Handle query parsing errors on QFE range split middleware

CHANGELOG.md

@@ -14,6 +14,7 @@ We use *breaking :warning:* to mark changes that are not backward compatible (re
 
 ### Fixed
 
+- [#6172](https://github.com/thanos-io/thanos/pull/6172) query-frontend: return JSON formatted errors for invalid PromQL expression in the split by interval middleware.
 - [#6171](https://github.com/thanos-io/thanos/pull/6171) Store: fix error handling on limits.
 
 ### Changed

internal/cortex/querier/queryrange/split_by_interval.go

@@ -99,7 +99,7 @@ func splitQuery(r Request, interval time.Duration) ([]Request, error) {
 func EvaluateAtModifierFunction(query string, start, end int64) (string, error) {
 	expr, err := parser.ParseExpr(query)
 	if err != nil {
-		return "", httpgrpc.Errorf(http.StatusBadRequest, "%s", err)
+		return "", httpgrpc.Errorf(http.StatusBadRequest, `{"status": "error", "error": "%s"}`, err)
 	}
 	parser.Inspect(expr, func(n parser.Node, _ []parser.Node) error {
 		if selector, ok := n.(*parser.VectorSelector); ok {

pkg/queryfrontend/split_by_interval_test.go

@@ -4,10 +4,13 @@
 package queryfrontend
 
 import (
+	"encoding/json"
 	"strconv"
 	"testing"
 	"time"
 
+	"github.com/weaveworks/common/httpgrpc"
+
 	"github.com/stretchr/testify/require"
 	"github.com/thanos-io/thanos/internal/cortex/querier/queryrange"
 )
@@ -238,3 +241,20 @@ func TestSplitQuery(t *testing.T) {
 		})
 	}
 }
+
+func TestSplitQuery_PromQLErrorReturnsJson(t *testing.T) {
+	input := &ThanosQueryRangeRequest{
+		Start: 2 * 3600 * seconds,
+		End:   3 * 3 * 3600 * seconds,
+		Step:  15 * seconds,
+		Query: "foo{",
+	}
+	queries, err := splitQuery(input, 1*time.Hour)
+	require.Error(t, err)
+	require.Nil(t, queries)
+
+	resp, ok := httpgrpc.HTTPResponseFromError(err)
+	require.True(t, ok, "could not assemble httpgrpc.HTTPResponse, is not status.Status")
+
+	require.True(t, json.Valid(resp.Body), "error message is not valid JSON: %s", resp.Body)
+}