[sim] Add WebSocket extension (client/server) (#2589)

This allows access to HAL-level simulation data via a WebSocket connection.

The server additionally serves local files.

The following environment variables can be used for configuration:
HALSIMWS_USERROOT (server) - local directory to use for file serving for /user/ URIs, defaults to ./sim/user
HALSIMWS_SYSROOT (server) - local directory to use for file serving for all other URIs, defaults to ./sim
HALSIMWS_URI (client or server) - WebSocket URI, defaults to /wpilibws
HALSIMWS_PORT (client or server) - port number, defaults to 8080
HALSIMWS_HOST (client) - host to connect to, defaults to localhost

Co-authored-by: Zhiquan Yeo <zyeo8@bloomberg.net>
Co-authored-by: Peter Johnson <johnson.peter@gmail.com>
Co-authored-by: jpokornyiii <jpokornyiii@gmail.com>

simulation/halsim_ws_server/src/test/native/include/WebServerClientTest.h

@@ -0,0 +1,72 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2020 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include <memory>
+#include <string>
+
+#include <wpi/WebSocket.h>
+#include <wpi/json.h>
+#include <wpi/uv/AsyncFunction.h>
+#include <wpi/uv/Buffer.h>
+#include <wpi/uv/Loop.h>
+#include <wpi/uv/Stream.h>
+#include <wpi/uv/Tcp.h>
+#include <wpi/uv/Timer.h>
+
+namespace uv = wpi::uv;
+
+namespace wpilibws {
+
+class WebServerClientTest {
+ public:
+  using BufferPool = wpi::uv::SimpleBufferPool<4>;
+  using LoopFunc = std::function<void(void)>;
+  using UvExecFunc = wpi::uv::AsyncFunction<void(LoopFunc)>;
+
+  static std::shared_ptr<WebServerClientTest> GetInstance() {
+    return g_instance;
+  }
+  static void SetInstance(std::shared_ptr<WebServerClientTest> inst) {
+    g_instance = inst;
+  }
+
+  WebServerClientTest() {}
+  WebServerClientTest(const WebServerClientTest&) = delete;
+  WebServerClientTest& operator=(const WebServerClientTest&) = delete;
+
+  bool Initialize(std::shared_ptr<uv::Loop>& loop);
+  void AttemptConnect();
+  void Exit();
+
+  void SendMessage(const wpi::json& msg);
+  const wpi::json& GetLastMessage();
+  bool IsConnectedWS() { return m_ws_connected; }
+  void SetTerminateFlag(bool flag) { m_terminateFlag = flag; }
+
+ private:
+  void InitializeWebSocket(const std::string& host, int port,
+                           const std::string& uri);
+
+  bool m_terminateFlag = false;
+  static std::shared_ptr<WebServerClientTest> g_instance;
+  bool m_tcp_connected = false;
+  std::shared_ptr<wpi::uv::Timer> m_connect_timer;
+  int m_connect_attempts = 0;
+  std::shared_ptr<wpi::uv::Loop> m_loop;
+  std::shared_ptr<wpi::uv::Tcp> m_tcp_client;
+  wpi::json m_json;
+
+  bool m_ws_connected = false;
+  std::shared_ptr<wpi::WebSocket> m_websocket;
+  std::shared_ptr<UvExecFunc> m_exec;
+  std::unique_ptr<BufferPool> m_buffers;
+  std::mutex m_buffers_mutex;
+};
+
+}  // namespace wpilibws