fix race on disconnect

between the time we check for `is_open` and the actual disconnect, the
socket could already have been disconnected by another thread (TOCTOU).
Furthermore, the disconnects can race causing a segfault or similar
issue in the asio's internals.

include/Client.h

@@ -18,6 +18,7 @@
 
 #pragma once
 
+#include <atomic>
 #include <chrono>
 #include <memory>
 #include <optional>
@@ -68,8 +69,11 @@ public:
     std::string GetCarPositionRaw(int Ident);
     void SetUDPAddr(const ip::udp::endpoint& Addr) { mUDPAddress = Addr; }
     void SetTCPSock(ip::tcp::socket&& CSock) { mSocket = std::move(CSock); }
-    void Disconnect(std::string_view Reason);
-    bool IsDisconnected() const { return !mSocket.is_open(); }
+    // Returns true only for the thread that actually performs socket shutdown/close.
+    [[nodiscard]] bool Disconnect(std::string_view Reason);
+    bool IsDisconnected() const {
+        return mDisconnectState.load(std::memory_order_acquire) != EDisconnectState::Connected;
+    }
     // locks
     void DeleteCar(int Ident);
     [[nodiscard]] const std::unordered_map<std::string, std::string>& GetIdentifiers() const { return mIdentifiers; }
@@ -106,6 +110,12 @@ public:
     [[nodiscard]] const std::vector<uint8_t>& GetMagic() const { return mMagic; }
 
 private:
+    enum class EDisconnectState {
+        Connected,
+        Disconnecting,
+        Disconnected
+    };
+
     void InsertVehicle(int ID, const std::string& Data);
 
     TServer& mServer;
@@ -121,6 +131,8 @@ private:
     TSetOfVehicleData mVehicleData;
     SparseArray<std::string> mVehiclePosition;
     std::string mName = "Unknown Client";
+    // Once disconnect starts, this client is terminal and its socket must be treated as dead.
+    std::atomic<EDisconnectState> mDisconnectState { EDisconnectState::Connected };
     ip::tcp::socket mSocket;
     ip::udp::endpoint mUDPAddress {};
     int mUnicycleID = -1;