fix: reject logs after response flush failure

tests/proxy/robustness_test.go

@@ -55,6 +55,18 @@ func (w *failingResponseWriter) Write(p []byte) (int, error) {
 	return 0, errors.New("write failed")
 }
 
+type failingFlushResponseWriter struct{ header http.Header }
+
+func (w *failingFlushResponseWriter) Header() http.Header { return w.header }
+func (*failingFlushResponseWriter) WriteHeader(int)       {}
+func (*failingFlushResponseWriter) Write(p []byte) (int, error) {
+	return len(p), nil
+}
+func (*failingFlushResponseWriter) Flush() {}
+func (*failingFlushResponseWriter) FlushError() error {
+	return errors.New("flush failed")
+}
+
 func newTestHandler(t *testing.T, upstream *url.URL, sub *captureSubmitter, opts proxy.Options) *proxy.Handler {
 	t.Helper()
 	filter, err := config.NewFilter(config.FilterDisabled, nil)
@@ -168,6 +180,22 @@ func TestResponseWriteFailurePreventsCommit(t *testing.T) {
 	}
 }
 
+func TestResponseFlushFailurePreventsCommit(t *testing.T) {
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, _ = io.WriteString(w, "response")
+	}))
+	defer upstream.Close()
+	u, _ := url.Parse(upstream.URL)
+
+	sub := &captureSubmitter{}
+	h := newTestHandler(t, u, sub, proxy.Options{})
+	w := &failingFlushResponseWriter{header: make(http.Header)}
+	h.ServeHTTP(w, httptest.NewRequest(http.MethodGet, "http://proxy.test/v1/test", nil))
+	if sub.Len() != 0 {
+		t.Fatal("failed response flush was committed")
+	}
+}
+
 func TestTrustedProxyClientIP(t *testing.T) {
 	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		_, _ = io.WriteString(w, "ok")