If frontend copy body return error, log it (#4533)

* if frontend copy body return error, log it

Signed-off-by: Mengmeng Yang <[email protected]>

Author: mengmengy

pkg/frontend/transport/handler.go

@@ -3,6 +3,7 @@ package transport
 import (
 	"bytes"
 	"context"
+	"errors"
 	"flag"
 	"fmt"
 	"io"
@@ -11,6 +12,7 @@ import (
 	"net/url"
 	"strconv"
 	"strings"
+	"syscall"
 	"time"
 
 	"github.com/go-kit/log"
@@ -143,8 +145,11 @@ func (f *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	}
 
 	w.WriteHeader(resp.StatusCode)
-	// we don't check for copy error as there is no much we can do at this point
-	_, _ = io.Copy(w, resp.Body)
+	// log copy response body error so that we will know even though success response code returned
+	bytesCopied, err := io.Copy(w, resp.Body)
+	if err != nil && !errors.Is(err, syscall.EPIPE) {
+		level.Error(util_log.WithContext(r.Context(), f.log)).Log("msg", "write response body error", "bytesCopied", bytesCopied, "err", err)
+	}
 
 	// Check whether we should parse the query string.
 	shouldReportSlowQuery := f.cfg.LogQueriesLongerThan != 0 && queryResponseTime > f.cfg.LogQueriesLongerThan