python - Python中子进程读取线超时

Question

我有一个小问题，我不太确定如何解决。这是一个最小的例子：

我有的

scan_process = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
while(some_criterium):
    line = scan_process.stdout.readline()
    some_criterium = do_something(line)

我想要什么

scan_process = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
while(some_criterium):
    line = scan_process.stdout.readline()
    if nothing_happens_after_10s:
        break
    else:
        some_criterium = do_something(line)

我从子进程中读取了一行并对其进行了处理。如果在固定时间间隔后没有线路到达，我该如何退出？

Answer 1

感谢所有的答案！

我找到了一种方法来解决我的问题，只需使用 select.poll 来查看标准输出。

import select
...
scan_process = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
poll_obj = select.poll()
poll_obj.register(scan_process.stdout, select.POLLIN)
while(some_criterium and not time_limit):
    poll_result = poll_obj.poll(0)
    if poll_result:
        line = scan_process.stdout.readline()
        some_criterium = do_something(line)
    update(time_limit)

Answer 2

这是一个便携式解决方案，它使用以下命令强制读取单行超时asyncio：

#!/usr/bin/env python3
import asyncio
import sys
from asyncio.subprocess import PIPE, STDOUT

async def run_command(*args, timeout=None):
    # Start child process
    # NOTE: universal_newlines parameter is not supported
    process = await asyncio.create_subprocess_exec(*args,
            stdout=PIPE, stderr=STDOUT)

    # Read line (sequence of bytes ending with b'\n') asynchronously
    while True:
        try:
            line = await asyncio.wait_for(process.stdout.readline(), timeout)
        except asyncio.TimeoutError:
            pass
        else:
            if not line: # EOF
                break
            elif do_something(line):
                continue # While some criterium is satisfied
        process.kill() # Timeout or some criterion is not satisfied
        break
    return await process.wait() # Wait for the child process to exit


if sys.platform == "win32":
    loop = asyncio.ProactorEventLoop() # For subprocess' pipes on Windows
    asyncio.set_event_loop(loop)
else:
    loop = asyncio.get_event_loop()

returncode = loop.run_until_complete(run_command("cmd", "arg 1", "arg 2",
                                                 timeout=10))
loop.close()

Answer 3

我在 Python 中使用了一些更通用的东西（如果我没记错的话，也是从 Stack Overflow 问题拼凑而成的，但我不记得是哪些问题了）。

import thread
from threading import Timer

def run_with_timeout(timeout, default, f, *args, **kwargs):
    if not timeout:
        return f(*args, **kwargs)
    try:
        timeout_timer = Timer(timeout, thread.interrupt_main)
        timeout_timer.start()
        result = f(*args, **kwargs)
        return result
    except KeyboardInterrupt:
        return default
    finally:
        timeout_timer.cancel()

不过请注意。这使用中断来停止你给它的任何功能。这对于所有函数来说可能不是一个好主意，它还会阻止您在超时期间使用Ctrl+关闭程序C（即Ctrl+C将被视为超时）。

您可以使用它并将其称为：

scan_process = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
while(some_criterium):
    line = run_with_timeout(timeout, None, scan_process.stdout.readline)
    if line is None:
        break
    else:
        some_criterium = do_something(line)

不过，这可能有点矫枉过正。我怀疑您的情况有一个我不知道的更简单的选择。

Answer 4

While Tom's solution works, using select() in the C idiom is more compact, this is the equivalent of your answer:

from select import select
scan_process = subprocess.Popen(command,
                                stdout=subprocess.PIPE,
                                stderr=subprocess.STDOUT,
                                bufsize=1)  # Line buffered
while some_criterium and not time_limit:
    poll_result = select([scan_process.stdout], [], [], time_limit)[0]

The rest is the same.

See pydoc select.select.

[Note: this is Unix-specific, as are some of the other answers.]

[Note 2: edited to add line buffering as per OP request]

[Note 3: the line buffering may not be reliable in all circumstances, leading to readline() blocking]

Answer 5

如果读取一行花费的时间太长，一个可移植的解决方案是使用线程杀死子进程：

#!/usr/bin/env python3
from subprocess import Popen, PIPE, STDOUT

timeout = 10
with Popen(command, stdout=PIPE, stderr=STDOUT,
           universal_newlines=True) as process:  # text mode
    # kill process in timeout seconds unless the timer is restarted
    watchdog = WatchdogTimer(timeout, callback=process.kill, daemon=True)
    watchdog.start()
    for line in process.stdout:
        # don't invoke the watcthdog callback if do_something() takes too long
        with watchdog.blocked:
            if not do_something(line):  # some criterium is not satisfied
                process.kill()
                break
            watchdog.restart()  # restart timer just before reading the next line
    watchdog.cancel()

像这样的WatchdogTimer类threading.Timer可以重新启动和/或阻止：

from threading import Event, Lock, Thread
from subprocess import Popen, PIPE, STDOUT
from time import monotonic  # use time.time or monotonic.monotonic on Python 2

class WatchdogTimer(Thread):
    """Run *callback* in *timeout* seconds unless the timer is restarted."""

    def __init__(self, timeout, callback, *args, timer=monotonic, **kwargs):
        super().__init__(**kwargs)
        self.timeout = timeout
        self.callback = callback
        self.args = args
        self.timer = timer
        self.cancelled = Event()
        self.blocked = Lock()

    def run(self):
        self.restart() # don't start timer until `.start()` is called
        # wait until timeout happens or the timer is canceled
        while not self.cancelled.wait(self.deadline - self.timer()):
            # don't test the timeout while something else holds the lock
            # allow the timer to be restarted while blocked
            with self.blocked:
                if self.deadline <= self.timer() and not self.cancelled.is_set():
                    return self.callback(*self.args)  # on timeout

    def restart(self):
        """Restart the watchdog timer."""
        self.deadline = self.timer() + self.timeout

    def cancel(self):
        self.cancelled.set()

Answer 6

尝试使用 signal.alarm：

#timeout.py
import signal, sys

def timeout(sig, frm):
  print "This is taking too long..."
  sys.exit(1)

signal.signal(signal.SIGALRM, timeout)
signal.alarm(10)
byte = 0

while 'IT' not in open('/dev/urandom').read(2):
  byte += 2
print "I got IT in %s byte(s)!" % byte

运行几次以显示它有效：

$ python timeout.py 
This is taking too long...
$ python timeout.py 
I got IT in 4672 byte(s)!

有关更详细的示例，请参阅pGuides。