Automated cherry pick of #107452: Fix header mutation race in timeout filter

staging/src/k8s.io/apiserver/pkg/server/filters/timeout.go

@@ -149,7 +149,7 @@ type timeoutWriter interface {
 }
 
 func newTimeoutWriter(w http.ResponseWriter) (timeoutWriter, http.ResponseWriter) {
-	base := &baseTimeoutWriter{w: w}
+	base := &baseTimeoutWriter{w: w, handlerHeaders: w.Header().Clone()}
 	wrapped := responsewriter.WrapForHTTP1Or2(base)
 
 	return base, wrapped
@@ -161,6 +161,9 @@ var _ responsewriter.UserProvidedDecorator = &baseTimeoutWriter{}
 type baseTimeoutWriter struct {
 	w http.ResponseWriter
 
+	// headers written by the normal handler
+	handlerHeaders http.Header
+
 	mu sync.Mutex
 	// if the timeout handler has timeout
 	timedOut bool
@@ -182,7 +185,7 @@ func (tw *baseTimeoutWriter) Header() http.Header {
 		return http.Header{}
 	}
 
-	return tw.w.Header()
+	return tw.handlerHeaders
 }
 
 func (tw *baseTimeoutWriter) Write(p []byte) (int, error) {
@@ -196,7 +199,10 @@ func (tw *baseTimeoutWriter) Write(p []byte) (int, error) {
 		return 0, http.ErrHijacked
 	}
 
-	tw.wroteHeader = true
+	if !tw.wroteHeader {
+		copyHeaders(tw.w.Header(), tw.handlerHeaders)
+		tw.wroteHeader = true
+	}
 	return tw.w.Write(p)
 }
 
@@ -221,10 +227,17 @@ func (tw *baseTimeoutWriter) WriteHeader(code int) {
 		return
 	}
 
+	copyHeaders(tw.w.Header(), tw.handlerHeaders)
 	tw.wroteHeader = true
 	tw.w.WriteHeader(code)
 }
 
+func copyHeaders(dst, src http.Header) {
+	for k, v := range src {
+		dst[k] = v
+	}
+}
+
 func (tw *baseTimeoutWriter) timeout(err *apierrors.StatusError) {
 	tw.mu.Lock()
 	defer tw.mu.Unlock()

staging/src/k8s.io/apiserver/pkg/server/filters/timeout_test.go

@@ -203,6 +203,52 @@ func TestTimeout(t *testing.T) {
 	}
 }
 
+func TestTimeoutHeaders(t *testing.T) {
+	origReallyCrash := runtime.ReallyCrash
+	runtime.ReallyCrash = false
+	defer func() {
+		runtime.ReallyCrash = origReallyCrash
+	}()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	withDeadline := func(handler http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+			handler.ServeHTTP(w, req.WithContext(ctx))
+		})
+	}
+
+	ts := httptest.NewServer(
+		withDeadline(
+			WithTimeout(
+				http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+					h := w.Header()
+					// trigger the timeout
+					cancel()
+					// mutate response Headers
+					for j := 0; j < 1000; j++ {
+						h.Set("Test", "post")
+					}
+				}),
+				func(req *http.Request) (*http.Request, bool, func(), *apierrors.StatusError) {
+					return req, false, func() {}, apierrors.NewServerTimeout(schema.GroupResource{Group: "foo", Resource: "bar"}, "get", 0)
+				},
+			),
+		),
+	)
+	defer ts.Close()
+
+	res, err := http.Get(ts.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if res.StatusCode != http.StatusGatewayTimeout {
+		t.Errorf("got res.StatusCode %d; expected %d", res.StatusCode, http.StatusServiceUnavailable)
+	}
+	res.Body.Close()
+}
+
 func captureStdErr() (func() string, func(), error) {
 	var buf bytes.Buffer
 	reader, writer, err := os.Pipe()