martian: add http 304 test

Test ensures that after responding with status code 304 the connection is left intact.

internal/martian/proxy_test.go

@@ -485,6 +485,77 @@ func TestIntegrationHTTP101SwitchingProtocols(t *testing.T) {
 	}
 }
 
+func TestIntegrationHTTP304NotModified(t *testing.T) {
+	t.Parallel()
+
+	tm := martiantest.NewModifier()
+	h := testHelper{
+		Proxy: func(p *Proxy) {
+			p.ReadTimeout = 200 * time.Millisecond
+			p.WriteTimeout = 200 * time.Millisecond
+			p.RequestModifier = tm
+			p.ResponseModifier = tm
+			p.AllowHTTP = true
+		},
+	}
+
+	sl, err := net.Listen("tcp", "localhost:0")
+	if err != nil {
+		t.Fatalf("net.Listen(): got %v, want no error", err)
+	}
+
+	go func() {
+		conn, err := sl.Accept()
+		if err != nil {
+			log.Errorf(context.TODO(), "proxy_test: failed to accept connection: %v", err)
+			return
+		}
+		defer conn.Close()
+
+		log.Infof(context.TODO(), "proxy_test: accepted connection: %s", conn.RemoteAddr())
+
+		req, err := http.ReadRequest(bufio.NewReader(conn))
+		if err != nil {
+			log.Errorf(context.TODO(), "proxy_test: failed to read request: %v", err)
+			return
+		}
+
+		res := proxyutil.NewResponse(304, http.NoBody, req)
+		res.Header.Set("Content-Length", "13")
+		res.Write(conn)
+		log.Infof(context.TODO(), "proxy_test: sent 304 response")
+
+		log.Infof(context.TODO(), "proxy_test: closed connection")
+	}()
+
+	conn, cancel := h.proxyConn(t)
+	defer cancel()
+	defer conn.Close()
+
+	host := sl.Addr().String()
+
+	req, err := http.NewRequest(http.MethodGet, "http://"+host, http.NoBody)
+	if err != nil {
+		t.Fatalf("http.NewRequest(): got %v, want no error", err)
+	}
+	if err := req.WriteProxy(conn); err != nil {
+		t.Fatalf("req.WriteProxy(): got %v, want no error", err)
+	}
+
+	res, err := http.ReadResponse(bufio.NewReader(conn), nil)
+	if err != nil {
+		t.Fatalf("http.ReadResponse(): got %v, want no error", err)
+	}
+	defer res.Body.Close()
+
+	if got, want := res.StatusCode, 304; got != want {
+		t.Fatalf("res.StatusCode: got %d, want %d", got, want)
+	}
+	if _, err := conn.Read(make([]byte, 0)); err != nil {
+		t.Fatalf("conn.Read(): got %v, want no error", err)
+	}
+}
+
 func TestIntegrationUnexpectedUpstreamFailure(t *testing.T) {
 	t.Parallel()