[asvetlov]

On Windows, a client that resets its connection while it waits in the listen queue makes the pending AcceptEx fail with ERROR_NETNAME_DELETED (WinError 64). BaseProactorEventLoop._start_serving treated every OSError as fatal for the listening socket and closed it. After that the server refused all new connections, while Server.is_serving() still returned True.

Changes:

• IocpProactor.accept() closes the accept socket when AcceptEx fails, and turns ERROR_NETNAME_DELETED into ConnectionResetError, as recv() and friends already do. accept_coro no longer re-raises that error: the caller already receives it through the future, so re-raising only logged a second "Task exception was never retrieved" traceback.
• The proactor serving loop treats ConnectionResetError and ConnectionAbortedError as per-connection errors: it logs them at debug level and keeps accepting, as SelectorEventLoop._accept_connection already does. Other OSErrors still close the listening socket.

Tests:

• test_windows_events.test_accept_connection_reset reproduces the bug reliably, using the reproducer from the issue: two connections are made while the loop is blocked, and the second is reset in the listen queue. On a Windows 11 debug build it failed 3 times out of 3 without the fix (WinError 64, then ConnectionRefusedError) and passed 10 times out of 10 with it. The full test_asyncio suite passes on Windows.
• test_proactor_events.test_create_server_connection_reset covers the serving-loop change with mocks.

Notes for reviewers:

• This supersedes gh-93821: Fix bug that accepting a socket connection and ERROR_NETNAME_DELETED occurs causes serving socket to be closed #124032 and gh-93821: Handle connection resets on Windows #124779, which are both stale:
  • gh-93821: Fix bug that accepting a socket connection and ERROR_NETNAME_DELETED occurs causes serving socket to be closed #124032 also mapped ERROR_OPERATION_ABORTED, which usually means the listening socket itself was closed. That error isn't mapped here.
  • gh-93821: Handle connection resets on Windows #124779 doesn't fix the bug: ConnectionResetError still reaches the except OSError branch, which closes the listener.

• Issue: When accepting a socket connection and ERROR_NETNAME_DELETED occurs, leads this into a closing of the serving socket (BaseProactorEventLoop) #93821

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