Python和asyncio：封闭的命名管道始终可供读取

I am working on a script that reads data through a named pipe from another piece of software. I would like to read data only when available, and I was trying to use add_reader from asyncio.

我正在编写一个脚本,通过另一个软件的命名管道读取数据。我想只在可用时读取数据,我试图使用asyncio的add_reader。

I noticed that, on Linux, the reader I registered is called continuously after the pipe is closed. On macOS, this doesn't happen.

我注意到,在Linux上,我注册的阅读器在管道关闭后连续调用。在macOS上,这不会发生。

This puzzles me, because after the writing end of the pipe has hung up, I would not expect the reading end to be available for reading, especially because clearly there can be no data.

这让我感到很困惑,因为在管道写入结束后,我不希望读取端可以读取,特别是因为显然没有数据。

This script illustrates the problem:

此脚本说明了此问题:

#!/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)

On OSX, this is the behavior I observe:

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

While on Linux:

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

So, why is a closed pipe available for reading although it has no data?

那么,为什么闭合管道可用于读取,尽管它没有数据?

Also, in my understanding, add_reader would trigger when the stream can be read from and there is some data to read; is this interpretation correct?

另外,根据我的理解,当可以读取流并且有一些数据需要读取时,add_reader会触发;这种解释是否正确?

Python and OS versions:

Python和OS版本:

Python 3.6.4 (MacPorts), macOS High Sierra 10.13.3 (17D102)

Python 3.6.4(MacPorts),macOS High Sierra 10.13.3(17D102)

Python 3.6.1 (manually compiled) CentOS Linux release 7.4.1708 (Core)

Python 3.6.1(手动编译)CentOS Linux 7.4.1708版(核心版)

Python 3.5.2 (from repo) Linux Mint 18.2 Sonya

Python 3.5.2(来自repo)Linux Mint 18.2 Sonya

1 个解决方案

#1

In python reading an empty data is a sign for socket/pipe closing.

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

Also please switch the pipe to non-blocking mode:

还请将管道切换到非阻塞模式:

os.set_blocking(fd, False)

1 个解决方案

#1

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