Async subprocess for Linux and Windows, revisited

'''
Modern recipe for async subprocesses in Linux and Windows. Works in Python 3.3+.
Written March 28, 2014 by Josiah Carlson

Released into the public domain.

From these two functions you can build buffered reads, writes, timeouts on
reads or writes, readline, etc.
'''

import errno
import os
import subprocess
import time

if subprocess.mswindows:
    from _winapi import PeekNamedPipe, ReadFile, WriteFile
    from msvcrt import get_osfhandle
else:
    import fcntl
    import select

def read(sp, stderr=False):
    """
    Reads all available data from the provided subprocess.

    Pass stderr=True if you want to read from stderr.
    """

    pipe = sp.stderr if stderr else sp.stdout
    if pipe.closed:
        return b''

    flags = fcntl.fcntl(pipe, fcntl.F_GETFL)
    fcntl.fcntl(pipe, fcntl.F_SETFL, flags | os.O_NONBLOCK)
    ret = []
    try:
        while select.select([pipe], [], [], 0)[0]:
            ret.append(pipe.read(4096))
            if not ret[-1]:
                break
        return b''.join(ret)
    finally:
        if not pipe.closed:
            fcntl.fcntl(pipe, fcntl.F_SETFL, flags)

def write(sp, data):
    """
    Tries to write the data to the provided subprocess.

    Returns the number of bytes written.
    """
    if not select.select([], [sp.stdin], [], 0)[1]:
        return 0

    try:
        return os.write(sp.stdin.fileno(), data)
    except OSError as why:
        if why[0] == errno.EPIPE:
            return 0
        raise

if subprocess.mswindows:
    def read(sp, stderr=False):
        """
        Reads all available data from the provided subprocess.

        Pass stderr=True if you want to read from stderr.
        """
        pipe = sp.stderr if stderr else sp.stdout
        if pipe.closed:
            return b''

        try:
            handle = get_osfhandle(pipe.fileno())
            avail = PeekNamedPipe(handle, 0)[0]
            if avail > 0:
                return ReadFile(handle, avail)[0]
            return b''
        except ValueError:
            return b''
        except (subprocess.pywintypes.error, Exception) as why:
            if why[0] in (109, errno.ESHUTDOWN):
                return b''
            raise

    def write(sp, data):
        """
        Tries to write the data to the provided subprocess.

        Returns the number of bytes written.
        """
        try:
            handle = get_osfhandle(sp.stdin.fileno())
            return WriteFile(handle, data)[0]
        except ValueError:
            return 0
        except (subprocess.pywintypes.error, Exception) as why:
            if why[0] in (109, errno.ESHUTDOWN):
                return 0