Destruct ConptyConnection on a background thread

src/cascadia/TerminalConnection/ConptyConnection.cpp

@@ -455,4 +455,23 @@ namespace winrt::Microsoft::Terminal::TerminalConnection::implementation
         return 0;
     }
 
+    // Function Description:
+    // - This function will be called (by C++/WinRT) after the final outstanding reference to
+    //   any given connection instance is released.
+    //   When a client application exits, its termination will wait for the output thread to
+    //   run down. However, because our teardown is somewhat complex, our last reference may
+    //   be owned by the very output thread that the client wait threadpool is blocked on.
+    //   During destruction, we'll try to release any outstanding handles--including the one
+    //   we have to the threadpool wait. As you might imagine, this takes us right to deadlock
+    //   city.
+    //   Deferring the final destruction of the connection to a background thread that can't
+    //   be awaiting our destruction breaks the deadlock.
+    // Arguments:
+    // - connection: the final living reference to an outgoing connection
+    winrt::fire_and_forget ConptyConnection::final_release(std::unique_ptr<ConptyConnection> connection)
+    {
+        co_await winrt::resume_background(); // move to background
+        connection.reset(); // explicitly destruct
+    }
+
 }

src/cascadia/TerminalConnection/ConptyConnection.h

@@ -27,6 +27,7 @@ namespace winrt::Microsoft::Terminal::TerminalConnection::implementation
             const uint32_t rows,
             const uint32_t cols,
             const guid& guid);
+        static winrt::fire_and_forget final_release(std::unique_ptr<ConptyConnection> connection);
 
         void Start();
         void WriteInput(hstring const& data);