericSeriesSetAdapter{s})
 
 	}
-	return &seriesSetAdapter{newGenericMergeSeriesSet(genericSets, nil, (&seriesMergerAdapter{VerticalSeriesMergeFunc: merger}).Merge)}
+	return &seriesSetAdapter{newGenericMergeSeriesSet(genericSets, (&seriesMergerAdapter{VerticalSeriesMergeFunc: merger}).Merge)}
 }
 
 // NewMergeChunkSeriesSet returns a new ChunkSeriesSet that merges results of chkQuerierSeries ChunkSeriesSets.
@@ -457,21 +424,30 @@ func NewMergeChunkSeriesSet(sets []ChunkSeriesSet, merger VerticalChunkSeriesMer
 		genericSets = append(genericSets, &genericChunkSeriesSetAdapter{s})
 
 	}
-	return &chunkSeriesSetAdapter{newGenericMergeSeriesSet(genericSets, nil, (&chunkSeriesMergerAdapter{VerticalChunkSeriesMergerFunc: merger}).Merge)}
+	return &chunkSeriesSetAdapter{newGenericMergeSeriesSet(genericSets, (&chunkSeriesMergerAdapter{VerticalChunkSeriesMergerFunc: merger}).Merge)}
+}
+
+// genericMergeSeriesSet implements genericSeriesSet.
+type genericMergeSeriesSet struct {
+	currentLabels labels.Labels
+	mergeFn       genericSeriesMergeFunc
+
+	heap        genericSeriesSetHeap
+	sets        []genericSeriesSet
+	currentSets []genericSeriesSet
 }
 
 // newGenericMergeSeriesSet returns a new genericSeriesSet that merges (and deduplicates)
-// series returned by the chkQuerierSeries series sets when iterating.
-// Each chkQuerierSeries series set must return its series in labels order, otherwise
+// series returned by the series sets when iterating.
+// Each series set must return its series in labels order, otherwise
 // merged series set will be incorrect.
-// Argument 'querier' is optional and can be nil. Pass Querier if you want to retry query in case of failing series set.
-// Overlapped situations are merged using provided mergeFunc.
-func newGenericMergeSeriesSet(sets []genericSeriesSet, querier *mergeGenericQuerier, mergeFunc genericSeriesMergeFunc) genericSeriesSet {
+// Overlapping cases are merged using provided mergeFn.
+func newGenericMergeSeriesSet(sets []genericSeriesSet, mergeFn genericSeriesMergeFunc) genericSeriesSet {
 	if len(sets) == 1 {
 		return sets[0]
 	}
 
-	// Sets need to be pre-advanced, so we can introspect the label of the
+	// We are pre-advancing sets, so we can introspect the label of the
 	// series under the cursor.
 	var h genericSeriesSetHeap
 	for _, set := range sets {
@@ -483,26 +459,25 @@ func newGenericMergeSeriesSet(sets []genericSeriesSet, querier *mergeGenericQuer
 		}
 	}
 	return &genericMergeSeriesSet{
-		mergeFunc: mergeFunc,
-		heap:      h,
-		sets:      sets,
-		querier:   querier,
+		mergeFn: mergeFn,
+		sets:    sets,
+		heap:    h,
 	}
 }
 
 func (c *genericMergeSeriesSet) Next() bool {
 	// Run in a loop because the "next" series sets may not be valid anymore.
-	// If a remote querier fails, we discard all series sets from that querier.
 	// If, for the current label set, all the next series sets come from
 	// failed remote storage sources, we want to keep trying with the next label set.
 	for {
-		// Firstly advance all the current series sets.  If any of them have run out
+		// Firstly advance all the current series sets. If any of them have run out
 		// we can drop them, otherwise they should be inserted back into the heap.
 		for _, set := range c.currentSets {
 			if set.Next() {
 				heap.Push(&c.heap, set)
 			}
 		}
+
 		if len(c.heap) == 0 {
 			return false
 		}
@@ -512,9 +487,6 @@ func (c *genericMergeSeriesSet) Next() bool {
 		c.currentLabels = c.heap[0].At().Labels()
 		for len(c.heap) > 0 && labels.Equal(c.currentLabels, c.heap[0].At().Labels()) {
 			set := heap.Pop(&c.heap).(genericSeriesSet)
-			if c.querier != nil && c.querier.IsFailedSet(set) {
-				continue
-			}
 			c.currentSets = append(c.currentSets, set)
 		}
 
@@ -535,7 +507,7 @@ func (c *genericMergeSeriesSet) At() Labels {
 	for _, seriesSet := range c.currentSets {
 		series = append(series, seriesSet.At())
 	}
-	return c.mergeFunc(series...)
+	return c.mergeFn(series...)
 }
 
 func (c *genericMergeSeriesSet) Err() error {
@@ -547,6 +519,14 @@ func (c *genericMergeSeriesSet) Err() error {
 	return nil
 }
 
+func (c *genericMergeSeriesSet) Warnings() Warnings {
+	var ws Warnings
+	for _, set := range c.sets {
+		ws = append(ws, set.Warnings()...)
+	}
+	return ws
+}
+
 type genericSeriesSetHeap []genericSeriesSet
 
 func (h genericSeriesSetHeap) Len() int      { return len(h) }

storage/fanout/fanout_test.go

@@ -16,14 +16,14 @@ package storage
 import (
 	"context"
 	"errors"
+	"testing"
 
 	"github.com/prometheus/common/model"
+
 	"github.com/prometheus/prometheus/pkg/labels"
 	"github.com/prometheus/prometheus/storage"
 	"github.com/prometheus/prometheus/util/teststorage"
 	"github.com/prometheus/prometheus/util/testutil"
-
-	"testing"
 )
 
 func TestSelectSorted(t *testing.T) {
@@ -79,8 +79,7 @@ func TestSelectSorted(t *testing.T) {
 	matcher, err := labels.NewMatcher(labels.MatchEqual, model.MetricNameLabel, "a")
 	testutil.Ok(t, err)
 
-	seriesSet, _, err := querier.Select(true, nil, matcher)
-	testutil.Ok(t, err)
+	seriesSet := querier.Select(true, nil, matcher)
 
 	result := make(map[int64]float64)
 	var labelsResult labels.Labels
@@ -95,6 +94,7 @@ func TestSelectSorted(t *testing.T) {
 		}
 	}
 
+	testutil.Ok(t, seriesSet.Err())
 	testutil.Equals(t, labelsResult, outputLabel)
 	testutil.Equals(t, inputTotalSize, len(result))
 }
@@ -131,19 +131,12 @@ func TestFanoutErrors(t *testing.T) {
 		defer querier.Close()
 
 		matcher := labels.MustNewMatcher(labels.MatchEqual, "a", "b")
-		ss, warnings, err := querier.Select(true, nil, matcher)
-		testutil.Equals(t, tc.err, err)
-		testutil.Equals(t, tc.warnings, warnings)
-
-		// Only test series iteration if there are no errors.
-		if err != nil {
-			continue
-		}
-
+		ss := querier.Select(true, nil, matcher)
 		for ss.Next() {
 			ss.At()
 		}
-		testutil.Ok(t, ss.Err())
+		testutil.Equals(t, tc.err, ss.Err())
+		testutil.Equals(t, tc.warnings, ss.Warnings())
 	}
 }
 
@@ -169,8 +162,8 @@ func (errStorage) Close() error {
 
 type errQuerier struct{}
 
-func (errQuerier) Select(bool, *storage.SelectHints, ...*labels.Matcher) (storage.SeriesSet, storage.Warnings, error) {
-	return nil, nil, errSelect
+func (errQuerier) Select(bool, *storage.SelectHints, ...*labels.Matcher) storage.SeriesSet {
+	return storage.ErrSeriesSet(errSelect)
 }
 
 func (errQuerier) LabelValues(name string) ([]string, storage.Warnings, error) {

storage/fanout_test.go

@@ -17,8 +17,11 @@ import (
 	"fmt"
 	"math"
 	"sort"
+	"sync"
 	"testing"
 
+	"github.com/pkg/errors"
+
 	"github.com/prometheus/prometheus/pkg/labels"
 	"github.com/prometheus/prometheus/tsdb/chunkenc"
 	"github.com/prometheus/prometheus/tsdb/tsdbutil"
@@ -56,24 +59,37 @@ func TestMergeTwoStringSlices(t *testing.T) {
 
 func TestMergeQuerierWithChainMerger(t *testing.T) {
 	for _, tc := range []struct {
-		name          string
-		querierSeries [][]Series
-		extraQueriers []Querier
+		name                 string
+		primaryQuerierSeries []Series
+		querierSeries        [][]Series
+		extraQueriers        []Querier
 
 		expected SeriesSet
 	}{
 		{
-			name:          "1 querier with no series",
+			name:                 "one primary querier with no series",
+			primaryQuerierSeries: []Series{},
+			expected:             NewMockSeriesSet(),
+		},
+		{
+			name:          "one secondary querier with no series",
 			querierSeries: [][]Series{{}},
 			expected:      NewMockSeriesSet(),
 		},
 		{
-			name:          "many queriers with no series",
+			name:          "many secondary queriers with no series",
 			querierSeries: [][]Series{{}, {}, {}, {}, {}, {}, {}},
 			expected:      NewMockSeriesSet(),
 		},
 		{
-			name: "1 querier, two series",
+			name:                 "mix of queriers with no series",
+			primaryQuerierSeries: []Series{},
+			querierSeries:        [][]Series{{}, {}, {}, {}, {}, {}, {}},
+			expected:             NewMockSeriesSet(),
+		},
+		// Test rest of cases on secondary queriers as the different between primary vs secondary is just error handling.
+		{
+			name: "one querier, two series",
 			querierSeries: [][]Series{{
 				NewListSeries(labels.FromStrings("bar", "baz"), []tsdbutil.Sample{sample{1, 1}, sample{2, 2}, sample{3, 3}}),
 				NewListSeries(labels.FromStrings("foo", "bar"), []tsdbutil.Sample{sample{0, 0}, sample{1, 1}, sample{2, 2}}),
@@ -84,7 +100,7 @@ func TestMergeQuerierWithChainMerger(t *testing.T) {
 			),
 		},
 		{
-			name: "2 queriers, 1 different series each",
+			name: "two queriers, one different series each",
 			querierSeries: [][]Series{{
 				NewListSeries(labels.FromStrings("bar", "baz"), []tsdbutil.Sample{sample{1, 1}, sample{2, 2}, sample{3, 3}}),
 			}, {
@@ -96,7 +112,7 @@ func TestMergeQuerierWithChainMerger(t *testing.T) {
 			),
 		},
 		{
-			name: "2 time unsorted queriers, 2 series each",
+			name: "two time unsorted queriers, two series each",
 			querierSeries: [][]Series{{
 				NewListSeries(labels.FromStrings("bar", "baz"), []tsdbutil.Sample{sample{5, 5}, sample{6, 6}}),
 				NewListSeries(labels.FromStrings("foo", "bar"), []tsdbutil.Sample{sample{0, 0}, sample{1, 1}, sample{2, 2}}),
@@ -116,7 +132,7 @@ func TestMergeQuerierWithChainMerger(t *testing.T) {
 			),
 		},
 		{
-			name: "5 queriers, only 2 queriers have 2 time unsorted series each",
+			name: "five queriers, only two queriers have two time unsorted series each",
 			querierSeries: [][]Series{{}, {}, {
 				NewListSeries(labels.FromStrings("bar", "baz"), []tsdbutil.Sample{sample{5, 5}, sample{6, 6}}),
 				NewListSeries(labels.FromStrings("foo", "bar"), []tsdbutil.Sample{sample{0, 0}, sample{1, 1}, sample{2, 2}}),
@@ -136,7 +152,7 @@ func TestMergeQuerierWithChainMerger(t *testing.T) {
 			),
 		},
 		{
-			name: "2 queriers, only 2 queriers have 2 time unsorted series each, with 3 noop and one nil querier together",
+			name: "two queriers, only two queriers have two time unsorted series each, with 3 noop and one nil querier together",
 			querierSeries: [][]Series{{}, {}, {
 				NewListSeries(labels.FromStrings("bar", "baz"), []tsdbutil.Sample{sample{5, 5}, sample{6, 6}}),
 				NewListSeries(labels.FromStrings("foo", "bar"), []tsdbutil.Sample{sample{0, 0}, sample{1, 1}, sample{2, 2}}),
@@ -157,7 +173,7 @@ func TestMergeQuerierWithChainMerger(t *testing.T) {
 			),
 		},
 		{
-			name: "2 queriers, with 2 series, one is overlapping",
+			name: "two queriers, with two series, one is overlapping",
 			querierSeries: [][]Series{{}, {}, {
 				NewListSeries(labels.FromStrings("bar", "baz"), []tsdbutil.Sample{sample{2, 21}, sample{3, 31}, sample{5, 5}, sample{6, 6}}),
 				NewListSeries(labels.FromStrings("foo", "bar"), []tsdbutil.Sample{sample{0, 0}, sample{1, 1}, sample{2, 2}}),
@@ -177,7 +193,7 @@ func TestMergeQuerierWithChainMerger(t *testing.T) {
 			),
 		},
 		{
-			name: "2 queries, one with NaN samples series",
+			name: "two queries, one with NaN samples series",
 			querierSeries: [][]Series{{
 				NewListSeries(labels.FromStrings("foo", "bar"), []tsdbutil.Sample{sample{0, math.NaN()}}),
 			}, {
@@ -189,13 +205,17 @@ func TestMergeQuerierWithChainMerger(t *testing.T) {
 		},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
+			var p Querier
+			if tc.primaryQuerierSeries != nil {
+				p = &mockQuerier{toReturn: tc.primaryQuerierSeries}
+			}
 			var qs []Querier
 			for _, in := range tc.querierSeries {
 				qs = append(qs, &mockQuerier{toReturn: in})
 			}
 			qs = append(qs, tc.extraQueriers...)
 
