Log errors when write fails, not 200

See discussion here: cortexproject/cortex#2483 (comment)

Signed-off-by: Goutham Veeramachaneni <gouthamve@gmail.com>

middleware/logging.go

@@ -39,7 +39,12 @@ func (l Log) Wrap(next http.Handler) http.Handler {
 		var buf bytes.Buffer
 		wrapped := newBadResponseLoggingWriter(w, &buf)
 		next.ServeHTTP(wrapped, r)
-		statusCode := wrapped.statusCode
+
+		statusCode, err := wrapped.statusCode, wrapped.writeError
+		if err != nil {
+			l.logWithRequest(r).Warnf("%s %s %s, error: %s ws: %v; %s", r.Method, uri, time.Since(begin), err, IsWSHandshakeRequest(r), headers)
+			return
+		}
 		if 100 <= statusCode && statusCode < 500 || statusCode == http.StatusBadGateway || statusCode == http.StatusServiceUnavailable {
 			l.logWithRequest(r).Debugf("%s %s (%d) %s", r.Method, uri, statusCode, time.Since(begin))
 			if l.LogRequestHeaders && headers != nil {

middleware/response.go

@@ -20,6 +20,7 @@ type badResponseLoggingWriter struct {
 	logBody       bool
 	bodyBytesLeft int
 	statusCode    int
+	writeError    error // The error returned when downstream Write() fails.
 }
 
 // newBadResponseLoggingWriter makes a new badResponseLoggingWriter.
@@ -50,6 +51,9 @@ func (b *badResponseLoggingWriter) Write(data []byte) (int, error) {
 	if b.logBody {
 		b.captureResponseBody(data)
 	}
+	if err != nil {
+		b.writeError = err
+	}
 	return n, err
 }