TSDB: Extract parts out of populateSeries (#6983)

This addresses fabxc's TODO.

More importantly, it now properly defers the
querier.Close(). Previously, if a panic happened after creation of the
querier within the populateSeries function, querier.Close() was never called.

The latter was responsible for #6977.

Signed-off-by: beorn7 <beorn@grafana.com>

promql/engine.go

@@ -502,15 +502,16 @@ func durationMilliseconds(d time.Duration) int64 {
 // execEvalStmt evaluates the expression of an evaluation statement for the given time range.
 func (ng *Engine) execEvalStmt(ctx context.Context, query *query, s *parser.EvalStmt) (parser.Value, storage.Warnings, error) {
 	prepareSpanTimer, ctxPrepare := query.stats.GetSpanTimer(ctx, stats.QueryPreparationTime, ng.metrics.queryPrepareTime)
-	querier, warnings, err := ng.populateSeries(ctxPrepare, query.queryable, s)
+	mint := ng.findMinTime(s)
+	querier, err := query.queryable.Querier(ctxPrepare, timestamp.FromTime(mint), timestamp.FromTime(s.End))
+	if err != nil {
 		prepareSpanTimer.Finish()
-
-	// XXX(fabxc): the querier returned by populateSeries might be instantiated
-	// we must not return without closing irrespective of the error.
-	// TODO: make this semantically saner.
-	if querier != nil {
-		defer querier.Close()
+		return nil, nil, err
 	}
+	defer querier.Close()
+
+	warnings, err := ng.populateSeries(ctxPrepare, querier, s)
+	prepareSpanTimer.Finish()
 
 	if err != nil {
 		return nil, warnings, err
@@ -616,7 +617,7 @@ func (ng *Engine) cumulativeSubqueryOffset(path []parser.Node) time.Duration {
 	return subqOffset
 }
 
-func (ng *Engine) populateSeries(ctx context.Context, q storage.Queryable, s *parser.EvalStmt) (storage.Querier, storage.Warnings, error) {
+func (ng *Engine) findMinTime(s *parser.EvalStmt) time.Time {
 	var maxOffset time.Duration
 	parser.Inspect(s.Expr, func(node parser.Node, path []parser.Node) error {
 		subqOffset := ng.cumulativeSubqueryOffset(path)
@@ -638,20 +639,18 @@ func (ng *Engine) populateSeries(ctx context.Context, q storage.Queryable, s *pa
 		}
 		return nil
 	})
+	return s.Start.Add(-maxOffset)
+}
 
-	mint := s.Start.Add(-maxOffset)
-
-	querier, err := q.Querier(ctx, timestamp.FromTime(mint), timestamp.FromTime(s.End))
-	if err != nil {
-		return nil, nil, err
-	}
-
-	var warnings storage.Warnings
-
-	// Whenever a MatrixSelector is evaluated this variable is set to the corresponding range.
+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.
-	var evalRange time.Duration
+		evalRange time.Duration
+		warnings  storage.Warnings
+		err       error
+	)
 
 	parser.Inspect(s.Expr, func(node parser.Node, path []parser.Node) error {
 		var set storage.SeriesSet
@@ -703,7 +702,7 @@ func (ng *Engine) populateSeries(ctx context.Context, q storage.Queryable, s *pa
 		}
 		return nil
 	})
-	return querier, warnings, err
+	return warnings, err
 }
 
 // extractFuncFromPath walks up the path and searches for the first instance of