Python 和异步 : closed named pipe always available for reading

Question

我正在编写一个脚本，该脚本通过命名管道从另一个软件读取数据。我只想在可用时读取数据，并且我正在尝试使用 add_reader from asyncio .

我注意到，在 Linux 上，我注册的阅读器会在管道关闭后继续被调用。在 macOS 上，这不会发生。

这让我百思不得其解，因为在管道的写端挂掉之后，我不会指望读端还能读，尤其是明明不可能没有数据。

这个脚本说明了问题:

#!/usr/bin/env python3
import os, asyncio, threading, time
NAMED_PIPE = 'write.pipe'

# Setup the named pipe
if os.path.exists(NAMED_PIPE):
    os.unlink(NAMED_PIPE)
os.mkfifo(NAMED_PIPE)

loop = asyncio.get_event_loop()

def simulate_write():
    # Open the pipe for writing and write something into it.
    # This simulates another process
    print('waiting for opening pipe for writing')
    with open(NAMED_PIPE, 'w') as write_stream:
        print('writing pipe opened')
        time.sleep(1)
        print('writing some data')
        print('<some data>', file=write_stream)
        time.sleep(1)
    print('exiting simulated write')


async def open_pipe_for_reading():
    print('waiting for opening pipe for reading')
    # This needs to run asynchronously because open will
    # not reuturn until on the other end, someone tries
    # to write
    return open(NAMED_PIPE)

count = 0
def read_data_block(fd):
    global count
    count += 1
    print('reading data', fd.read())
    if count > 10:
        print('reached maximum number of calls')
        loop.remove_reader(fd.fileno())

# Spawn a thread that will simulate writing
threading.Thread(target=simulate_write).start()
# Get the result of open_pipe_for_reading
stream = loop.run_until_complete(open_pipe_for_reading())
print('reading pipe opened')
# Schedule the reader
loop.add_reader(stream.fileno(), read_data_block, stream)
try:
    loop.run_forever()
except KeyboardInterrupt:
    pass
finally:
    print('closing stream')
    stream.close()
print('removing pipe')
os.unlink(NAMED_PIPE)

在 OSX 上，这是我观察到的行为:

waiting for opening pipe for writing
waiting for opening pipe for reading
reading pipe opened
writing pipe opened
writing some data
exiting simulated write
reading data <some data>

^Cclosing stream
removing pipe

在 Linux 上:

waiting for opening pipe for writing
waiting for opening pipe for reading
reading pipe opened
writing pipe opened
writing some data
exiting simulated write
reading data <some data>

reading data
reading data
reading data
reading data
reading data
reading data
reading data
reading data
reading data
reading data
reached maximum number of calls
^C<closing stream
removing pipe

那么，为什么一个没有数据的封闭管道可以读取呢？

此外，据我了解，add_reader 会在可以从读取流并且有一些数据要读取时触发； 这个解释正确吗？

---

Python 和操作系统版本:

• Python 3.6.4 (MacPorts)、macOS High Sierra 10.13.3 (17D102)
• Python 3.6.1(手动编译)CentOS Linux release 7.4.1708 (Core)
• Python 3.5.2(来自 repo)Linux Mint 18.2 Sonya

Answer 1

在 python 中读取空数据是套接字/管道关闭的标志。

data = fd.read()
if not data:
    return

另外请将管道切换到非阻塞模式:

os.set_blocking(fd, False)