API: change HTTP status code tracked in metrics form 503/422 to 499 if a request is canceled

Signed-off-by: Marco Pracucci <marco@pracucci.com>

web/api/v1/api.go

@@ -63,6 +63,10 @@ type status string
 const (
 	statusSuccess status = "success"
 	statusError   status = "error"
+
+	// Non-standard status code (originally introduced by nginx) for the case when a client closes
+	// the connection while the server is still processing the request.
+	statusClientClosedConnection = 499
 )
 
 type errorType string
@@ -593,6 +597,10 @@ func returnAPIError(err error) *apiError {
 		return &apiError{errorInternal, err}
 	}
 
+	if errors.Is(err, context.Canceled) {
+		return &apiError{errorCanceled, err}
+	}
+
 	return &apiError{errorExec, err}
 }
 
@@ -1599,7 +1607,9 @@ func (api *API) respondError(w http.ResponseWriter, apiErr *apiError, data inter
 		code = http.StatusBadRequest
 	case errorExec:
 		code = http.StatusUnprocessableEntity
-	case errorCanceled, errorTimeout:
+	case errorCanceled:
+		code = statusClientClosedConnection
+	case errorTimeout:
 		code = http.StatusServiceUnavailable
 	case errorInternal:
 		code = http.StatusInternalServerError

web/api/v1/errors_test.go

@@ -58,7 +58,7 @@ func TestApiStatusCodes(t *testing.T) {
 		"promql.ErrQueryCanceled": {
 			err:            promql.ErrQueryCanceled("some error"),
 			expectedString: "query was canceled",
-			expectedCode:   http.StatusServiceUnavailable,
+			expectedCode:   statusClientClosedConnection,
 		},
 
 		"promql.ErrQueryTimeout": {
@@ -76,7 +76,7 @@ func TestApiStatusCodes(t *testing.T) {
 		"context.Canceled": {
 			err:            context.Canceled,
 			expectedString: "context canceled",
-			expectedCode:   http.StatusUnprocessableEntity,
+			expectedCode:   statusClientClosedConnection,
 		},
 	} {
 		for k, q := range map[string]storage.SampleAndChunkQueryable{