mcp: use http.ResponseController to ensure writes are flushed

[toofishes]

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.

[maciej-kisiel]

This assumes that the wrapper implements Unwrap, right? Is that a common practice?

[toofishes]

This assumes that the wrapper implements Unwrap, right? Is that a common practice?

The wrapper can implement Unwrap, implement http.Flusher directly, or a FlushError() method that returns an error, and it will continue to work. https://go.dev/src/net/http/responsecontroller.go shows the relatively straightforward switch on interface type.

It seems like it is still not as well known as it should be, but it isn't uncommon:

• https://github.com/aarondl/authboss/blob/e94e98622f4fe26395e13bf4b7d18931dd0c71cc/client_state.go#L247
• https://github.com/nijaru/aku/blob/5bd05728314fa6a0b46cb596713cef9c5dd1d700/aku.go#L224
• https://github.com/klauspost/compress/blob/15967def6890f5494829394274686436976c4936/gzhttp/compress.go#L222
• https://github.com/urfave/negroni/blob/1af389b71c452984b3fb1820121bac8849442bfe/response_writer.go#L88
• https://github.com/SigNoz/signoz/blob/2163e1ce41b23476526cb32a2ef3ce0317d6f2c7/pkg/http/middleware/response.go#L56
• https://github.com/elastic/apm-agent-go/blob/53b32c7bfa8cbb2c91eb36b9317eb41de5b8c8e1/module/apmhttp/handler.go#L246

Note that several of these also implement Flush, either because they need to do additional work, or predate the ResponseController Unwrap interface existing.

An additional change I plan to propose (in a future PR) is also calling SetWriteDeadline(time.Time{}) on the response once a connection has entered the event stream phase, which also benefits from being exposed on the response controller. This would prevent a http.Server WriteTimeout from closing SSE connections. I currently need to install this middleware on the MCP route otherwise:

func DisableWriteDeadlineMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		rc := http.NewResponseController(w)
		if err := rc.SetWriteDeadline(time.Time{}); err != nil {
			logging.ContextLogger(r.Context()).Warn("unable to disable write deadline", zap.Error(err))
		}
		next.ServeHTTP(w, r)
	})
}

[maciej-kisiel]

Please merge main.

[toofishes]

Added the suggested comments and rebased on main.