[wpiutil] Add HttpWebSocketServerConnection (#2505)

This is a derived class of HttpServerConnection that implements the
WebSocket upgrade pieces.  This combination is pretty common so is
worth refactoring here.

wpiutil/src/main/native/include/wpi/HttpWebSocketServerConnection.inl

@@ -0,0 +1,54 @@
+/*----------------------------------------------------------------------------*/
+/* 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#ifndef WPIUTIL_WPI_HTTPWEBSOCKETSERVERCONNECTION_INL_
+#define WPIUTIL_WPI_HTTPWEBSOCKETSERVERCONNECTION_INL_
+
+#include <memory>
+
+namespace wpi {
+
+template <typename Derived>
+HttpWebSocketServerConnection<Derived>::HttpWebSocketServerConnection(
+    std::shared_ptr<uv::Stream> stream, ArrayRef<StringRef> protocols)
+    : HttpServerConnection{stream},
+      m_helper{m_request},
+      m_protocols{protocols.begin(), protocols.end()} {
+  // Handle upgrade event
+  m_helper.upgrade.connect([this] {
+    // Negotiate sub-protocol
+    SmallVector<StringRef, 2> protocols{m_protocols.begin(), m_protocols.end()};
+    StringRef protocol = m_helper.MatchProtocol(protocols).second;
+
+    // Check that the upgrade is valid
+    if (!IsValidWsUpgrade(protocol)) return;
+
+    // Disconnect HttpServerConnection header reader
+    m_dataConn.disconnect();
+    m_messageCompleteConn.disconnect();
+
+    // Accepting the stream may destroy this (as it replaces the stream user
+    // data), so grab a shared pointer first.
+    auto self = this->shared_from_this();
+
+    // Accept the upgrade
+    auto ws = m_helper.Accept(m_stream, protocol);
+
+    // Set this as the websocket user data to keep it around
+    ws->SetData(self);
+
+    // Store in member
+    m_websocket = ws.get();
+
+    // Call derived class function
+    ProcessWsUpgrade();
+  });
+}
+
+}  // namespace wpi
+
+#endif  // WPIUTIL_WPI_HTTPWEBSOCKETSERVERCONNECTION_INL_