Propagate more errors during rule evaluation.

main.go

@@ -94,8 +94,10 @@ func main() {
 			}
 
 		case ruleResult := <-ruleResults:
-			for _, sample := range ruleResult.Samples {
+			if ruleResult.Err == nil {
-				ts.AppendSample(sample)
+				for _, sample := range ruleResult.Samples {
+					ts.AppendSample(sample)
+				}
 			}
 		}
 	}

rules/ast/ast.go

@@ -258,12 +258,13 @@ func labelsToKey(labels model.Metric) string {
 	return strings.Join(keyParts, ",") // TODO not safe when label value contains comma.
 }
 
-func EvalVectorInstant(node VectorNode, timestamp time.Time) (vector Vector) {
+func EvalVectorInstant(node VectorNode, timestamp time.Time) (vector Vector, err error) {
 	viewAdapter, err := viewAdapterForInstantQuery(node, timestamp)
 	if err != nil {
 		return
 	}
-	return node.Eval(timestamp, viewAdapter)
+	vector = node.Eval(timestamp, viewAdapter)
+	return
 }
 
 func EvalVectorRange(node VectorNode, start time.Time, end time.Time, interval time.Duration) (matrix Matrix, err error) {

rules/manager.go

@@ -68,9 +68,10 @@ func (m *ruleManager) runIteration(results chan *Result) {
 	now := time.Now()
 	for _, rule := range m.rules {
 		go func() {
-			vector := rule.Eval(&now)
+			vector, err := rule.Eval(&now)
 			m.results <- &Result{
 				Samples: vector,
+				Err:     err,
 			}
 		}()
 	}

rules/rules.go

@@ -30,13 +30,16 @@ type Rule struct {
 
 func (rule *Rule) Name() string { return rule.name }
 
-func (rule *Rule) EvalRaw(timestamp *time.Time) (vector ast.Vector) {
+func (rule *Rule) EvalRaw(timestamp *time.Time) (vector ast.Vector, err error) {
 	return ast.EvalVectorInstant(rule.vector, *timestamp)
 }
 
-func (rule *Rule) Eval(timestamp *time.Time) ast.Vector {
+func (rule *Rule) Eval(timestamp *time.Time) (vector ast.Vector, err error) {
 	// Get the raw value of the rule expression.
-	vector := rule.EvalRaw(timestamp)
+	vector, err = rule.EvalRaw(timestamp)
+	if err != nil {
+		return
+	}
 
 	// Override the metric name and labels.
 	for _, sample := range vector {
@@ -49,7 +52,7 @@ func (rule *Rule) Eval(timestamp *time.Time) ast.Vector {
 			}
 		}
 	}
-	return vector
+	return
 }
 
 func (rule *Rule) RuleToDotGraph() string {