create_subprocess_exec stdin writer does not write everything #312

# Standard Library
import asyncio.subprocess
import pytest

@pytest.mark.parametrize("num_lines", range(8188, 8194,1))
async def test_uvloopwrite(loop, num_lines):
    proc = await asyncio.create_subprocess_exec("python", __file__,
                                                        stdout=asyncio.subprocess.PIPE,
                                                        stdin=asyncio.subprocess.PIPE)
    data =  b"\n" * num_lines + b"END\n"
    print(num_lines, len(data))
    print("writing", proc.stdin.transport.get_write_buffer_limits())
    proc.stdin.write(data)
    print("drain")
    await proc.stdin.drain()
    # proc.stdin.close()
    print("waiting")
    await proc.wait()
    out = await proc.stdout.read()
    assert int(out) == num_lines

# this small program is called by the test
if __name__ == "__main__":
    import sys
    n = 0
    while True:
        line = sys.stdin.readline()
        if n > 8186:
            print(n, repr(line), file=sys.stderr)
        if line == "END\n":
            break
        n+=1
    print(n)