> - I've learned that MSG_WAITALL may be unreliable on certain systems, so any implementation of recvall depending on MSG_WAITALL may inexplicably fail on such systems

Something else occurred since 5 years: the PEP 475 was accepted, it makes Python more reliable when it receives signals.

If recv(WAIT_ALL) is interrupted by a signal and returns less bytes, we must call PyErr_CheckSignal(). If the signal handler raises an exception, drop read data and raises the exception. If the signal handler does not raise an exception, we now *must* retry recv(WAIT_ALL) (with a shorter length, to not read too much data).

The IncompleteRead exception is still needed if the socket is closed before receiving the requested number of bytes.