import asyncio
import sys
from asyncio.subprocess import PIPE


CHILD_CMD = """import sys; sys.stdout.flush(); sys.stdout.buffer.write(b'x' * (2**17+1)); sys.stdout.flush()"""

@asyncio.coroutine
def fail():
    child = yield from asyncio.create_subprocess_exec(
        sys.executable, '-c', CHILD_CMD, stdout=PIPE)

    total = 0
    while True:
        chunk = yield from child.stdout.read(2048)
        if not chunk:
            break
        total += len(chunk)
        yield from asyncio.sleep(0.001)

    if (yield from child.wait()) != 0:
        raise ValueError()

    return total


loop = asyncio.get_event_loop()
try:
    print('read {} bytes from child'.format(loop.run_until_complete(fail())))
finally:
    loop.close()