-			mergedQuerier, _, _ := NewMergeQuerier(qs[0], qs, ChainedSeriesMerge).Select(false, nil)
+			mergedQuerier := NewMergeQuerier(p, qs, ChainedSeriesMerge).Select(false, nil)
 
 			// Get all merged series upfront to make sure there are no incorrectly retained shared
 			// buffers causing bugs.
@@ -222,22 +242,35 @@ func TestMergeQuerierWithChainMerger(t *testing.T) {
 
 func TestMergeChunkQuerierWithNoVerticalChunkSeriesMerger(t *testing.T) {
 	for _, tc := range []struct {
-		name             string
-		chkQuerierSeries [][]ChunkSeries
-		extraQueriers    []ChunkQuerier
+		name                    string
+		primaryChkQuerierSeries []ChunkSeries
+		chkQuerierSeries        [][]ChunkSeries
+		extraQueriers           []ChunkQuerier
 
 		expected ChunkSeriesSet
 	}{
 		{
-			name:             "one querier with no series",
+			name:                    "one primary querier with no series",
+			primaryChkQuerierSeries: []ChunkSeries{},
+			expected:                NewMockChunkSeriesSet(),
+		},
+		{
+			name:             "one secondary querier with no series",
 			chkQuerierSeries: [][]ChunkSeries{{}},
 			expected:         NewMockChunkSeriesSet(),
 		},
 		{
-			name:             "many queriers with no series",
+			name:             "many secondary queriers with no series",
 			chkQuerierSeries: [][]ChunkSeries{{}, {}, {}, {}, {}, {}, {}},
 			expected:         NewMockChunkSeriesSet(),
 		},
+		{
+			name:                    "mix of queriers with no series",
+			primaryChkQuerierSeries: []ChunkSeries{},
+			chkQuerierSeries:        [][]ChunkSeries{{}, {}, {}, {}, {}, {}, {}},
+			expected:                NewMockChunkSeriesSet(),
+		},
+		// Test rest of cases on secondary queriers as the different between primary vs secondary is just error handling.
 		{
 			name: "one querier, two series",
 			chkQuerierSeries: [][]ChunkSeries{{
@@ -347,13 +380,19 @@ func TestMergeChunkQuerierWithNoVerticalChunkSeriesMerger(t *testing.T) {
 		},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
+			var p ChunkQuerier
+			if tc.primaryChkQuerierSeries != nil {
+				p = &mockChunkQurier{toReturn: tc.primaryChkQuerierSeries}
+			}
+
 			var qs []ChunkQuerier
 			for _, in := range tc.chkQuerierSeries {
 				qs = append(qs, &mockChunkQurier{toReturn: in})
 			}
 			qs = append(qs, tc.extraQueriers...)
 
-			merged, _, _ := NewMergeChunkQuerier(qs[0], qs, NewVerticalChunkSeriesMerger(nil)).Select(false, nil)
+			// TODO(bwplotka): Add case of overlap to check if those are handled well.
+			merged := NewMergeChunkQuerier(p, qs, NewVerticalChunkSeriesMerger(nil)).Select(false, nil)
 			for merged.Next() {
 				testutil.Assert(t, tc.expected.Next(), "Expected Next() to be true")
 				actualSeries := merged.At()
@@ -384,14 +423,14 @@ func (a seriesByLabel) Len() int           { return len(a) }
 func (a seriesByLabel) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
 func (a seriesByLabel) Less(i, j int) bool { return labels.Compare(a[i].Labels(), a[j].Labels()) < 0 }
 
-func (m *mockQuerier) Select(sortSeries bool, _ *SelectHints, _ ...*labels.Matcher) (SeriesSet, Warnings, error) {
+func (m *mockQuerier) Select(sortSeries bool, _ *SelectHints, _ ...*labels.Matcher) SeriesSet {
 	cpy := make([]Series, len(m.toReturn))
 	copy(cpy, m.toReturn)
 	if sortSeries {
 		sort.Sort(seriesByLabel(cpy))
 	}
 
-	return NewMockSeriesSet(cpy...), nil, nil
+	return NewMockSeriesSet(cpy...)
 }
 
 type mockChunkQurier struct {
@@ -408,14 +447,14 @@ func (a chunkSeriesByLabel) Less(i, j int) bool {
 	return labels.Compare(a[i].Labels(), a[j].Labels()) < 0
 }
 
-func (m *mockChunkQurier) Select(sortSeries bool, _ *SelectHints, _ ...*labels.Matcher) (ChunkSeriesSet, Warnings, error) {
+func (m *mockChunkQurier) Select(sortSeries bool, _ *SelectHints, _ ...*labels.Matcher) ChunkSeriesSet {
 	cpy := make([]ChunkSeries, len(m.toReturn))
 	copy(cpy, m.toReturn)
 	if sortSeries {
 		sort.Sort(chunkSeriesByLabel(cpy))
 	}
 
-	return NewMockChunkSeriesSet(cpy...), nil, nil
+	return NewMockChunkSeriesSet(cpy...)
 }
 
 type mockSeriesSet struct {
@@ -439,6 +478,8 @@ func (m *mockSeriesSet) At() Series { return m.series[m.idx] }
 
 func (m *mockSeriesSet) Err() error { return nil }
 
+func (m *mockSeriesSet) Warnings() Warnings { return nil }
+
 type mockChunkSeriesSet struct {
 	idx    int
 	series []ChunkSeries
@@ -460,6 +501,8 @@ func (m *mockChunkSeriesSet) At() ChunkSeries { return m.series[m.idx] }
 
 func (m *mockChunkSeriesSet) Err() error { return nil }
 
+func (m *mockChunkSeriesSet) Warnings() Warnings { return nil }
+
 func TestChainSampleIterator(t *testing.T) {
 	for _, tc := range []struct {
 		input    []chunkenc.Iterator
@@ -571,7 +614,7 @@ func makeMergeSeriesSet(numSeriesSets, numSeries, numSamples int) SeriesSet {
 	for i := 0; i < numSeriesSets; i++ {
 		seriesSets = append(seriesSets, &genericSeriesSetAdapter{makeSeriesSet(numSeries, numSamples)})
 	}
-	return &seriesSetAdapter{newGenericMergeSeriesSet(seriesSets, nil, (&seriesMergerAdapter{VerticalSeriesMergeFunc: ChainedSeriesMerge}).Merge)}
+	return &seriesSetAdapter{newGenericMergeSeriesSet(seriesSets, (&seriesMergerAdapter{VerticalSeriesMergeFunc: ChainedSeriesMerge}).Merge)}
 }
 
