Fix "double socket close" issue with Windows version of TCPConnection

.release-notes/4437.md

@@ -0,0 +1,15 @@
+## Fix "double socket close" issue with Windows version of TCPConnection
+
+When fixing a number of "smaller" Windows TCP networking issues a couple years back, in addition to fixing those issues, we introduced a new bug. That bug lingered for two years. It lingered in large part because it would only become apparent in a low resource environment.
+
+When we recently switched our Windows CI from CirrusCI to GitHub Actions, we went from a high-resource environment to a low-resource environment and started getting a ton of "random" Windows TCP test failures.
+
+The problem that was fairly easy to recreate in a test environment would be fairly unlikely in most applications but existed nontheless. The scenario in our test environment was like this:
+
+- Test 1 runs and completes but hasn't done test teardown yet
+- Test 2 starts up
+- Test 1 runs the "buggy" line of code and closes the socket it has been using with OS, but doesn't reset its own internal record of the file descriptor for the socket.
+- Test 2 is gets a socket from the OS with the file descriptor for the socket just closed by Test 1
+- Test 1 still has a "valid" file descriptor and as part of full shutdown, closes the socket associated with "its" file descriptor. When Test 1 does this, test 2's socket closes and the test fails to complete successfully.
+
+The problem would appear "in the wild" if a Windows application was quickly closing and opening TCP sockets in a manner similiar to the Pony standard library TCP tests.

packages/net/tcp_connection.pony

@@ -1059,11 +1059,7 @@ actor TCPConnection is AsioEventNotify
       _shutdown = true
 
       if _connected then
-        ifdef windows then
-          @pony_os_socket_close(_fd)
-        else
-          @pony_os_socket_shutdown(_fd)
-        end
+        @pony_os_socket_shutdown(_fd)
       else
         _shutdown_peer = true
       end