chore: avoid superfluous WriteHeader call errors (#1275)

rest/internal/cors/handlers_test.go

@@ -1,6 +1,8 @@
 package cors
 
 import (
+	"bufio"
+	"net"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -129,3 +131,48 @@ func TestCorsHandlerWithOrigins(t *testing.T) {
 		}
 	}
 }
+
+func TestGuardedResponseWriter_Flush(t *testing.T) {
+	req := httptest.NewRequest(http.MethodGet, "http://localhost", nil)
+	handler := NotAllowedHandler(func(w http.ResponseWriter) {
+		w.Header().Set("X-Test", "test")
+		w.WriteHeader(http.StatusServiceUnavailable)
+		_, err := w.Write([]byte("content"))
+		assert.Nil(t, err)
+
+		flusher, ok := w.(http.Flusher)
+		assert.True(t, ok)
+		flusher.Flush()
+	}, "foo.com")
+
+	resp := httptest.NewRecorder()
+	handler.ServeHTTP(resp, req)
+	assert.Equal(t, http.StatusServiceUnavailable, resp.Code)
+	assert.Equal(t, "test", resp.Header().Get("X-Test"))
+	assert.Equal(t, "content", resp.Body.String())
+}
+
+func TestGuardedResponseWriter_Hijack(t *testing.T) {
+	resp := httptest.NewRecorder()
+	writer := &guardedResponseWriter{
+		w: resp,
+	}
+	assert.NotPanics(t, func() {
+		writer.Hijack()
+	})
+
+	writer = &guardedResponseWriter{
+		w: mockedHijackable{resp},
+	}
+	assert.NotPanics(t, func() {
+		writer.Hijack()
+	})
+}
+
+type mockedHijackable struct {
+	*httptest.ResponseRecorder
+}
+
+func (m mockedHijackable) Hijack() (net.Conn, *bufio.ReadWriter, error) {
+	return nil, nil, nil
+}