 func benchmarkDrain(seriesSet SeriesSet, b *testing.B) {
@@ -603,3 +646,260 @@ func BenchmarkMergeSeriesSet(b *testing.B) {
 		})
 	}
 }
+
+type mockGenericQuerier struct {
+	mtx sync.Mutex
+
+	closed                bool
+	labelNamesCalls       int
+	labelNamesRequested   []string
+	sortedSeriesRequested []bool
+
+	resp     []string
+	warnings Warnings
+	err      error
+}
+
+func (m *mockGenericQuerier) Select(b bool, _ *SelectHints, _ ...*labels.Matcher) genericSeriesSet {
+	m.mtx.Lock()
+	m.sortedSeriesRequested = append(m.sortedSeriesRequested, b)
+	m.mtx.Unlock()
+	return &mockGenericSeriesSet{resp: m.resp, warnings: m.warnings, err: m.err}
+}
+
+func (m *mockGenericQuerier) LabelValues(name string) ([]string, Warnings, error) {
+	m.mtx.Lock()
+	m.labelNamesRequested = append(m.labelNamesRequested, name)
+	m.mtx.Unlock()
+	return m.resp, m.warnings, m.err
+}
+
+func (m *mockGenericQuerier) LabelNames() ([]string, Warnings, error) {
+	m.mtx.Lock()
+	m.labelNamesCalls++
+	m.mtx.Unlock()
+	return m.resp, m.warnings, m.err
+}
+
+func (m *mockGenericQuerier) Close() error {
+	m.closed = true
+	return nil
+}
+
+type mockGenericSeriesSet struct {
+	resp     []string
+	warnings Warnings
+	err      error
+
+	curr int
+}
+
+func (m *mockGenericSeriesSet) Next() bool {
+	if m.err != nil {
+		return false
+	}
+	if m.curr >= len(m.resp) {
+		return false
+	}
+	m.curr++
+	return true
+}
+
+func (m *mockGenericSeriesSet) Err() error         { return m.err }
+func (m *mockGenericSeriesSet) Warnings() Warnings { return m.warnings }
+
+func (m *mockGenericSeriesSet) At() Labels {
+	return mockLabels(m.resp[m.curr-1])
+}
+
+type mockLabels string
+
+func (l mockLabels) Labels() labels.Labels {
+	return labels.FromStrings("test", string(l))
+}
+
+func unwrapMockGenericQuerier(t *testing.T, qr genericQuerier) *mockGenericQuerier {
+	m, ok := qr.(*mockGenericQuerier)
+	if !ok {
+		s, ok := qr.(*secondaryQuerier)
+		testutil.Assert(t, ok, "expected secondaryQuerier got something else")
+		m, ok = s.genericQuerier.(*mockGenericQuerier)
+		testutil.Assert(t, ok, "expected mockGenericQuerier got something else")
+	}
+	return m
+}
+
+func TestMergeGenericQuerierWithSecondaries_ErrorHandling(t *testing.T) {
+	var (
+		errStorage  = errors.New("storage error")
+		warnStorage = errors.New("storage warning")
+	)
+	for _, tcase := range []struct {
+		name     string
+		queriers []genericQuerier
+
+		expectedSelectsSeries []labels.Labels
+		expectedLabels        []string
+
+		expectedWarnings [3]Warnings
+		expectedErrs     [3]error
+	}{
+		{},
+		{
+			name:     "one successful primary querier",
+			queriers: []genericQuerier{&mockGenericQuerier{resp: []string{"a", "b"}, warnings: nil, err: nil}},
+			expectedSelectsSeries: []labels.Labels{
+				labels.FromStrings("test", "a"),
+				labels.FromStrings("test", "b"),
+			},
+			expectedLabels: []string{"a", "b"},
+		},
+		{
+			name: "multiple successful primary queriers",
+			queriers: []genericQuerier{
+				&mockGenericQuerier{resp: []string{"a", "b"}, warnings: nil, err: nil},
+				&mockGenericQuerier{resp: []string{"b", "c"}, warnings: nil, err: nil},
+			},
+			expectedSelectsSeries: []labels.Labels{
+				labels.FromStrings("test", "a"),
+				labels.FromStrings("test", "b"),
+				labels.FromStrings("test", "c"),
+			},
+			expectedLabels: []string{"a", "b", "c"},
+		},
+		{
+			name:         "one failed primary querier",
+			queriers:     []genericQuerier{&mockGenericQuerier{warnings: nil, err: errStorage}},
+			expectedErrs: [3]error{errStorage, errStorage, errStorage},
+		},
+		{
+			name: "one successful primary querier with successful secondaries",
+			queriers: []genericQuerier{
+				&mockGenericQuerier{resp: []string{"a", "b"}, warnings: nil, err: nil},
+				&secondaryQuerier{genericQuerier: &mockGenericQuerier{resp: []string{"b"}, warnings: nil, err: nil}},
+				&secondaryQuerier{genericQuerier: &mockGenericQuerier{resp: []string{"c"}, warnings: nil, err: nil}},
+			},
+			expectedSelectsSeries: []labels.Labels{
+				labels.FromStrings("test", "a"),
+				labels.FromStrings("test", "b"),
+				labels.FromStrings("test", "c"),
+			},
+			expectedLabels: []string{"a", "b", "c"},
+		},
+		{
+			name: "one successful primary querier with empty response and successful secondaries",
+			queriers: []genericQuerier{
+				&mockGenericQuerier{resp: []string{}, warnings: nil, err: nil},
+				&secondaryQuerier{genericQuerier: &mockGenericQuerier{resp: []string{"b"}, warnings: nil, err: nil}},
+				&secondaryQuerier{genericQuerier: &mockGenericQuerier{resp: []string{"c"}, warnings: nil, err: nil}},
+			},
+			expectedSelectsSeries: []labels.Labels{
+				labels.FromStrings("test", "b"),
+				labels.FromStrings("test", "c"),
+			},
+			expectedLabels: []string{"b", "c"},
+		},
+		{
+			name: "one failed primary querier with successful secondaries",
+			queriers: []genericQuerier{
+				&mockGenericQuerier{warnings: nil, err: errStorage},
+				&secondaryQuerier{genericQuerier: &mockGenericQuerier{resp: []string{"b"}, warnings: nil, err: nil}},
+				&secondaryQuerier{genericQuerier: &mockGenericQuerier{resp: []string{"c"}, warnings: nil, err: nil}},
+			},
+			expectedErrs: [3]error{errStorage, errStorage, errStorage},
+		},
+		{
+			name: "one successful primary querier with failed secondaries",
+			queriers: []genericQuerier{
+				&mockGenericQuerier{resp: []string{"a"}, warnings: nil, err: nil},
+				&secondaryQuerier{genericQuerier: &mockGenericQuerier{resp: []string{"b"}, warnings: nil, err: errStorage}},
+				&secondaryQuerier{genericQuerier: &mockGenericQuerier{resp: []string{"c"}, warnings: nil, err: errStorage}},
+			},
+			expectedSelectsSeries: []labels.Labels{
+				labels.FromStrings("test", "a"),
+			},
+			expectedLabels: []string{"a"},
+			expectedWarnings: [3]Warnings{
+				[]error{errStorage, errStorage},
+				[]error{errStorage, errStorage},
+				[]error{errStorage, errStorage},
+			},
+		},
+		{
+			name: "successful queriers with warnings",
+			queriers: []genericQuerier{
+				&mockGenericQuerier{resp: []string{"a"}, warnings: []error{warnStorage}, err: nil},
+				&secondaryQuerier{genericQuerier: &mockGenericQuerier{resp: []string{"b"}, warnings: []error{warnStorage}, err: nil}},
+			},
+			expectedSelectsSeries: []labels.Labels{
+				labels.FromStrings("test", "a"),
+				labels.FromStrings("test", "b"),
+			},
+			expectedLabels: []string{"a", "b"},
+			expectedWarnings: [3]Warnings{
+				[]error{warnStorage, warnStorage},
+				[]error{warnStorage, warnStorage},
+				[]error{warnStorage, warnStorage},
+			},
+		},
+	} {
+		t.Run(tcase.name, func(t *testing.T) {
+			q := &mergeGenericQuerier{
+				queriers: tcase.queriers,
+				mergeFn:  func(l ...Labels) Labels { return l[0] },
+			}
+
+			t.Run("Select", func(t *testing.T) {
+				res := q.Select(false, nil)
+				var lbls []labels.Labels
+				for res.Next() {
+					lbls = append(lbls, res.At().Labels())
+				}
+				testutil.Equals(t, tcase.expectedWarnings[0], res.Warnings())
+				testutil.Equals(t, tcase.expectedErrs[0], res.Err())
+				testutil.Assert(t, errors.Is(res.Err(), tcase.expectedErrs[0]), "expected error doesn't match")
+				testutil.Equals(t, tcase.expectedSelectsSeries, lbls)
+
+				for _, qr := range q.queriers {
+					m := unwrapMockGenericQuerier(t, qr)
+
+					exp := []bool{true}
+					if len(q.queriers) == 1 {
+						exp[0] = false
+					}
+					testutil.Equals(t, exp, m.sortedSeriesRequested)
+				}
+			})
+			t.Run("LabelNames", func(t *testing.T) {
+				res, w, err := q.LabelNames()
+				testutil.Equals(t, tcase.expectedWarnings[1], w)
+				testutil.Assert(t, errors.Is(err, tcase.expectedErrs[1]), "expected error doesn't match")
+				testutil.Equals(t, tcase.expectedLabels, res)
+
+				if err != nil {
+					return
+				}
+				for _, qr := range q.queriers {
+					m := unwrapMockGenericQuerier(t, qr)
+
+					testutil.Equals(t, 1, m.labelNamesCalls)
+				}
+			})
+			t.Run("LabelValues", func(t *testing.T) {
+				res, w, err := q.LabelValues("test")
+				testutil.Equals(t, tcase.expectedWarnings[2], w)
+				testutil.Assert(t, errors.Is(err, tcase.expectedErrs[2]), "expected error doesn't match")
+				testutil.Equals(t, tcase.expectedLabels, res)
+
+				if err != nil {
+					return
+				}
+				for _, qr := range q.queriers {
+					m := unwrapMockGenericQuerier(t, qr)
+
+					testutil.Equals(t, []string{"test"}, m.labelNamesRequested)
+				}
+			})
+		})
+	}
+}

storage/generic.go

@@ -20,13 +20,14 @@ import "github.com/prometheus/prometheus/pkg/labels"
 
 type genericQuerier interface {
 	baseQuerier
-	Select(bool, *SelectHints, ...*labels.Matcher) (genericSeriesSet, Warnings, error)
+	Select(bool, *SelectHints, ...*labels.Matcher) genericSeriesSet
 }
 
 type genericSeriesSet interface {
 	Next() bool
 	At() Labels
 	Err() error
+	Warnings() Warnings
 }
 
 type genericSeriesMergeFunc func(...Labels) Labels
@@ -55,13 +56,11 @@ type genericQuerierAdapter struct {
 	cq ChunkQuerier
 }
 
-func (q *genericQuerierAdapter) Select(sortSeries bool, hints *SelectHints, matchers ...*labels.Matcher) (genericSeriesSet, Warnings, error) {
+func (q *genericQuerierAdapter) Select(sortSeries bool, hints *SelectHints, matchers ...*labels.Matcher) genericSeriesSet {
 	if q.q != nil {
-		s, w, err := q.q.Select(sortSeries, hints, matchers...)
-		return &genericSeriesSetAdapter{s}, w, err
+		return &genericSeriesSetAdapter{q.q.Select(sortSeries, hints, matchers...)}
 	}
-	s, w, err := q.cq.Select(sortSeries, hints, matchers...)
-	return &genericChunkSeriesSetAdapter{s}, w, err
+	return &genericChunkSeriesSetAdapter{q.cq.Select(sortSeries, hints, matchers...)}
 }
 
 func newGenericQuerierFrom(q Querier) genericQuerier {
@@ -84,9 +83,8 @@ func (a *seriesSetAdapter) At() Series {
 	return a.genericSeriesSet.At().(Series)
 }
 
-func (q *querierAdapter) Select(sortSeries bool, hints *SelectHints, matchers ...*labels.Matcher) (SeriesSet, Warnings, error) {
-	s, w, err := q.genericQuerier.Select(sortSeries, hints, matchers...)
-	return &seriesSetAdapter{s}, w, err
+func (q *querierAdapter) Select(sortSeries bool, hints *SelectHints, matchers ...*labels.Matcher) SeriesSet {
+	return &seriesSetAdapter{q.genericQuerier.Select(sortSeries, hints, matchers...)}
 }
 
 type chunkQuerierAdapter struct {
@@ -101,9 +99,8 @@ func (a *chunkSeriesSetAdapter) At() ChunkSeries {
 	return a.genericSeriesSet.At().(ChunkSeries)
 }
 
-func (q *chunkQuerierAdapter) Select(sortSeries bool, hints *SelectHints, matchers ...*labels.Matcher) (ChunkSeriesSet, Warnings, error) {
-	s, w, err := q.genericQuerier.Select(sortSeries, hints, matchers...)
-	return &chunkSeriesSetAdapter{s}, w, err
+func (q *chunkQuerierAdapter) Select(sortSeries bool, hints *SelectHints, matchers ...*labels.Matcher) ChunkSeriesSet {
+	return &chunkSeriesSetAdapter{q.genericQuerier.Select(sortSeries, hints, matchers...)}
 }
 
 type seriesMergerAdapter struct {
@@ -129,3 +126,13 @@ func (a *chunkSeriesMergerAdapter) Merge(s ...Labels) Labels {
 	}
 	return a.VerticalChunkSeriesMergerFunc(buf...)
 }
+
+type noopGenericSeriesSet struct{}
+
+func (noopGenericSeriesSet) Next() bool { return false }
+
+func (noopGenericSeriesSet) At() Labels { return nil }
+
+func (noopGenericSeriesSet) Err() error { return nil }
+
+func (noopGenericSeriesSet) Warnings() Warnings { return nil }

storage/interface.go

@@ -65,7 +65,7 @@ type Querier interface {
 	// Select returns a set of series that matches the given label matchers.
 	// Caller can specify if it requires returned series to be sorted. Prefer not requiring sorting for better performance.
 	// It allows passing hints that can help in optimising select, but it's up to implementation how this is used if used at all.
-	Select(sortSeries bool, hints *SelectHints, matchers ...*labels.Matcher) (SeriesSet, Warnings, error)
+	Select(sortSeries bool, hints *SelectHints, matchers ...*labels.Matcher) SeriesSet
 }
 
 // A ChunkQueryable handles queries against a storage.
@@ -82,7 +82,7 @@ type ChunkQuerier interface {
 	// Select returns a set of series that matches the given label matchers.
 	// Caller can specify if it requires returned series to be sorted. Prefer not requiring sorting for better performance.
 	// It allows passing hints that can help in optimising select, but it's up to implementation how this is used if used at all.
-	Select(sortSeries bool, hints *SelectHints, matchers ...*labels.Matcher) (ChunkSeriesSet, Warnings, error)
+	Select(sortSeries bool, hints *SelectHints, matchers ...*labels.Matcher) ChunkSeriesSet
 }
 
 type baseQuerier interface {
@@ -153,7 +153,12 @@ type Appender interface {
 type SeriesSet interface {
 	Next() bool
 	At() Series
+	// The error that iteration as failed with.
+	// When an error occurs, set cannot continue to iterate.
 	Err() error
+	// A collection of warnings for the whole set.
+	// Warnings could be return even iteration has not failed with error.
+	Warnings() Warnings
 }
 
 var emptySeriesSet = errSeriesSet{}
@@ -164,12 +169,19 @@ func EmptySeriesSet() SeriesSet {
 }
 
 type errSeriesSet struct {
+	ws  Warnings
 	err error
 }
 
-func (s errSeriesSet) Next() bool { return false }
-func (s errSeriesSet) At() Series { return nil }
-func (s errSeriesSet) Err() error { return s.err }
+func (s errSeriesSet) Next() bool         { return false }
+func (s errSeriesSet) At() Series         { return nil }
+func (s errSeriesSet) Err() error         { return s.err }
+func (s errSeriesSet) Warnings() Warnings { return s.ws }
+
+// ErrSeriesSet returns a series set that wraps an error.
+func ErrSeriesSet(err error) SeriesSet {
+	return errSeriesSet{err: err}
+}
 
