Fix edge case in findPathRange that would occationally cause panics

https://github.com/jacksontj/promxy/issues/562

promql/engine_extra.go

@@ -13,6 +13,11 @@ func findPathRange(path []parser.Node, eRanges []evalRange) time.Duration {
 		depth     int
 	)
 	for _, r := range eRanges {
+		// If the prefix is longer then it can't be the parent of `child`
+		if len(r.Prefix) > len(path) {
+			continue
+		}
+
 		// Check if we are a child
 		child := true
 		for i, p := range r.Prefix {

promql/engine_extra_test.go

@@ -0,0 +1,56 @@
+package promql
+
+import (
+	"fmt"
+	"reflect"
+	"strconv"
+	"testing"
+	"time"
+
+	"github.com/prometheus/prometheus/promql/parser"
+)
+
+type StubNode struct {
+	start, end int
+}
+
+func (s StubNode) String() string {
+	return fmt.Sprintf("[%d,%d]", s.start, s.end)
+}
+
+func (s StubNode) PositionRange() parser.PositionRange {
+	return parser.PositionRange{
+		Start: parser.Pos(s.start),
+		End:   parser.Pos(s.end),
+	}
+}
+
+func TestFindPathRange(t *testing.T) {
+	tests := []struct {
+		path    []parser.Node
+		eRanges []evalRange
+		out     time.Duration
+	}{
+		// Test a case where the evalRange is longer than the path
+		{
+			path: []parser.Node{StubNode{0, 1}},
+			eRanges: []evalRange{
+				evalRange{
+					Prefix: []parser.PositionRange{
+						parser.PositionRange{parser.Pos(0), parser.Pos(1)},
+						parser.PositionRange{parser.Pos(1), parser.Pos(3)},
+					},
+				},
+			},
+		},
+	}
+
+	for i, test := range tests {
+		t.Run(strconv.Itoa(i), func(t *testing.T) {
+			out := findPathRange(test.path, test.eRanges)
+			if !reflect.DeepEqual(out, test.out) {
+				t.Fatalf("Mismatch in test output expected=%#v actual=%#v", test.out, out)
+			}
+		})
+	}
+}