Return 400 instead of 502 when VictoriaLogs rejects a query

A malformed LogsQL query is a user error, not a server failure.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

api.go

@@ -75,7 +75,7 @@ func (a *API) logs(w http.ResponseWriter, r *http.Request) {
 	start := time.Now()
 	rows, err := a.store.Query(r.Context(), q)
 	if err != nil {
-		writeErr(w, http.StatusBadGateway, err)
+		writeErr(w, queryStatus(err), err)
 		return
 	}
 	a.annotateHosts(rows)
@@ -105,7 +105,7 @@ func (a *API) histogram(w http.ResponseWriter, r *http.Request) {
 	q := filter + " | stats by (_time:" + step.expr + ") count() hits | sort by (_time)"
 	rows, err := a.store.Query(r.Context(), q)
 	if err != nil {
-		writeErr(w, http.StatusBadGateway, err)
+		writeErr(w, queryStatus(err), err)
 		return
 	}
 	type bucket struct {
@@ -168,7 +168,7 @@ func (a *API) facets(w http.ResponseWriter, r *http.Request) {
 		q := "_time:7d | stats by (" + field + ") count() hits | sort by (hits desc) | limit 300"
 		rows, err := a.store.Query(r.Context(), q)
 		if err != nil {
-			writeErr(w, http.StatusBadGateway, err)
+			writeErr(w, queryStatus(err), err)
 			return
 		}
 		vals := make([]string, 0, len(rows))

export.go

@@ -114,7 +114,7 @@ func (a *API) exportCSV(w http.ResponseWriter, r *http.Request) {
 	})
 	if err != nil {
 		if cw == nil {
-			writeErr(w, http.StatusBadGateway, err)
+			writeErr(w, queryStatus(err), err)
 			return
 		}
 		// Headers are already sent: the file is truncated, log why.

store.go

@@ -5,6 +5,7 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"log"
@@ -170,7 +171,7 @@ func (s *Store) QueryStream(ctx context.Context, query string, fn func(row map[s
 	defer resp.Body.Close()
 	if resp.StatusCode != http.StatusOK {
 		msg, _ := io.ReadAll(io.LimitReader(resp.Body, 2048))
-		return fmt.Errorf("%s", strings.TrimSpace(string(msg)))
+		return &queryError{status: resp.StatusCode, msg: strings.TrimSpace(string(msg))}
 	}
 
 	sc := bufio.NewScanner(resp.Body)
@@ -245,6 +246,24 @@ func (s *Store) deleteCall(ctx context.Context, path string) ([]byte, error) {
 	return body, nil
 }
 
+// queryError is an error answer of VictoriaLogs to a query.
+type queryError struct {
+	status int
+	msg    string
+}
+
+func (e *queryError) Error() string { return e.msg }
+
+// queryStatus is the HTTP status to return for a failed query: 400 when
+// VictoriaLogs rejected the query itself (bad LogsQL), 502 otherwise.
+func queryStatus(err error) int {
+	var qe *queryError
+	if errors.As(err, &qe) && qe.status >= 400 && qe.status < 500 {
+		return http.StatusBadRequest
+	}
+	return http.StatusBadGateway
+}
+
 func (s *Store) Stats() map[string]any {
 	return map[string]any{
 		"received":  s.received.Load(),