 // Series exposes a single time series and allows iterating over samples.
 type Series interface {
@@ -181,7 +193,12 @@ type Series interface {
 type ChunkSeriesSet interface {
 	Next() bool
 	At() ChunkSeries
+	// The error that iteration has failed with.
+	// When an error occurs, set cannot continue to iterate.
 	Err() error
+	// A collection of warnings for the whole set.
+	// Warnings could be return even iteration has not failed with error.
+	Warnings() Warnings
 }
 
 // ChunkSeries exposes a single time series and allows iterating over chunks.

storage/lazy.go

@@ -0,0 +1,67 @@
+// Copyright 2017 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package storage
+
+type lazySeriesSet struct {
+	create func() (s genericSeriesSet, ok bool)
+
+	set genericSeriesSet
+}
+
+func (c *lazySeriesSet) Next() bool {
+	if c.set != nil {
+		return c.set.Next()
+	}
+
+	var ok bool
+	c.set, ok = c.create()
+	return ok
+}
+
+func (c *lazySeriesSet) Err() error {
+	if c.set != nil {
+		return c.set.Err()
+	}
+	return nil
+}
+
+func (c *lazySeriesSet) At() Labels {
+	if c.set != nil {
+		return c.set.At()
+	}
+	return nil
+}
+
+func (c *lazySeriesSet) Warnings() Warnings {
+	if c.set != nil {
+		return c.set.Warnings()
+	}
+	return nil
+}
+
+type warningsOnlySeriesSet Warnings
+
+func (warningsOnlySeriesSet) Next() bool           { return false }
+func (warningsOnlySeriesSet) Err() error           { return nil }
+func (warningsOnlySeriesSet) At() Labels           { return nil }
+func (c warningsOnlySeriesSet) Warnings() Warnings { return Warnings(c) }
+
+type errorOnlySeriesSet struct {
+	err error
+}
+
+func (errorOnlySeriesSet) Next() bool         { return false }
+func (errorOnlySeriesSet) At() Labels         { return nil }
+func (s errorOnlySeriesSet) Err() error       { return s.err }
+func (errorOnlySeriesSet) Warnings() Warnings { return nil }

storage/noop.go

@@ -24,8 +24,8 @@ func NoopQuerier() Querier {
 	return noopQuerier{}
 }
 
-func (noopQuerier) Select(bool, *SelectHints, ...*labels.Matcher) (SeriesSet, Warnings, error) {
-	return NoopSeriesSet(), nil, nil
+func (noopQuerier) Select(bool, *SelectHints, ...*labels.Matcher) SeriesSet {
+	return NoopSeriesSet()
 }
 
 func (noopQuerier) LabelValues(string) ([]string, Warnings, error) {
@@ -47,8 +47,8 @@ func NoopChunkedQuerier() ChunkQuerier {
 	return noopChunkQuerier{}
 }
 
-func (noopChunkQuerier) Select(bool, *SelectHints, ...*labels.Matcher) (ChunkSeriesSet, Warnings, error) {
-	return NoopChunkedSeriesSet(), nil, nil
+func (noopChunkQuerier) Select(bool, *SelectHints, ...*labels.Matcher) ChunkSeriesSet {
+	return NoopChunkedSeriesSet()
 }
 
 func (noopChunkQuerier) LabelValues(string) ([]string, Warnings, error) {
@@ -76,6 +76,8 @@ func (noopSeriesSet) At() Series { return nil }
 
 func (noopSeriesSet) Err() error { return nil }
 
+func (noopSeriesSet) Warnings() Warnings { return nil }
+
 type noopChunkedSeriesSet struct{}
 
 // NoopChunkedSeriesSet is a ChunkSeriesSet that does nothing.
@@ -88,3 +90,5 @@ func (noopChunkedSeriesSet) Next() bool { return false }
 func (noopChunkedSeriesSet) At() ChunkSeries { return nil }
 
 func (noopChunkedSeriesSet) Err() error { return nil }
+
+func (noopChunkedSeriesSet) Warnings() Warnings { return nil }

storage/remote/codec.go

@@ -107,7 +107,7 @@ func ToQuery(from, to int64, matchers []*labels.Matcher, hints *storage.SelectHi
 }
 
 // ToQueryResult builds a QueryResult proto.
-func ToQueryResult(ss storage.SeriesSet, sampleLimit int) (*prompb.QueryResult, error) {
+func ToQueryResult(ss storage.SeriesSet, sampleLimit int) (*prompb.QueryResult, storage.Warnings, error) {
 	numSamples := 0
 	resp := &prompb.QueryResult{}
 	for ss.Next() {
@@ -118,7 +118,7 @@ func ToQueryResult(ss storage.SeriesSet, sampleLimit int) (*prompb.QueryResult,
 		for iter.Next() {
 			numSamples++
 			if sampleLimit > 0 && numSamples > sampleLimit {
-				return nil, HTTPError{
+				return nil, ss.Warnings(), HTTPError{
 					msg:    fmt.Sprintf("exceeded sample limit (%d)", sampleLimit),
 					status: http.StatusBadRequest,
 				}
@@ -130,7 +130,7 @@ func ToQueryResult(ss storage.SeriesSet, sampleLimit int) (*prompb.QueryResult,
 			})
 		}
 		if err := iter.Err(); err != nil {
-			return nil, err
+			return nil, ss.Warnings(), err
 		}
 
 		resp.Timeseries = append(resp.Timeseries, &prompb.TimeSeries{
@@ -139,9 +139,9 @@ func ToQueryResult(ss storage.SeriesSet, sampleLimit int) (*prompb.QueryResult,
 		})
 	}
 	if err := ss.Err(); err != nil {
-		return nil, err
+		return nil, ss.Warnings(), err
 	}
-	return resp, nil
+	return resp, ss.Warnings(), nil
 }
 
 // FromQueryResult unpacks and sorts a QueryResult proto.
@@ -195,7 +195,7 @@ func StreamChunkedReadResponses(
 	ss storage.SeriesSet,
 	sortedExternalLabels []prompb.Label,
 	maxBytesInFrame int,
-) error {
+) (storage.Warnings, error) {
 	var (
 		chks     []prompb.Chunk
 		lbls     []prompb.Label
@@ -218,7 +218,7 @@ func StreamChunkedReadResponses(
 			// TODO(bwplotka): Use ChunkIterator once available in TSDB instead of re-encoding: https://github.com/prometheus/prometheus/pull/5882
 			chks, err = encodeChunks(iter, chks, maxBytesInFrame-lblsSize)
 			if err != nil {
-				return err
+				return ss.Warnings(), err
 			}
 
 			if len(chks) == 0 {
@@ -234,25 +234,25 @@ func StreamChunkedReadResponses(
 				QueryIndex: queryIndex,
 			})
 			if err != nil {
-				return errors.Wrap(err, "marshal ChunkedReadResponse")
+				return ss.Warnings(), errors.Wrap(err, "marshal ChunkedReadResponse")
 			}
 
 			if _, err := stream.Write(b); err != nil {
-				return errors.Wrap(err, "write to stream")
+				return ss.Warnings(), errors.Wrap(err, "write to stream")
 			}
 
 			chks = chks[:0]
 		}
 
 		if err := iter.Err(); err != nil {
-			return err
+			return ss.Warnings(), err
 		}
 	}
 	if err := ss.Err(); err != nil {
-		return err
+		return ss.Warnings(), err
 	}
 
-	return nil
+	return ss.Warnings(), nil
 }
 
 // encodeChunks expects iterator to be ready to use (aka iter.Next() called before invoking).
@@ -365,6 +365,8 @@ func (e errSeriesSet) Err() error {
 	return e.err
 }
 
+func (e errSeriesSet) Warnings() storage.Warnings { return nil }
+
 // concreteSeriesSet implements storage.SeriesSet.
 type concreteSeriesSet struct {
 	cur    int
@@ -384,6 +386,8 @@ func (c *concreteSeriesSet) Err() error {
 	return nil
 }
 
+func (c *concreteSeriesSet) Warnings() storage.Warnings { return nil }
+
 // concreteSeries implements storage.Series.
 type concreteSeries struct {
 	labels  labels.Labels

storage/remote/queue_manager.go

@@ -115,7 +115,7 @@ func newQueueManagerMetrics(r prometheus.Registerer, rn, e string) *queueManager
 		Subsystem:   subsystem,
 		Name:        "sent_batch_duration_seconds",
 		Help:        "Duration of sample batch send calls to the remote storage.",
-		Buckets:     prometheus.DefBuckets,
+		Buckets:     append(prometheus.DefBuckets, 25, 60, 120, 300),
 		ConstLabels: constLabels,
 	})
 	m.highestSentTimestamp = &maxGauge{

storage/remote/read.go

@@ -71,10 +71,10 @@ type querier struct {
 }
 
 // Select implements storage.Querier and uses the given matchers to read series sets from the Client.
-func (q *querier) Select(sortSeries bool, hints *storage.SelectHints, matchers ...*labels.Matcher) (storage.SeriesSet, storage.Warnings, error) {
+func (q *querier) Select(sortSeries bool, hints *storage.SelectHints, matchers ...*labels.Matcher) storage.SeriesSet {
 	query, err := ToQuery(q.mint, q.maxt, matchers, hints)
 	if err != nil {
-		return nil, nil, err
+		return storage.ErrSeriesSet(err)
 	}
 
 	remoteReadGauge := remoteReadQueries.WithLabelValues(q.client.remoteName, q.client.url.String())
@@ -86,10 +86,10 @@ func (q *querier) Select(sortSeries bool, hints *storage.SelectHints, matchers .
 
 	res, err := q.client.Read(q.ctx, query)
 	if err != nil {
-		return nil, nil, fmt.Errorf("remote_read: %v", err)
+		return storage.ErrSeriesSet(fmt.Errorf("remote_read: %v", err))
 	}
 
-	return FromQueryResult(sortSeries, res), nil, nil
+	return FromQueryResult(sortSeries, res)
 }
 
 // LabelValues implements storage.Querier and is a noop.
@@ -132,13 +132,9 @@ type externalLabelsQuerier struct {
 // Select adds equality matchers for all external labels to the list of matchers
 // before calling the wrapped storage.Queryable. The added external labels are
 // removed from the returned series sets.
-func (q externalLabelsQuerier) Select(sortSeries bool, hints *storage.SelectHints, matchers ...*labels.Matcher) (storage.SeriesSet, storage.Warnings, error) {
+func (q externalLabelsQuerier) Select(sortSeries bool, hints *storage.SelectHints, matchers ...*labels.Matcher) storage.SeriesSet {
 	m, added := q.addExternalLabels(matchers)
-	s, warnings, err := q.Querier.Select(sortSeries, hints, m...)
-	if err != nil {
-		return nil, warnings, err
-	}
-	return newSeriesSetFilter(s, added), warnings, nil
+	return newSeriesSetFilter(q.Querier.Select(sortSeries, hints, m...), added)
 }
 
 // PreferLocalStorageFilter returns a QueryableFunc which creates a NoopQuerier
@@ -185,7 +181,7 @@ type requiredMatchersQuerier struct {
 
