Both asyncio.selector_events._SelectorTransport and asyncio.proactor_events._ProactorBasePipeTransport check if an associated socket is left unclosed and emit a warning on a positive answer.

However, further behavior differs. While _SelectorTransport closes the socket:

def __del__(self, _warn=warnings.warn):
    if self._sock is not None:
        _warn(f"unclosed transport {self!r}", ResourceWarning, source=self)
        self._sock.close()

the _ProactorBasePipeTransport attempts to close itself leaving the socked leaked:

def __del__(self, _warn=warnings.warn):
    if self._sock is not None:
        _warn(f"unclosed transport {self!r}", ResourceWarning, source=self)
        self.close()

It looks like there was no plan to close the transport itself causing <already closed parent loop>.call_soon().

The deleters were introduced by 978a9af with no explanatory comment about the implementation difference.

Fixes (no autoclose) gh-92841, gh-91233, and gh-83413. Probably fixes (no autoclose) gh-81562.