Avoid extraneous copy when `_recv`ing bytes through pipes in multiprocessing "connections"

Feature or enhancement

Avoid extraneous copy when _recving bytes through pipes in multiprocessing "connections"

Pitch

In the default IPC implementation of the multiprocessing module using pipes, the Connection._recv method is defined as follows:

https://github.com/python/cpython/blob/586fc02be5b3e103bfddd49654034a898a8d6dfc/Lib/multiprocessing/connection.py#L378-L392

Seems like we can avoid a copy if we can read directly into a preallocated byte array? This would be beneficial if the bytes we're sending between processes are large. Another benefit here is that the buf.write() call above doesn't release the GIL, so in my workloads I've seen it causing stalls in another thread when trying to read from a multiprocessing.Queue instance.

Sample implementation that avoids the extraneous copy is shown below (bit untested). If it makes sense, I can submit a patch, but probably need some guidance to deal with the Windows case:

import io


def unix_readinto(fd, buf):
    # Thanks https://stackoverflow.com/q/13919006/1572989
    return io.FileIO(fd, closefd=False).readinto(buf)


class Connection(...):
    ...

    def _recv(self, size, readinto=unix_readinto):
        buf = io.BytesIO(initial_bytes=bytes(size))
        handle = self._handle
    
        mem = buf.getbuffer()
        total = 0
        while total != size:
            n = readinto(handle, mem[total:])
    
            if n == 0:
                if total == 0:
                    raise EOFError
                else:
                    raise OSError("got end of file during message")
    
            total += n
    
        return buf

Previous discussion

N/A

Start with _recv in Lib/multiprocessing/connection.py, linked in the issue, and compare how pipe reads are handled across Unix and Windows. Verify a preallocated buffer can receive large messages without the extra copy while preserving the existing EOF and partial-read behavior on both platforms; the issue does not name specific tests.