 // Select returns a NoopSeriesSet if the given matchers don't match the label
 // set of the requiredMatchersQuerier. Otherwise it'll call the wrapped querier.
-func (q requiredMatchersQuerier) Select(sortSeries bool, hints *storage.SelectHints, matchers ...*labels.Matcher) (storage.SeriesSet, storage.Warnings, error) {
+func (q requiredMatchersQuerier) Select(sortSeries bool, hints *storage.SelectHints, matchers ...*labels.Matcher) storage.SeriesSet {
 	ms := q.requiredMatchers
 	for _, m := range matchers {
 		for i, r := range ms {
@@ -199,7 +195,7 @@ func (q requiredMatchersQuerier) Select(sortSeries bool, hints *storage.SelectHi
 		}
 	}
 	if len(ms) > 0 {
-		return storage.NoopSeriesSet(), nil, nil
+		return storage.NoopSeriesSet()
 	}
 	return q.Querier.Select(sortSeries, hints, matchers...)
 }

storage/remote/read_test.go

@@ -117,10 +117,8 @@ func TestExternalLabelsQuerierSelect(t *testing.T) {
 		},
 	}
 	want := newSeriesSetFilter(mockSeriesSet{}, q.externalLabels)
-	have, _, err := q.Select(false, nil, matchers...)
-	if err != nil {
-		t.Error(err)
-	}
+	have := q.Select(false, nil, matchers...)
+
 	if !reflect.DeepEqual(want, have) {
 		t.Errorf("expected series set %+v, got %+v", want, have)
 	}
@@ -218,16 +216,12 @@ func TestSeriesSetFilter(t *testing.T) {
 		},
 	}
 
-	for i, tc := range tests {
+	for _, tc := range tests {
 		filtered := newSeriesSetFilter(FromQueryResult(true, tc.in), tc.toRemove)
-		have, err := ToQueryResult(filtered, 1e6)
-		if err != nil {
-			t.Fatal(err)
-		}
-
-		if !reflect.DeepEqual(have, tc.expected) {
-			t.Fatalf("%d. unexpected labels; want %v, got %v", i, tc.expected, have)
-		}
+		act, ws, err := ToQueryResult(filtered, 1e6)
+		testutil.Ok(t, err)
+		testutil.Equals(t, 0, len(ws))
+		testutil.Equals(t, tc.expected, act)
 	}
 }
 
@@ -242,8 +236,8 @@ type mockSeriesSet struct {
 	storage.SeriesSet
 }
 
-func (mockQuerier) Select(bool, *storage.SelectHints, ...*labels.Matcher) (storage.SeriesSet, storage.Warnings, error) {
-	return mockSeriesSet{}, nil, nil
+func (mockQuerier) Select(bool, *storage.SelectHints, ...*labels.Matcher) storage.SeriesSet {
+	return mockSeriesSet{}
 }
 
 func TestPreferLocalStorageFilter(t *testing.T) {
@@ -398,10 +392,8 @@ func TestRequiredLabelsQuerierSelect(t *testing.T) {
 			requiredMatchers: test.requiredMatchers,
 		}
 
-		have, _, err := q.Select(false, nil, test.matchers...)
-		if err != nil {
-			t.Error(err)
-		}
+		have := q.Select(false, nil, test.matchers...)
+
 		if want := test.seriesSet; want != have {
 			t.Errorf("%d. expected series set %+v, got %+v", i, want, have)
 		}

storage/remote/storage.go

@@ -132,6 +132,9 @@ func (s *Storage) StartTime() (int64, error) {
 
 // Querier returns a storage.MergeQuerier combining the remote client queriers
 // of each configured remote read endpoint.
+// Returned querier will never return error as all queryables are assumed best effort.
+// Additionally all returned queriers ensure that its Select's SeriesSets have ready data after first `Next` invoke.
+// This is because Prometheus (fanout and secondary queries) can't handle the stream failing half way through by design.
 func (s *Storage) Querier(ctx context.Context, mint, maxt int64) (storage.Querier, error) {
 	s.mtx.Lock()
 	queryables := s.queryables

storage/secondary.go

@@ -0,0 +1,112 @@
+// Copyright 2017 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package storage
+
+import (
+	"sync"
+
+	"github.com/prometheus/prometheus/pkg/labels"
+)
+
+// secondaryQuerier is a wrapper that allows a querier to be treated in a best effort manner.
+// This means that an error on any method returned by Querier except Close will be returned as a warning,
+// and the result will be empty.
+//
+// Additionally, Querier ensures that if ANY SeriesSet returned by this querier's Select failed on an initial Next,
+// All other SeriesSet will be return no response as well. This ensures consistent partial response strategy, where you
+// have either full results or none from each secondary Querier.
+// NOTE: This works well only for implementations that only fail during first Next() (e.g fetch from network). If implementation fails
+// during further iterations, set will panic. If Select is invoked after first Next of any returned SeriesSet, querier will panic.
+//
+// Not go-routine safe.
+// NOTE: Prometheus treats all remote storages as secondary / best effort.
+type secondaryQuerier struct {
+	genericQuerier
+
+	once      sync.Once
+	done      bool
+	asyncSets []genericSeriesSet
+}
+
+func newSecondaryQuerierFrom(q Querier) genericQuerier {
+	return &secondaryQuerier{genericQuerier: newGenericQuerierFrom(q)}
+}
+
+func newSecondaryQuerierFromChunk(cq ChunkQuerier) genericQuerier {
+	return &secondaryQuerier{genericQuerier: newGenericQuerierFromChunk(cq)}
+}
+
+func (s *secondaryQuerier) LabelValues(name string) ([]string, Warnings, error) {
+	vals, w, err := s.genericQuerier.LabelValues(name)
+	if err != nil {
+		return nil, append([]error{err}, w...), nil
+	}
+	return vals, w, nil
+}
+
+func (s *secondaryQuerier) LabelNames() ([]string, Warnings, error) {
+	names, w, err := s.genericQuerier.LabelNames()
+	if err != nil {
+		return nil, append([]error{err}, w...), nil
+	}
+	return names, w, nil
+}
+
+func (s *secondaryQuerier) createFn(asyncSet genericSeriesSet) func() (genericSeriesSet, bool) {
+	s.asyncSets = append(s.asyncSets, asyncSet)
+	curr := len(s.asyncSets) - 1
+	return func() (genericSeriesSet, bool) {
+		s.once.Do(func() {
+			// At first create invocation we iterate over all sets and ensure its Next() returns some value without
+			// errors. This is to ensure we support consistent partial failures.
+			for i, set := range s.asyncSets {
+				if set.Next() {
+					continue
+				}
+				ws := set.Warnings()
+				// Failed set.
+				if err := set.Err(); err != nil {
+					ws = append([]error{err}, ws...)
+					// Promote the warnings to the current one.
+					s.asyncSets[curr] = warningsOnlySeriesSet(ws)
+					// One of the sets failed, ensure rest of the sets returns nothing. (All or nothing logic).
+					for i := range s.asyncSets {
+						if curr != i {
+							s.asyncSets[i] = noopGenericSeriesSet{}
+						}
+					}
+					break
+				}
+				// Exhausted set.
+				s.asyncSets[i] = warningsOnlySeriesSet(ws)
+			}
+
+			s.done = true
+		})
+
+		switch s.asyncSets[curr].(type) {
+		case warningsOnlySeriesSet, noopGenericSeriesSet:
+			return s.asyncSets[curr], false
+		default:
+			return s.asyncSets[curr], true
+		}
+	}
+}
+
+func (s *secondaryQuerier) Select(sortSeries bool, hints *SelectHints, matchers ...*labels.Matcher) genericSeriesSet {
+	if s.done {
+		panic("secondaryQuerier: Select invoked after first Next of any returned SeriesSet was done")
+	}
+	return &lazySeriesSet{create: s.createFn(s.genericQuerier.Select(sortSeries, hints, matchers...))}
+}

tsdb/block_test.go

@@ -203,9 +203,7 @@ func TestCorruptedChunk(t *testing.T) {
 			querier, err := NewBlockQuerier(b, 0, 1)
 			testutil.Ok(t, err)
 			defer func() { testutil.Ok(t, querier.Close()) }()
-			set, ws, err := querier.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "a", "b"))
-			testutil.Ok(t, err)
-			testutil.Equals(t, 0, len(ws))
+			set := querier.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "a", "b"))
 
 			// Check query err.
 			testutil.Equals(t, false, set.Next())

tsdb/cmd/tsdb/main.go

@@ -617,18 +617,7 @@ func dumpSamples(db *tsdb.DBReadOnly, mint, maxt int64) (err error) {
 		err = merr.Err()
 	}()
 
-	ss, ws, err := q.Select(false, nil, labels.MustNewMatcher(labels.MatchRegexp, "", ".*"))
-	if err != nil {
-		return err
-	}
-
-	if len(ws) > 0 {
-		var merr tsdb_errors.MultiError
-		for _, w := range ws {
-			merr.Add(w)
-		}
-		return merr.Err()
-	}
+	ss := q.Select(false, nil, labels.MustNewMatcher(labels.MatchRegexp, "", ".*"))
 
 	for ss.Next() {
 		series := ss.At()
@@ -643,6 +632,14 @@ func dumpSamples(db *tsdb.DBReadOnly, mint, maxt int64) (err error) {
 		}
 	}
 
+	if ws := ss.Warnings(); len(ws) > 0 {
+		var merr tsdb_errors.MultiError
+		for _, w := range ws {
+			merr.Add(w)
+		}
+		return merr.Err()
+	}
+
 	if ss.Err() != nil {
 		return ss.Err()
 	}

tsdb/db_test.go

@@ -67,15 +67,12 @@ func openTestDB(t testing.TB, opts *Options, rngs []int64) (db *DB, close func()
 
 // query runs a matcher query against the querier and fully expands its data.
 func query(t testing.TB, q storage.Querier, matchers ...*labels.Matcher) map[string][]tsdbutil.Sample {
-	ss, ws, err := q.Select(false, nil, matchers...)
+	ss := q.Select(false, nil, matchers...)
 	defer func() {
 		testutil.Ok(t, q.Close())
 	}()
-	testutil.Ok(t, err)
-	testutil.Equals(t, 0, len(ws))
 
 	result := map[string][]tsdbutil.Sample{}
-
 	for ss.Next() {
 		series := ss.At()
 
@@ -95,6 +92,7 @@ func query(t testing.TB, q storage.Querier, matchers ...*labels.Matcher) map[str
 		result[name] = samples
 	}
 	testutil.Ok(t, ss.Err())
+	testutil.Equals(t, 0, len(ss.Warnings()))
 
 	return result
 }
@@ -315,9 +313,7 @@ Outer:
 		q, err := db.Querier(context.TODO(), 0, numSamples)
 		testutil.Ok(t, err)
 
-		res, ws, err := q.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "a", "b"))
-		testutil.Ok(t, err)
-		testutil.Equals(t, 0, len(ws))
+		res := q.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "a", "b"))
 
 		expSamples := make([]tsdbutil.Sample, 0, len(c.remaint))
 		for _, ts := range c.remaint {
@@ -333,6 +329,7 @@ Outer:
 			testutil.Equals(t, eok, rok)
 
 			if !eok {
+				testutil.Equals(t, 0, len(res.Warnings()))
 				continue Outer
 			}
 			sexp := expss.At()
@@ -491,10 +488,7 @@ func TestDB_Snapshot(t *testing.T) {
 	defer func() { testutil.Ok(t, querier.Close()) }()
 
 	// sum values
-	seriesSet, ws, err := querier.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "foo", "bar"))
-	testutil.Ok(t, err)
-	testutil.Equals(t, 0, len(ws))
-
+	seriesSet := querier.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "foo", "bar"))
 	sum := 0.0
 	for seriesSet.Next() {
 		series := seriesSet.At().Iterator()
@@ -505,6 +499,7 @@ func TestDB_Snapshot(t *testing.T) {
 		testutil.Ok(t, series.Err())
 	}
 	testutil.Ok(t, seriesSet.Err())
+	testutil.Equals(t, 0, len(seriesSet.Warnings()))
 	testutil.Equals(t, 1000.0, sum)
 }
 
