test: verify request overload recovery

transport/native/server_test.go

@@ -722,6 +722,83 @@ func TestServerConcurrentRequestLimit(t *testing.T) {
 	}
 }
 
+func TestServerConcurrentRequestLimitRecovers(t *testing.T) {
+	started := make(chan struct{})
+	release := make(chan struct{})
+	var first sync.Once
+	application := omni.NewWithRouter(netrouter.New())
+	application.GET("/work", func(ctx *omni.Context) error {
+		first.Do(func() {
+			close(started)
+			<-release
+		})
+		return ctx.OK("done")
+	})
+	server := native.New(application)
+	server.MaxConcurrentRequests = 1
+	listener, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	done := make(chan error, 1)
+	go func() { done <- server.Serve(context.Background(), listener) }()
+	waitForServer(t, listener.Addr().String())
+	defer stopServer(t, server, listener, done)
+
+	firstConn, err := net.Dial("tcp", listener.Addr().String())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer firstConn.Close()
+	_, _ = io.WriteString(firstConn, "GET /work HTTP/1.1\r\nHost: test\r\nConnection: close\r\n\r\n")
+	select {
+	case <-started:
+	case <-time.After(2 * time.Second):
+		t.Fatal("first handler did not start")
+	}
+
+	secondConn, err := net.Dial("tcp", listener.Addr().String())
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, _ = io.WriteString(secondConn, "GET /work HTTP/1.1\r\nHost: test\r\nConnection: close\r\n\r\n")
+	response, err := http.ReadResponse(bufio.NewReader(secondConn), &http.Request{Method: http.MethodGet})
+	if err != nil {
+		secondConn.Close()
+		t.Fatal(err)
+	}
+	response.Body.Close()
+	secondConn.Close()
+	if response.StatusCode != http.StatusServiceUnavailable {
+		t.Fatalf("expected 503 at concurrency limit, got %d", response.StatusCode)
+	}
+
+	close(release)
+	response, err = http.ReadResponse(bufio.NewReader(firstConn), &http.Request{Method: http.MethodGet})
+	if err != nil {
+		t.Fatal(err)
+	}
+	response.Body.Close()
+	if response.StatusCode != http.StatusOK {
+		t.Fatalf("expected held request to complete, got %d", response.StatusCode)
+	}
+
+	thirdConn, err := net.Dial("tcp", listener.Addr().String())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer thirdConn.Close()
+	_, _ = io.WriteString(thirdConn, "GET /work HTTP/1.1\r\nHost: test\r\nConnection: close\r\n\r\n")
+	response, err = http.ReadResponse(bufio.NewReader(thirdConn), &http.Request{Method: http.MethodGet})
+	if err != nil {
+		t.Fatal(err)
+	}
+	response.Body.Close()
+	if response.StatusCode != http.StatusOK {
+		t.Fatalf("request slot was not reusable after overload, got %d", response.StatusCode)
+	}
+}
+
 func TestServerConnectionLimitReleasesAfterRequest(t *testing.T) {
 	started := make(chan struct{})
 	release := make(chan struct{})