proxy: HTTP status 499 for 'Context canceled' errors (#2297)

* Adding {when_unix_ms} requests placeholder (unix timestamp with a milliseconds precision)

* Add an 499 HTTP status code on user's cancel request as NGINX doing (instead of 502 Bad Gateway status with 'Context canceled' message)

* 499 HTTP status code was added as constant CustomStatusContextCancelled = 499

caddyhttp/proxy/proxy.go

@@ -262,6 +262,10 @@ func (p Proxy) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error) {
 			return http.StatusRequestEntityTooLarge, backendErr
 		}
 
+		if backendErr == context.Canceled {
+			return CustomStatusContextCancelled, backendErr
+		}
+
 		// failover; remember this failure for some time if
 		// request failure counting is enabled
 		timeout := host.FailTimeout
@@ -397,3 +401,5 @@ func mutateHeadersByRules(headers, rules http.Header, repl httpserver.Replacer)
 		}
 	}
 }
+
+const CustomStatusContextCancelled = 499

caddyhttp/proxy/proxy_test.go

@@ -1369,7 +1369,7 @@ func TestCancelRequest(t *testing.T) {
 
 	rec := httptest.NewRecorder()
 	status, err := p.ServeHTTP(rec, req)
-	expectedStatus, expectErr := http.StatusBadGateway, context.Canceled
+	expectedStatus, expectErr := CustomStatusContextCancelled, context.Canceled
 	if status != expectedStatus || err != expectErr {
 		t.Errorf("expect proxy handle return status[%d] with error[%v], but got status[%d] with error[%v]",
 			expectedStatus, expectErr, status, err)