@@ -546,10 +541,7 @@ func TestDB_Snapshot_ChunksOutsideOfCompactedRange(t *testing.T) {
 	defer func() { testutil.Ok(t, querier.Close()) }()
 
 	// Sum values.
-	seriesSet, ws, err := querier.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "foo", "bar"))
-	testutil.Ok(t, err)
-	testutil.Equals(t, 0, len(ws))
-
+	seriesSet := querier.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "foo", "bar"))
 	sum := 0.0
 	for seriesSet.Next() {
 		series := seriesSet.At().Iterator()
@@ -560,6 +552,7 @@ func TestDB_Snapshot_ChunksOutsideOfCompactedRange(t *testing.T) {
 		testutil.Ok(t, series.Err())
 	}
 	testutil.Ok(t, seriesSet.Err())
+	testutil.Equals(t, 0, len(seriesSet.Warnings()))
 
 	// Since we snapshotted with MaxTime - 10, so expect 10 less samples.
 	testutil.Equals(t, 1000.0-10, sum)
@@ -618,9 +611,7 @@ Outer:
 		testutil.Ok(t, err)
 		defer func() { testutil.Ok(t, q.Close()) }()
 
-		res, ws, err := q.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "a", "b"))
-		testutil.Ok(t, err)
-		testutil.Equals(t, 0, len(ws))
+		res := q.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "a", "b"))
 
 		expSamples := make([]tsdbutil.Sample, 0, len(c.remaint))
 		for _, ts := range c.remaint {
@@ -641,6 +632,7 @@ Outer:
 			testutil.Equals(t, eok, rok)
 
 			if !eok {
+				testutil.Equals(t, 0, len(res.Warnings()))
 				continue Outer
 			}
 			sexp := expss.At()
@@ -792,10 +784,7 @@ func TestDB_e2e(t *testing.T) {
 			q, err := db.Querier(context.TODO(), mint, maxt)
 			testutil.Ok(t, err)
 
-			ss, ws, err := q.Select(false, nil, qry.ms...)
-			testutil.Ok(t, err)
-			testutil.Equals(t, 0, len(ws))
-
+			ss := q.Select(false, nil, qry.ms...)
 			result := map[string][]tsdbutil.Sample{}
 
 			for ss.Next() {
@@ -810,6 +799,7 @@ func TestDB_e2e(t *testing.T) {
 			}
 
 			testutil.Ok(t, ss.Err())
+			testutil.Equals(t, 0, len(ss.Warnings()))
 			testutil.Equals(t, expected, result)
 
 			q.Close()
@@ -968,9 +958,7 @@ func TestTombstoneClean(t *testing.T) {
 		testutil.Ok(t, err)
 		defer q.Close()
 
-		res, ws, err := q.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "a", "b"))
-		testutil.Ok(t, err)
-		testutil.Equals(t, 0, len(ws))
+		res := q.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "a", "b"))
 
 		expSamples := make([]tsdbutil.Sample, 0, len(c.remaint))
 		for _, ts := range c.remaint {
@@ -1004,6 +992,7 @@ func TestTombstoneClean(t *testing.T) {
 			testutil.Equals(t, errExp, errRes)
 			testutil.Equals(t, smplExp, smplRes)
 		}
+		testutil.Equals(t, 0, len(res.Warnings()))
 
 		for _, b := range db.Blocks() {
 			testutil.Equals(t, tombstones.NewMemTombstones(), b.tombstones)
@@ -1306,19 +1295,17 @@ func TestNotMatcherSelectsLabelsUnsetSeries(t *testing.T) {
 	defer func() { testutil.Ok(t, q.Close()) }()
 
 	for _, c := range cases {
-		ss, ws, err := q.Select(false, nil, c.selector...)
+		ss := q.Select(false, nil, c.selector...)
+		lres, _, ws, err := expandSeriesSet(ss)
 		testutil.Ok(t, err)
 		testutil.Equals(t, 0, len(ws))
-
-		lres, _, err := expandSeriesSet(ss)
-		testutil.Ok(t, err)
 		testutil.Equals(t, c.series, lres)
 	}
 }
 
 // expandSeriesSet returns the raw labels in the order they are retrieved from
 // the series set and the samples keyed by Labels().String().
-func expandSeriesSet(ss storage.SeriesSet) ([]labels.Labels, map[string][]sample, error) {
+func expandSeriesSet(ss storage.SeriesSet) ([]labels.Labels, map[string][]sample, storage.Warnings, error) {
 	resultLabels := []labels.Labels{}
 	resultSamples := map[string][]sample{}
 	for ss.Next() {
@@ -1332,7 +1319,7 @@ func expandSeriesSet(ss storage.SeriesSet) ([]labels.Labels, map[string][]sample
 		resultLabels = append(resultLabels, series.Labels())
 		resultSamples[series.Labels().String()] = samples
 	}
-	return resultLabels, resultSamples, ss.Err()
+	return resultLabels, resultSamples, ss.Warnings(), ss.Err()
 }
 
 func TestOverlappingBlocksDetectsAllOverlaps(t *testing.T) {
@@ -2503,9 +2490,7 @@ func TestDBReadOnly_FlushWAL(t *testing.T) {
 	defer func() { testutil.Ok(t, querier.Close()) }()
 
 	// Sum the values.
-	seriesSet, ws, err := querier.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, defaultLabelName, "flush"))
-	testutil.Ok(t, err)
-	testutil.Equals(t, 0, len(ws))
+	seriesSet := querier.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, defaultLabelName, "flush"))
 
 	sum := 0.0
 	for seriesSet.Next() {
@@ -2517,6 +2502,7 @@ func TestDBReadOnly_FlushWAL(t *testing.T) {
 		testutil.Ok(t, series.Err())
 	}
 	testutil.Ok(t, seriesSet.Err())
+	testutil.Equals(t, 0, len(seriesSet.Warnings()))
 	testutil.Equals(t, 1000.0, sum)
 }
 
@@ -2568,11 +2554,11 @@ func TestDBCannotSeePartialCommits(t *testing.T) {
 			testutil.Ok(t, err)
 			defer querier.Close()
 
-			ss, _, err := querier.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "foo", "bar"))
+			ss := querier.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "foo", "bar"))
+			_, seriesSet, ws, err := expandSeriesSet(ss)
 			testutil.Ok(t, err)
+			testutil.Equals(t, 0, len(ws))
 
-			_, seriesSet, err := expandSeriesSet(ss)
-			testutil.Ok(t, err)
 			values := map[float64]struct{}{}
 			for _, series := range seriesSet {
 				values[series[len(series)-1].v] = struct{}{}
@@ -2608,16 +2594,16 @@ func TestDBQueryDoesntSeeAppendsAfterCreation(t *testing.T) {
 	defer querierAfterAddButBeforeCommit.Close()
 
 	// None of the queriers should return anything after the Add but before the commit.
-	ss, _, err := querierBeforeAdd.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "foo", "bar"))
-	testutil.Ok(t, err)
-	_, seriesSet, err := expandSeriesSet(ss)
+	ss := querierBeforeAdd.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "foo", "bar"))
+	_, seriesSet, ws, err := expandSeriesSet(ss)
 	testutil.Ok(t, err)
+	testutil.Equals(t, 0, len(ws))
 	testutil.Equals(t, map[string][]sample{}, seriesSet)
 
-	ss, _, err = querierAfterAddButBeforeCommit.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "foo", "bar"))
-	testutil.Ok(t, err)
-	_, seriesSet, err = expandSeriesSet(ss)
+	ss = querierAfterAddButBeforeCommit.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "foo", "bar"))
+	_, seriesSet, ws, err = expandSeriesSet(ss)
 	testutil.Ok(t, err)
+	testutil.Equals(t, 0, len(ws))
 	testutil.Equals(t, map[string][]sample{}, seriesSet)
 
 	// This commit is after the queriers are created, so should not be returned.
@@ -2625,17 +2611,17 @@ func TestDBQueryDoesntSeeAppendsAfterCreation(t *testing.T) {
 	testutil.Ok(t, err)
 
 	// Nothing returned for querier created before the Add.
-	ss, _, err = querierBeforeAdd.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "foo", "bar"))
-	testutil.Ok(t, err)
-	_, seriesSet, err = expandSeriesSet(ss)
+	ss = querierBeforeAdd.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "foo", "bar"))
+	_, seriesSet, ws, err = expandSeriesSet(ss)
 	testutil.Ok(t, err)
+	testutil.Equals(t, 0, len(ws))
 	testutil.Equals(t, map[string][]sample{}, seriesSet)
 
 	// Series exists but has no samples for querier created after Add.
-	ss, _, err = querierAfterAddButBeforeCommit.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "foo", "bar"))
-	testutil.Ok(t, err)
-	_, seriesSet, err = expandSeriesSet(ss)
+	ss = querierAfterAddButBeforeCommit.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "foo", "bar"))
+	_, seriesSet, ws, err = expandSeriesSet(ss)
 	testutil.Ok(t, err)
+	testutil.Equals(t, 0, len(ws))
 	testutil.Equals(t, map[string][]sample{`{foo="bar"}`: {}}, seriesSet)
 
 	querierAfterCommit, err := db.Querier(context.Background(), 0, 1000000)
@@ -2643,10 +2629,10 @@ func TestDBQueryDoesntSeeAppendsAfterCreation(t *testing.T) {
 	defer querierAfterCommit.Close()
 
 	// Samples are returned for querier created after Commit.
-	ss, _, err = querierAfterCommit.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "foo", "bar"))
-	testutil.Ok(t, err)
-	_, seriesSet, err = expandSeriesSet(ss)
+	ss = querierAfterCommit.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "foo", "bar"))
+	_, seriesSet, ws, err = expandSeriesSet(ss)
 	testutil.Ok(t, err)
+	testutil.Equals(t, 0, len(ws))
 	testutil.Equals(t, map[string][]sample{`{foo="bar"}`: {{t: 0, v: 0}}}, seriesSet)
 }
 

tsdb/head_test.go

@@ -562,9 +562,7 @@ func TestHeadDeleteSimple(t *testing.T) {
 				for _, h := range []*Head{head, reloadedHead} {
 					q, err := NewBlockQuerier(h, h.MinTime(), h.MaxTime())
 					testutil.Ok(t, err)
-					actSeriesSet, ws, err := q.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, lblDefault.Name, lblDefault.Value))
-					testutil.Ok(t, err)
-					testutil.Equals(t, 0, len(ws))
+					actSeriesSet := q.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, lblDefault.Name, lblDefault.Value))
 					testutil.Ok(t, q.Close())
 					expSeriesSet := newMockSeriesSet([]storage.Series{
 						newSeries(map[string]string{lblDefault.Name: lblDefault.Value}, func() []tsdbutil.Sample {
@@ -583,6 +581,8 @@ func TestHeadDeleteSimple(t *testing.T) {
 
 						if !eok {
 							testutil.Ok(t, h.Close())
+							testutil.Ok(t, actSeriesSet.Err())
+							testutil.Equals(t, 0, len(actSeriesSet.Warnings()))
 							continue Outer
 						}
 						expSeries := expSeriesSet.At()
@@ -623,13 +623,15 @@ func TestDeleteUntilCurMax(t *testing.T) {
 	// Test the series returns no samples. The series is cleared only after compaction.
 	q, err := NewBlockQuerier(hb, 0, 100000)
 	testutil.Ok(t, err)
-	res, ws, err := q.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "a", "b"))
-	testutil.Ok(t, err)
-	testutil.Equals(t, 0, len(ws))
+	res := q.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "a", "b"))
 	testutil.Assert(t, res.Next(), "series is not present")
 	s := res.At()
 	it := s.Iterator()
 	testutil.Assert(t, !it.Next(), "expected no samples")
+	for res.Next() {
+	}
+	testutil.Ok(t, res.Err())
+	testutil.Equals(t, 0, len(res.Warnings()))
 
