http3: automatically add content-length for small responses (#3989)

* response writer: add content-length automatically when response is small enough and doesn't call Flush

* fix comment

* add integration test

* Update http3/response_writer.go

* Update integrationtests/self/http_test.go

---------

Co-authored-by: Marten Seemann <martenseemann@gmail.com>

http3/server_test.go

@@ -180,6 +180,47 @@ var _ = Describe("Server", func() {
 			Expect(hfs).To(HaveKeyWithValue(":status", []string{"200"}))
 		})
 
+		It("sets Content-Length when the handler doesn't flush to the client", func() {
+			s.Handler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				w.Write([]byte("foobar"))
+			})
+
+			responseBuf := &bytes.Buffer{}
+			setRequest(encodeRequest(exampleGetRequest))
+			str.EXPECT().Context().Return(reqContext)
+			str.EXPECT().Write(gomock.Any()).DoAndReturn(responseBuf.Write).AnyTimes()
+			str.EXPECT().CancelRead(gomock.Any())
+
+			serr := s.handleRequest(conn, str, qpackDecoder, nil)
+			Expect(serr.err).ToNot(HaveOccurred())
+			hfs := decodeHeader(responseBuf)
+			Expect(hfs).To(HaveKeyWithValue(":status", []string{"200"}))
+			Expect(hfs).To(HaveKeyWithValue("content-length", []string{"6"}))
+			// status, content-length, date, content-type
+			Expect(hfs).To(HaveLen(4))
+		})
+
+		It("not sets Content-Length when the handler flushes to the client", func() {
+			s.Handler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				w.Write([]byte("foobar"))
+				// force flush
+				w.(http.Flusher).Flush()
+			})
+
+			responseBuf := &bytes.Buffer{}
+			setRequest(encodeRequest(exampleGetRequest))
+			str.EXPECT().Context().Return(reqContext)
+			str.EXPECT().Write(gomock.Any()).DoAndReturn(responseBuf.Write).AnyTimes()
+			str.EXPECT().CancelRead(gomock.Any())
+
+			serr := s.handleRequest(conn, str, qpackDecoder, nil)
+			Expect(serr.err).ToNot(HaveOccurred())
+			hfs := decodeHeader(responseBuf)
+			Expect(hfs).To(HaveKeyWithValue(":status", []string{"200"}))
+			// status, date, content-type
+			Expect(hfs).To(HaveLen(3))
+		})
+
 		It("handles a aborting handler", func() {
 			s.Handler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 				panic(http.ErrAbortHandler)