Make findMinTime more exact for MatrixSelector

When a Range is smaller than lookbackDelta the previous version
of findMinTime used to offset the minTime by lookbackDelta instead
of Range. In reality, a MatrixSelector never needs to use the
LookbackDelta and it only did so because the Inspect code cascaded
down to the VectorSelector of the Matrix, which is incorrect.

Thus prevent the walk from descending to a MatrixSelector's
underlying VectorSelector.

Author: cevian

pkg/promql/engine.go

@@ -533,7 +533,11 @@ 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)
-	mint := ng.findMinTime(s)
+	mint, err := ng.findMinTime(s)
+	if err != nil {
+		prepareSpanTimer.Finish()
+		return nil, nil, err
+	}
 	querier, err := query.queryable.Querier(ctxPrepare, timestamp.FromTime(mint), timestamp.FromTime(s.End))
 	if err != nil {
 		prepareSpanTimer.Finish()
@@ -650,9 +654,21 @@ func (ng *Engine) cumulativeSubqueryOffset(path []parser.Node) time.Duration {
 	return subqOffset
 }
 
-func (ng *Engine) findMinTime(s *parser.EvalStmt) time.Time {
+type inspector func(parser.Node, []parser.Node) (bool, error)
+
+func (f inspector) Visit(node parser.Node, path []parser.Node) (parser.Visitor, error) {
+	if v, err := f(node, path); !v || err != nil {
+		return nil, err
+	}
+
+	return f, nil
+}
+
+// This is modified from upstream to avoid descending into a MatrixSelector's VectorSelector
+func (ng *Engine) findMinTime(s *parser.EvalStmt) (time.Time, error) {
 	var maxOffset time.Duration
-	parser.Inspect(s.Expr, func(node parser.Node, path []parser.Node) error {
+
+	err := parser.Walk(inspector(func(node parser.Node, path []parser.Node) (bool, error) {
 		subqOffset := ng.cumulativeSubqueryOffset(path)
 		switch n := node.(type) {
 		case *parser.VectorSelector:
@@ -669,10 +685,13 @@ func (ng *Engine) findMinTime(s *parser.EvalStmt) time.Time {
 			if m := n.VectorSelector.(*parser.VectorSelector).Offset + n.Range + subqOffset; m > maxOffset {
 				maxOffset = m
 			}
+			//do not process the child VectorSelector since we don't want to add the LookbackDelta etc for the underlying
+			//VectorSelector.
+			return false, nil
 		}
-		return nil
-	})
-	return s.Start.Add(-maxOffset)
+		return true, nil
+	}), s.Expr, nil)
+	return s.Start.Add(-maxOffset), err
 }
 
 func (ng *Engine) populateSeries(ctx context.Context, querier Querier, s *parser.EvalStmt) (storage.Warnings, parser.Node, error) {