 	// Add again and test for presence.
 	app = hb.Appender()
@@ -638,15 +640,17 @@ func TestDeleteUntilCurMax(t *testing.T) {
 	testutil.Ok(t, app.Commit())
 	q, err = NewBlockQuerier(hb, 0, 100000)
 	testutil.Ok(t, err)
-	res, ws, err = q.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "a", "b"))
-	testutil.Ok(t, err)
-	testutil.Equals(t, 0, len(ws))
+	res = q.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "a", "b"))
 	testutil.Assert(t, res.Next(), "series don't exist")
 	exps := res.At()
 	it = exps.Iterator()
 	resSamples, err := expandSeriesIterator(it)
 	testutil.Ok(t, err)
 	testutil.Equals(t, []tsdbutil.Sample{sample{11, 1}}, resSamples)
+	for res.Next() {
+	}
+	testutil.Ok(t, res.Err())
+	testutil.Equals(t, 0, len(res.Warnings()))
 }
 
 func TestDeletedSamplesAndSeriesStillInWALAfterCheckpoint(t *testing.T) {
@@ -807,9 +811,7 @@ func TestDelete_e2e(t *testing.T) {
 			q, err := NewBlockQuerier(hb, 0, 100000)
 			testutil.Ok(t, err)
 			defer q.Close()
-			ss, ws, err := q.Select(true, nil, del.ms...)
-			testutil.Ok(t, err)
-			testutil.Equals(t, 0, len(ws))
+			ss := q.Select(true, nil, del.ms...)
 			// Build the mockSeriesSet.
 			matchedSeries := make([]storage.Series, 0, len(matched))
 			for _, m := range matched {
@@ -850,6 +852,8 @@ func TestDelete_e2e(t *testing.T) {
 				testutil.Equals(t, errExp, errRes)
 				testutil.Equals(t, smplExp, smplRes)
 			}
+			testutil.Ok(t, ss.Err())
+			testutil.Equals(t, 0, len(ss.Warnings()))
 		}
 	}
 }
@@ -1118,11 +1122,12 @@ func TestUncommittedSamplesNotLostOnTruncate(t *testing.T) {
 	testutil.Ok(t, err)
 	defer q.Close()
 
-	ss, ws, err := q.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "a", "1"))
-	testutil.Ok(t, err)
-	testutil.Equals(t, 0, len(ws))
-
+	ss := q.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "a", "1"))
 	testutil.Equals(t, true, ss.Next())
+	for ss.Next() {
+	}
+	testutil.Ok(t, ss.Err())
+	testutil.Equals(t, 0, len(ss.Warnings()))
 }
 
 func TestRemoveSeriesAfterRollbackAndTruncate(t *testing.T) {
@@ -1148,11 +1153,9 @@ func TestRemoveSeriesAfterRollbackAndTruncate(t *testing.T) {
 	testutil.Ok(t, err)
 	defer q.Close()
 
-	ss, ws, err := q.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "a", "1"))
-	testutil.Ok(t, err)
-	testutil.Equals(t, 0, len(ws))
-
+	ss := q.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "a", "1"))
 	testutil.Equals(t, false, ss.Next())
+	testutil.Equals(t, 0, len(ss.Warnings()))
 
 	// Truncate again, this time the series should be deleted
 	testutil.Ok(t, h.Truncate(2050))
@@ -1434,11 +1437,11 @@ func TestMemSeriesIsolation(t *testing.T) {
 		testutil.Ok(t, err)
 		defer querier.Close()
 
-		ss, _, err := querier.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "foo", "bar"))
+		ss := querier.Select(false, nil, labels.MustNewMatcher(labels.MatchEqual, "foo", "bar"))
+		_, seriesSet, ws, err := expandSeriesSet(ss)
 		testutil.Ok(t, err)
+		testutil.Equals(t, 0, len(ws))
 
-		_, seriesSet, err := expandSeriesSet(ss)
-		testutil.Ok(t, err)
 		for _, series := range seriesSet {
 			return int(series[len(series)-1].v)
 		}
@@ -1790,8 +1793,9 @@ func testHeadSeriesChunkRace(t *testing.T) {
 		h.gc()
 		wg.Done()
 	}()
-	ss, _, err := q.Select(false, nil, matcher)
-	testutil.Ok(t, err)
+	ss := q.Select(false, nil, matcher)
+	for ss.Next() {
+	}
 	testutil.Ok(t, ss.Err())
 	wg.Wait()
 }

tsdb/querier.go

@@ -85,9 +85,9 @@ func (q *querier) lvals(qs []storage.Querier, n string) ([]string, storage.Warni
 	return mergeStrings(s1, s2), ws, nil
 }
 
-func (q *querier) Select(sortSeries bool, hints *storage.SelectHints, ms ...*labels.Matcher) (storage.SeriesSet, storage.Warnings, error) {
+func (q *querier) Select(sortSeries bool, hints *storage.SelectHints, ms ...*labels.Matcher) storage.SeriesSet {
 	if len(q.blocks) == 0 {
-		return storage.EmptySeriesSet(), nil, nil
+		return storage.EmptySeriesSet()
 	}
 	if len(q.blocks) == 1 {
 		// Sorting Head series is slow, and unneeded when only the
@@ -96,18 +96,12 @@ func (q *querier) Select(sortSeries bool, hints *storage.SelectHints, ms ...*lab
 	}
 
 	ss := make([]storage.SeriesSet, len(q.blocks))
-	var ws storage.Warnings
 	for i, b := range q.blocks {
 		// We have to sort if blocks > 1 as MergedSeriesSet requires it.
-		s, w, err := b.Select(true, hints, ms...)
-		ws = append(ws, w...)
-		if err != nil {
-			return nil, ws, err
-		}
-		ss[i] = s
+		ss[i] = b.Select(true, hints, ms...)
 	}
 
-	return NewMergedSeriesSet(ss), ws, nil
+	return NewMergedSeriesSet(ss)
 }
 
 func (q *querier) Close() error {
@@ -125,31 +119,23 @@ type verticalQuerier struct {
 	querier
 }
 
-func (q *verticalQuerier) Select(sortSeries bool, hints *storage.SelectHints, ms ...*labels.Matcher) (storage.SeriesSet, storage.Warnings, error) {
+func (q *verticalQuerier) Select(sortSeries bool, hints *storage.SelectHints, ms ...*labels.Matcher) storage.SeriesSet {
 	return q.sel(sortSeries, hints, q.blocks, ms)
 }
 
-func (q *verticalQuerier) sel(sortSeries bool, hints *storage.SelectHints, qs []storage.Querier, ms []*labels.Matcher) (storage.SeriesSet, storage.Warnings, error) {
+func (q *verticalQuerier) sel(sortSeries bool, hints *storage.SelectHints, qs []storage.Querier, ms []*labels.Matcher) storage.SeriesSet {
 	if len(qs) == 0 {
-		return storage.EmptySeriesSet(), nil, nil
+		return storage.EmptySeriesSet()
 	}
 	if len(qs) == 1 {
 		return qs[0].Select(sortSeries, hints, ms...)
 	}
 	l := len(qs) / 2
 
-	var ws storage.Warnings
-	a, w, err := q.sel(sortSeries, hints, qs[:l], ms)
-	ws = append(ws, w...)
-	if err != nil {
-		return nil, ws, err
-	}
-	b, w, err := q.sel(sortSeries, hints, qs[l:], ms)
-	ws = append(ws, w...)
-	if err != nil {
-		return nil, ws, err
-	}
-	return newMergedVerticalSeriesSet(a, b), ws, nil
+	return newMergedVerticalSeriesSet(
+		q.sel(sortSeries, hints, qs[:l], ms),
+		q.sel(sortSeries, hints, qs[l:], ms),
+	)
 }
 
 // NewBlockQuerier returns a querier against the reader.
@@ -189,7 +175,7 @@ type blockQuerier struct {
 	mint, maxt int64
 }
 
-func (q *blockQuerier) Select(sortSeries bool, hints *storage.SelectHints, ms ...*labels.Matcher) (storage.SeriesSet, storage.Warnings, error) {
+func (q *blockQuerier) Select(sortSeries bool, hints *storage.SelectHints, ms ...*labels.Matcher) storage.SeriesSet {
 	var base storage.DeprecatedChunkSeriesSet
 	var err error
 
@@ -199,7 +185,7 @@ func (q *blockQuerier) Select(sortSeries bool, hints *storage.SelectHints, ms ..
 		base, err = LookupChunkSeries(q.index, q.tombstones, ms...)
 	}
 	if err != nil {
-		return nil, nil, err
+		return storage.ErrSeriesSet(err)
 	}
 
 	mint := q.mint
@@ -218,7 +204,7 @@ func (q *blockQuerier) Select(sortSeries bool, hints *storage.SelectHints, ms ..
 
 		mint: mint,
 		maxt: maxt,
-	}, nil, nil
+	}
 }
 
 func (q *blockQuerier) LabelValues(name string) ([]string, storage.Warnings, error) {
@@ -501,6 +487,14 @@ func (s *mergedSeriesSet) Err() error {
 	return s.err
 }
 
+func (s *mergedSeriesSet) Warnings() storage.Warnings {
+	var ws storage.Warnings
+	for _, ss := range s.all {
+		ws = append(ws, ss.Warnings()...)
+	}
+	return ws
+}
+
 // nextAll is to call Next() for all SeriesSet.
 // Because the order of the SeriesSet slice will affect the results,
 // we need to use an buffer slice to hold the order.
@@ -509,7 +503,10 @@ func (s *mergedSeriesSet) nextAll() {
 	for _, ss := range s.all {
 		if ss.Next() {
 			s.buf = append(s.buf, ss)
-		} else if ss.Err() != nil {
+			continue
+		}
+
+		if ss.Err() != nil {
 			s.done = true
 			s.err = ss.Err()
 			break
@@ -623,6 +620,13 @@ func (s *mergedVerticalSeriesSet) Err() error {
 	return s.b.Err()
 }
 
+func (s *mergedVerticalSeriesSet) Warnings() storage.Warnings {
+	var ws storage.Warnings
+	ws = append(ws, s.a.Warnings()...)
+	ws = append(ws, s.b.Warnings()...)
+	return ws
+}
+
 func (s *mergedVerticalSeriesSet) compare() int {
 	if s.adone {
 		return 1
@@ -848,8 +852,9 @@ func (s *blockSeriesSet) Next() bool {
 	return false
 }
 
-func (s *blockSeriesSet) At() storage.Series { return s.cur }
-func (s *blockSeriesSet) Err() error         { return s.err }
+func (s *blockSeriesSet) At() storage.Series         { return s.cur }
+func (s *blockSeriesSet) Err() error                 { return s.err }
+func (s *blockSeriesSet) Warnings() storage.Warnings { return nil }
 
 // chunkSeries is a series that is backed by a sequence of chunks holding
 // time series data.

tsdb/querier_bench_test.go

@@ -155,8 +155,7 @@ func BenchmarkQuerierSelect(b *testing.B) {
 
 				b.ResetTimer()
 				for i := 0; i < b.N; i++ {
-					ss, _, err := q.Select(sorted, nil, matcher)
-					testutil.Ok(b, err)
+					ss := q.Select(sorted, nil, matcher)
 					for ss.Next() {
 					}
 					testutil.Ok(b, ss.Err())

tsdb/querier_test.go

@@ -39,12 +39,14 @@ import (
 type mockSeriesSet struct {
 	next   func() bool
 	series func() storage.Series
+	ws     func() storage.Warnings
 	err    func() error
 }
 
-func (m *mockSeriesSet) Next() bool         { return m.next() }
-func (m *mockSeriesSet) At() storage.Series { return m.series() }
-func (m *mockSeriesSet) Err() error         { return m.err() }
+func (m *mockSeriesSet) Next() bool                 { return m.next() }
+func (m *mockSeriesSet) At() storage.Series         { return m.series() }
+func (m *mockSeriesSet) Err() error                 { return m.err() }
+func (m *mockSeriesSet) Warnings() storage.Warnings { return m.ws() }
 
 func newMockSeriesSet(list []storage.Series) *mockSeriesSet {
 	i := -1
@@ -57,11 +59,11 @@ func newMockSeriesSet(list []storage.Series) *mockSeriesSet {
 			return list[i]
 		},
 		err: func() error { return nil },
+		ws:  func() storage.Warnings { return nil },
 	}
 }
 
 func TestMergedSeriesSet(t *testing.T) {
-
 	cases := []struct {
 		// The input sets in order (samples in series in b are strictly
 		// after those in a).
@@ -373,15 +375,14 @@ Outer:
 			maxt: c.maxt,
 		}
 
