mcp: use http.ResponseController to ensure writes are flushed (#870)

If the ResponseWriter is a wrapped one (which is quite common when using
middleware to log things like response sizes), the direct cast to
`http.Flusher` doesn't find the flush capability. Instead, use
`http.ResponseController` as introduced in go1.20 which handles
unwrapping to find the base response writer.

mcp/event.go

@@ -41,7 +41,7 @@ func (e Event) Empty() bool {
 }
 
 // writeEvent writes the event to w, and flushes.
-func writeEvent(w io.Writer, evt Event) (int, error) {
+func writeEvent(w http.ResponseWriter, evt Event) (int, error) {
 	var b bytes.Buffer
 	if evt.Name != "" {
 		fmt.Fprintf(&b, "event: %s\n", evt.Name)
@@ -54,9 +54,9 @@ func writeEvent(w io.Writer, evt Event) (int, error) {
 	}
 	fmt.Fprintf(&b, "data: %s\n\n", string(evt.Data))
 	n, err := w.Write(b.Bytes())
-	if f, ok := w.(http.Flusher); ok {
-		f.Flush()
-	}
+	rc := http.NewResponseController(w)
+	// Ignore returned error as flushing is best-effort.
+	_ = rc.Flush()
 	return n, err
 }
 

mcp/streamable.go

@@ -1053,9 +1053,9 @@ func (c *streamableServerConn) acquireStream(ctx context.Context, w http.Respons
 		// Issue #410: the standalone SSE stream is likely not to receive messages
 		// for a long time. Ensure that headers are flushed.
 		w.WriteHeader(http.StatusOK)
-		if f, ok := w.(http.Flusher); ok {
-			f.Flush()
-		}
+		rc := http.NewResponseController(w)
+		// Ignore returned error as flushing is best-effort.
+		_ = rc.Flush()
 	}
 
 	for _, data := range toReplay {

mcp/streamable_client_test.go

@@ -119,14 +119,15 @@ func (s *fakeStreamableServer) ServeHTTP(w http.ResponseWriter, req *http.Reques
 	for k, v := range resp.header {
 		w.Header().Set(k, v)
 	}
+	rc := http.NewResponseController(w)
 	w.WriteHeader(status)
-	w.(http.Flusher).Flush() // flush response headers
+	rc.Flush() // flush response headers
 
 	if v := req.Header.Get(protocolVersionHeader); v != resp.wantProtocolVersion && resp.wantProtocolVersion != "" {
 		s.t.Errorf("%v: bad protocol version header: got %q, want %q", key, v, resp.wantProtocolVersion)
 	}
 	w.Write([]byte(body))
-	w.(http.Flusher).Flush() // flush response
+	rc.Flush() // flush response
 
 	if resp.done != nil {
 		<-resp.done