refactor: handle ErrAbortHandler in gin recovery middleware instead of per-handler recover (#134)

Move SSE panic handling from a per-handler recover() in proxy.go and
ErrorHandler in transparent.go to a centralized CustomRecoveryWithZap
middleware. This provides consistent ErrAbortHandler handling across the
entire app without masking legitimate copy errors in the reverse proxy.

Relates to #128

Author: hrntknr

pkg/backend/transparent.go

@@ -125,27 +125,7 @@ func (p *TransparentBackend) Run(ctx context.Context) (http.Handler, error) {
 			base:       http.DefaultTransport,
 			targetHost: p.url.Host,
 		},
-		// FlushInterval -1 enables immediate flushing for SSE streams.
-		// Without this the proxy buffers the response, breaking
-		// Server-Sent Events used by MCP Streamable HTTP.
 		FlushInterval: -1,
-		// ErrorHandler prevents the default ReverseProxy behavior of
-		// panicking when the backend closes the connection (e.g. when
-		// an SSE stream ends or the client disconnects). Instead we
-		// log the error and return a 502 to the client.
-		ErrorHandler: func(w http.ResponseWriter, r *http.Request, err error) {
-			p.logger.Warn("reverse proxy error",
-				zap.String("method", r.Method),
-				zap.String("path", r.URL.Path),
-				zap.Error(err),
-			)
-			// Only write an error response if headers haven't been sent yet
-			// (i.e. the stream hasn't started). For in-flight SSE streams
-			// the ResponseWriter is already flushed, so just return.
-			if !isHeadersSent(w) {
-				http.Error(w, "Bad Gateway", http.StatusBadGateway)
-			}
-		},
 		Rewrite: func(pr *httputil.ProxyRequest) {
 			pr.SetURL(p.url)
 			if p.isTrusted(pr.In.RemoteAddr) {
@@ -168,17 +148,6 @@ func (p *TransparentBackend) Run(ctx context.Context) (http.Handler, error) {
 	return &rp, nil
 }
 
-// isHeadersSent checks whether the ResponseWriter has already started sending
-// the response (status + headers). Once flushed we can no longer write an
-// error status, so callers should just close the connection instead.
-func isHeadersSent(w http.ResponseWriter) bool {
-	// A non-zero status in the header map means WriteHeader was called.
-	// The standard http.response tracks this internally; the only reliable
-	// external signal is whether Content-Type (or any header set by the
-	// backend) is already present in the response.
-	return w.Header().Get("Content-Type") != ""
-}
-
 func (p *TransparentBackend) isTrusted(hostport string) bool {
 	if host, _, err := net.SplitHostPort(hostport); err == nil {
 		hostport = host

pkg/mcp-proxy/main.go

@@ -305,7 +305,13 @@ func Run(
 	})
 
 	router.Use(ginzap.Ginzap(logger, time.RFC3339, true))
-	router.Use(ginzap.RecoveryWithZap(logger, true))
+	router.Use(ginzap.CustomRecoveryWithZap(logger, true, func(c *gin.Context, err any) {
+		if err == http.ErrAbortHandler {
+			c.Abort()
+			return
+		}
+		c.AbortWithStatus(http.StatusInternalServerError)
+	}))
 	store := cookie.NewStore(secret)
 	store.Options(sessions.Options{
 		Path:     "/",

pkg/proxy/proxy.go

@@ -87,22 +87,6 @@ func (p *ProxyRouter) handleProxy(c *gin.Context) {
 		}
 	}
 
-	// Recover from http.ErrAbortHandler panics that occur when SSE
-	// streams end (backend closes connection, client disconnects).
-	// The default ReverseProxy panics with ErrAbortHandler on flush
-	// errors — this is expected for SSE and should not crash the proxy.
-	defer func() {
-		if r := recover(); r != nil {
-			// http.ErrAbortHandler is the sentinel panic used by
-			// net/http to silently abort a handler. Let it go.
-			if r == http.ErrAbortHandler {
-				return
-			}
-			// Re-panic for anything unexpected
-			panic(r)
-		}
-	}()
-
 	p.proxy.ServeHTTP(c.Writer, c.Request)
 }