-		res, ws, err := querier.Select(false, nil, c.ms...)
-		testutil.Ok(t, err)
-		testutil.Equals(t, 0, len(ws))
+		res := querier.Select(false, nil, c.ms...)
 
 		for {
 			eok, rok := c.exp.Next(), res.Next()
 			testutil.Equals(t, eok, rok)
 
 			if !eok {
+				testutil.Equals(t, 0, len(res.Warnings()))
 				continue Outer
 			}
 			sexp := c.exp.At()
@@ -536,15 +537,14 @@ Outer:
 			maxt: c.maxt,
 		}
 
-		res, ws, err := querier.Select(false, nil, c.ms...)
-		testutil.Ok(t, err)
-		testutil.Equals(t, 0, len(ws))
+		res := querier.Select(false, nil, c.ms...)
 
 		for {
 			eok, rok := c.exp.Next(), res.Next()
 			testutil.Equals(t, eok, rok)
 
 			if !eok {
+				testutil.Equals(t, 0, len(res.Warnings()))
 				continue Outer
 			}
 			sexp := c.exp.At()
@@ -1654,7 +1654,7 @@ func BenchmarkQuerySeek(b *testing.B) {
 				b.ResetTimer()
 				b.ReportAllocs()
 
-				ss, ws, err := sq.Select(false, nil, labels.MustNewMatcher(labels.MatchRegexp, "__name__", ".*"))
+				ss := sq.Select(false, nil, labels.MustNewMatcher(labels.MatchRegexp, "__name__", ".*"))
 				for ss.Next() {
 					it := ss.At().Iterator()
 					for t := mint; t <= maxt; t++ {
@@ -1664,7 +1664,7 @@ func BenchmarkQuerySeek(b *testing.B) {
 				}
 				testutil.Ok(b, ss.Err())
 				testutil.Ok(b, err)
-				testutil.Equals(b, 0, len(ws))
+				testutil.Equals(b, 0, len(ss.Warnings()))
 			})
 		}
 	}
@@ -1792,9 +1792,11 @@ func BenchmarkSetMatcher(b *testing.B) {
 			b.ResetTimer()
 			b.ReportAllocs()
 			for n := 0; n < b.N; n++ {
-				_, ws, err := que.Select(false, nil, labels.MustNewMatcher(labels.MatchRegexp, "test", c.pattern))
-				testutil.Ok(b, err)
-				testutil.Equals(b, 0, len(ws))
+				ss := que.Select(false, nil, labels.MustNewMatcher(labels.MatchRegexp, "test", c.pattern))
+				for ss.Next() {
+				}
+				testutil.Ok(b, ss.Err())
+				testutil.Equals(b, 0, len(ss.Warnings()))
 			}
 		})
 	}
@@ -2252,9 +2254,7 @@ func benchQuery(b *testing.B, expExpansions int, q storage.Querier, selectors la
 	b.ResetTimer()
 	b.ReportAllocs()
 	for i := 0; i < b.N; i++ {
-		ss, ws, err := q.Select(false, nil, selectors...)
-		testutil.Ok(b, err)
-		testutil.Equals(b, 0, len(ws))
+		ss := q.Select(false, nil, selectors...)
 		var actualExpansions int
 		for ss.Next() {
 			s := ss.At()
@@ -2264,6 +2264,8 @@ func benchQuery(b *testing.B, expExpansions int, q storage.Querier, selectors la
 			}
 			actualExpansions++
 		}
+		testutil.Ok(b, ss.Err())
+		testutil.Equals(b, 0, len(ss.Warnings()))
 		testutil.Equals(b, expExpansions, actualExpansions)
 		testutil.Ok(b, ss.Err())
 	}

web/api/v1/api.go

@@ -596,13 +596,8 @@ func (api *API) series(r *http.Request) (result apiFuncResult) {
 	}
 
 	var sets []storage.SeriesSet
-	var warnings storage.Warnings
 	for _, mset := range matcherSets {
-		s, wrn, err := q.Select(false, nil, mset...)
-		warnings = append(warnings, wrn...)
-		if err != nil {
-			return apiFuncResult{nil, &apiError{errorExec, err}, warnings, closer}
-		}
+		s := q.Select(false, nil, mset...)
 		sets = append(sets, s)
 	}
 
@@ -611,6 +606,8 @@ func (api *API) series(r *http.Request) (result apiFuncResult) {
 	for set.Next() {
 		metrics = append(metrics, set.At().Labels())
 	}
+
+	warnings := set.Warnings()
 	if set.Err() != nil {
 		return apiFuncResult{nil, &apiError{errorExec, set.Err()}, warnings, closer}
 	}
@@ -1226,12 +1223,9 @@ func (api *API) remoteRead(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 		for i, query := range req.Queries {
-			err := api.remoteReadQuery(ctx, query, externalLabels, func(querier storage.Querier, hints *storage.SelectHints, filteredMatchers []*labels.Matcher) error {
+			ws, err := api.remoteReadQuery(ctx, query, externalLabels, func(querier storage.Querier, hints *storage.SelectHints, filteredMatchers []*labels.Matcher) (storage.Warnings, error) {
 				// The streaming API has to provide the series sorted.
-				set, _, err := querier.Select(true, hints, filteredMatchers...)
-				if err != nil {
-					return err
-				}
+				set := querier.Select(true, hints, filteredMatchers...)
 
 				return remote.StreamChunkedReadResponses(
 					remote.NewChunkedWriter(w, f),
@@ -1241,6 +1235,9 @@ func (api *API) remoteRead(w http.ResponseWriter, r *http.Request) {
 					api.remoteReadMaxBytesInFrame,
 				)
 			})
+			for _, w := range ws {
+				level.Warn(api.logger).Log("msg", "warnings on remote read query", "err", w.Error())
+			}
 			if err != nil {
 				if httpErr, ok := err.(remote.HTTPError); ok {
 					http.Error(w, httpErr.Error(), httpErr.Status())
@@ -1259,22 +1256,26 @@ func (api *API) remoteRead(w http.ResponseWriter, r *http.Request) {
 			Results: make([]*prompb.QueryResult, len(req.Queries)),
 		}
 		for i, query := range req.Queries {
-			err := api.remoteReadQuery(ctx, query, externalLabels, func(querier storage.Querier, hints *storage.SelectHints, filteredMatchers []*labels.Matcher) error {
-				set, _, err := querier.Select(false, hints, filteredMatchers...)
-				if err != nil {
-					return err
-				}
+			ws, err := api.remoteReadQuery(ctx, query, externalLabels, func(querier storage.Querier, hints *storage.SelectHints, filteredMatchers []*labels.Matcher) (storage.Warnings, error) {
+				set := querier.Select(false, hints, filteredMatchers...)
 
-				resp.Results[i], err = remote.ToQueryResult(set, api.remoteReadSampleLimit)
+				var (
+					ws  storage.Warnings
+					err error
+				)
+				resp.Results[i], ws, err = remote.ToQueryResult(set, api.remoteReadSampleLimit)
 				if err != nil {
-					return err
+					return ws, err
 				}
 
 				for _, ts := range resp.Results[i].Timeseries {
 					ts.Labels = remote.MergeLabels(ts.Labels, sortedExternalLabels)
 				}
-				return nil
+				return ws, nil
 			})
+			for _, w := range ws {
+				level.Warn(api.logger).Log("msg", "warnings on remote read query", "err", w.Error())
+			}
 			if err != nil {
 				if httpErr, ok := err.(remote.HTTPError); ok {
 					http.Error(w, httpErr.Error(), httpErr.Status())
@@ -1318,15 +1319,15 @@ func filterExtLabelsFromMatchers(pbMatchers []*prompb.LabelMatcher, externalLabe
 	return filteredMatchers, nil
 }
 
-func (api *API) remoteReadQuery(ctx context.Context, query *prompb.Query, externalLabels map[string]string, seriesHandleFn func(querier storage.Querier, hints *storage.SelectHints, filteredMatchers []*labels.Matcher) error) error {
+func (api *API) remoteReadQuery(ctx context.Context, query *prompb.Query, externalLabels map[string]string, seriesHandleFn func(querier storage.Querier, hints *storage.SelectHints, filteredMatchers []*labels.Matcher) (storage.Warnings, error)) (storage.Warnings, error) {
 	filteredMatchers, err := filterExtLabelsFromMatchers(query.Matchers, externalLabels)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	querier, err := api.Queryable.Querier(ctx, query.StartTimestampMs, query.EndTimestampMs)
 	if err != nil {
-		return err
+		return nil, err
 	}
 	defer func() {
 		if err := querier.Close(); err != nil {

web/api/v1/api_test.go

@@ -517,12 +517,8 @@ func setupRemote(s storage.Storage) *httptest.Server {
 			}
 			defer querier.Close()
 
-			set, _, err := querier.Select(false, hints, matchers...)
-			if err != nil {
-				http.Error(w, err.Error(), http.StatusInternalServerError)
-				return
-			}
-			resp.Results[i], err = remote.ToQueryResult(set, 1e6)
+			set := querier.Select(false, hints, matchers...)
+			resp.Results[i], _, err = remote.ToQueryResult(set, 1e6)
 			if err != nil {
 				http.Error(w, err.Error(), http.StatusInternalServerError)
 				return

web/federate.go

@@ -95,16 +95,7 @@ func (h *Handler) federation(w http.ResponseWriter, req *http.Request) {
 
 	var sets []storage.SeriesSet
 	for _, mset := range matcherSets {
-		s, wrns, err := q.Select(false, hints, mset...)
-		if wrns != nil {
-			level.Debug(h.logger).Log("msg", "Federation select returned warnings", "warnings", wrns)
-			federationWarnings.Add(float64(len(wrns)))
-		}
-		if err != nil {
-			federationErrors.Inc()
-			http.Error(w, err.Error(), http.StatusInternalServerError)
-			return
-		}
+		s := q.Select(false, hints, mset...)
 		sets = append(sets, s)
 	}
 
@@ -142,6 +133,10 @@ func (h *Handler) federation(w http.ResponseWriter, req *http.Request) {
 			Point:  promql.Point{T: t, V: v},
 		})
 	}
+	if ws := set.Warnings(); len(ws) > 0 {
+		level.Debug(h.logger).Log("msg", "Federation select returned warnings", "warnings", ws)
+		federationWarnings.Add(float64(len(ws)))
+	}
 	if set.Err() != nil {
 		federationErrors.Inc()
 		http.Error(w, set.Err().Error(), http.StatusInternalServerError)

web/ui/react-app/yarn.lock

@@ -11222,9 +11222,9 @@ websocket-driver@>=0.5.1:
     websocket-extensions ">=0.1.1"
 
 websocket-extensions@>=0.1.1:
-  version "0.1.3"
-  resolved "https://registry.yarnpkg.com/websocket-extensions/-/websocket-extensions-0.1.3.tgz#5d2ff22977003ec687a4b87073dfbbac146ccf29"
-  integrity sha512-nqHUnMXmBzT0w570r2JpJxfiSD1IzoI+HGVdd3aZ0yNi3ngvQ4jv1dtHt5VGxfI2yj5yqImPhOK4vmIh2xMbGg==
+  version "0.1.4"
+  resolved "https://registry.yarnpkg.com/websocket-extensions/-/websocket-extensions-0.1.4.tgz#7f8473bc839dfd87608adb95d7eb075211578a42"
+  integrity sha512-OqedPIGOfsDlo31UNwYbCFMSaO9m9G/0faIHj5/dZFDMFqPTcx6UwqyOy3COEaEOg/9VsGIpdqn62W5KhoKSpg==
 
 whatwg-encoding@^1.0.1, whatwg-encoding@^1.0.3, whatwg-encoding@^1.0.5:
   version "1.0.5"