fix(api): Don't send multiple responses for one request (#21651)

fix(api): Don't send responses twice.

In some cases multiple responses are being sent for one request, causing
the API server to incorrectly drop connections.

This change introduces a ResponseWriter which tracks whether a
response has already been sent. This is used to prevent a response being
sent if something already has (e.g. by a preconditions check function).

Fixes #21633.

Co-authored-by: Menno Finlay-Smits <hello@menno.io>

cmd/api-router.go

@@ -218,6 +218,8 @@ func s3APIMiddleware(f http.HandlerFunc, flags ...s3HFlag) http.HandlerFunc {
 	handlerName := getHandlerName(f, "objectAPIHandlers")
 
 	var handler http.HandlerFunc = func(w http.ResponseWriter, r *http.Request) {
+		w = &trackingResponseWriter{ResponseWriter: w}
+
 		// Wrap the actual handler with the appropriate tracing middleware.
 		var tracedHandler http.HandlerFunc
 		if handlerFlags.has(traceHdrsS3HFlag) {