Avoid CompleteMultipart freeze with unexpected network issue (#15102)

If sending a white space during a long S3 handler call fails,
the whitespace goroutine forgets to return a result to the caller.
Therefore, the complete multipart handler will be blocked.

Remember to send the header written result to the caller 
or/and close the channel.

cmd/object-handlers.go

@@ -3101,10 +3101,12 @@ func (w *whiteSpaceWriter) WriteHeader(statusCode int) {
 // we do background append as and when the parts arrive and completeMultiPartUpload
 // is quick. Only in a rare case where parts would be out of order will
 // FS:completeMultiPartUpload() take a longer time.
-func sendWhiteSpace(w http.ResponseWriter) <-chan bool {
+func sendWhiteSpace(ctx context.Context, w http.ResponseWriter) <-chan bool {
 	doneCh := make(chan bool)
 	go func() {
+		defer close(doneCh)
 		ticker := time.NewTicker(time.Second * 10)
+		defer ticker.Stop()
 		headerWritten := false
 		for {
 			select {
@@ -3124,7 +3126,8 @@ func sendWhiteSpace(w http.ResponseWriter) <-chan bool {
 				}
 				w.(http.Flusher).Flush()
 			case doneCh <- headerWritten:
-				ticker.Stop()
+				return
+			case <-ctx.Done():
 				return
 			}
 		}
@@ -3298,7 +3301,7 @@ func (api objectAPIHandlers) CompleteMultipartUploadHandler(w http.ResponseWrite
 	}
 
 	w = &whiteSpaceWriter{ResponseWriter: w, Flusher: w.(http.Flusher)}
-	completeDoneCh := sendWhiteSpace(w)
+	completeDoneCh := sendWhiteSpace(ctx, w)
 	objInfo, err := completeMultiPartUpload(ctx, bucket, object, uploadID, complMultipartUpload.Parts, opts)
 	// Stop writing white spaces to the client. Note that close(doneCh) style is not used as it
 	// can cause white space to be written after we send XML response in a race condition.