Avoid panics for unsupported optional response interfaces

Wrapped response writers such as http.TimeoutHandler may not implement
Hijacker or CloseNotifier. Match the safe optional-interface checks used by
Flush and Pusher before calling through.

Tested: docker run --rm -v "$PWD":/src -w /src golang:1.25 go test -run 'TestResponseWriterHijack|TestResponseWriterHijackAfterWrite|TestResponseWriterHijackAfterWriteHeaderNow' ./...
Tested: docker run --rm -v "$PWD":/src -w /src golang:1.25 go test ./...

response_writer.go

@@ -114,15 +114,22 @@ func (w *responseWriter) Hijack() (net.Conn, *bufio.ReadWriter, error) {
 	if w.size > 0 {
 		return nil, nil, errHijackAlreadyWritten
 	}
+	hijacker, ok := w.ResponseWriter.(http.Hijacker)
+	if !ok {
+		return nil, nil, http.ErrNotSupported
+	}
 	if w.size < 0 {
 		w.size = 0
 	}
-	return w.ResponseWriter.(http.Hijacker).Hijack()
+	return hijacker.Hijack()
 }
 
 // CloseNotify implements the http.CloseNotifier interface.
 func (w *responseWriter) CloseNotify() <-chan bool {
-	return w.ResponseWriter.(http.CloseNotifier).CloseNotify()
+	if notifier, ok := w.ResponseWriter.(http.CloseNotifier); ok {
+		return notifier.CloseNotify()
+	}
+	return nil
 }
 
 // Flush implements the http.Flusher interface.

response_writer_test.go

@@ -10,6 +10,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -113,19 +114,36 @@ func TestResponseWriterHijack(t *testing.T) {
 	writer.reset(testWriter)
 	w := ResponseWriter(writer)
 
-	assert.Panics(t, func() {
+	_, _, err := w.Hijack()
-		_, _, err := w.Hijack()
+	require.ErrorIs(t, err, http.ErrNotSupported)
-		require.NoError(t, err)
+	assert.False(t, w.Written())
-	})
-	assert.True(t, w.Written())
 
-	assert.Panics(t, func() {
+	assert.Nil(t, w.CloseNotify())
-		w.CloseNotify()
-	})
 
 	w.Flush()
 }
 
+func TestResponseWriterHijackWithTimeoutHandler(t *testing.T) {
+	var hijackErr error
+	var closeNotify <-chan bool
+	var written bool
+
+	handler := http.TimeoutHandler(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		writer := &responseWriter{}
+		writer.reset(w)
+
+		_, _, hijackErr = writer.Hijack()
+		closeNotify = writer.CloseNotify()
+		written = writer.Written()
+	}), time.Second, "")
+
+	handler.ServeHTTP(httptest.NewRecorder(), httptest.NewRequest(http.MethodGet, "/", nil))
+
+	require.ErrorIs(t, hijackErr, http.ErrNotSupported)
+	assert.Nil(t, closeNotify)
+	assert.False(t, written)
+}
+
 type mockHijacker struct {
 	*httptest.ResponseRecorder
 	hijacked bool