块 - 将输入发送到 python 子进程管道

I'm on linux, but since I was testing with threads, my first approach was to replicate the exact windows threading code seen on subprocess.py's communicate() method, but for two processes instead of one. Here's the entire listing of what I tried:

import os
from subprocess import Popen, PIPE
import threading

def get_output(fobj, buffer):
    while True:
        chunk = fobj.read() # BLOCKS HERE
        if not chunk:
            break
        buffer.append(chunk)

p1 = Popen(["grep", "-v", "not"], stdin=PIPE, stdout=PIPE)
p2 = Popen(["cut", "-c", "1-10"], stdin=p1.stdout, stdout=PIPE)

b = [] # create a buffer
t = threading.Thread(target=get_output, args=(p2.stdout, b))
t.start() # start reading thread

for x in xrange(100000):
    p1.stdin.write('hello world
') # write data
    p1.stdin.flush()
p1.stdin.close() # close input...
t.join()

嗯.它没有用.即使在调用 p1.stdin.close() 之后，p2.stdout.read() 仍然会阻塞.

Well. It didn't work. Even after p1.stdin.close() was called, p2.stdout.read() still blocks.

然后我在 subprocess.py 上尝试了 posix 代码:

Then I tried the posix code on subprocess.py:

import os
from subprocess import Popen, PIPE
import select

p1 = Popen(["grep", "-v", "not"], stdin=PIPE, stdout=PIPE)
p2 = Popen(["cut", "-c", "1-10"], stdin=p1.stdout, stdout=PIPE)

numwrites = 100000
to_read = [p2.stdout]
to_write = [p1.stdin]
b = [] # create buffer

while to_read or to_write:
    read_now, write_now, xlist = select.select(to_read, to_write, [])
    if read_now:
        data = os.read(p2.stdout.fileno(), 1024)
        if not data:
            p2.stdout.close()
            to_read = []
        else:
            b.append(data)

    if write_now:
        if numwrites > 0:
            numwrites -= 1
            p1.stdin.write('hello world!
'); p1.stdin.flush()
        else:
            p1.stdin.close()
            to_write = []

print b

在 select.select() 上也会阻塞.通过传播prints，我发现了这一点:

Also blocks on select.select(). By spreading prints around, I found out this:

• 正在阅读.代码在执行过程中多次读取.
• 写作也很有效.数据写入p1.stdin.
• 在numwrites结束时，调用p1.stdin.close().
• 当 select() 开始阻塞时，只有 to_read 有东西，p2.stdout.to_write 已经为空.
• os.read() 调用总是返回一些东西，所以 p2.stdout.close() 永远不会被调用.

• Reading is working. Code reads many times during execution.
• Writing is also working. Data is written to p1.stdin.
• At the end of numwrites, p1.stdin.close() is called.
• When select() starts blocking, only to_read has something, p2.stdout. to_write is already empty.
• os.read() call always returns something, so p2.stdout.close() is never called.

两个测试的结论:关闭管道上第一个进程的 stdin(示例中为 grep)不会使其转储它的缓冲输出到下一个并死掉.

Conclusion from both tests: Closing the stdin of the first process on the pipeline (grep in the example) is not making it dump its buffered output to the next and die.

没有办法让它工作?

PS:我不想使用临时文件，我已经用文件进行了测试，我知道它可以工作.而且我不想使用 windows.

PS: I don't want to use a temporary file, I've already tested with files and I know it works. And I don't want to use windows.

推荐答案

我知道怎么做.

这与线程无关，也与 select() 无关.

It is not about threads, and not about select().

当我运行第一个进程 (grep) 时，它会创建两个低级文件描述符，每个管道一个.让我们调用这些 a 和 b.

When I run the first process (grep), it creates two low-level file descriptors, one for each pipe. Lets call those a and b.

当我运行第二个进程时，b 被传递给 cut sdtin.但是 Popen 有一个脑死的默认值 - close_fds=False.

When I run the second process, b gets passed to cut sdtin. But there is a brain-dead default on Popen - close_fds=False.

这样做的效果是cut也继承了a.所以即使我关闭了a grep 也不会死掉，因为stdin 在cut 的进程(cut 忽略它).

The effect of that is that cut also inherits a. So grep can't die even if I close a, because stdin is still open on cut's process (cut ignores it).

下面的代码现在可以完美运行了.

The following code now runs perfectly.

from subprocess import Popen, PIPE

p1 = Popen(["grep", "-v", "not"], stdin=PIPE, stdout=PIPE)
p2 = Popen(["cut", "-c", "1-10"], stdin=p1.stdout, stdout=PIPE, close_fds=True)
p1.stdin.write('Hello World
')
p1.stdin.close()
result = p2.stdout.read() 
assert result == "Hello Worl
"

close_fds=True 应该是 unix 系统上的默认值.在 Windows 上，它会关闭 所有 fds，因此它会阻止管道.

close_fds=True SHOULD BE THE DEFAULT on unix systems. On windows it closes all fds, so it prevents piping.

PS:对于阅读此答案时遇到类似问题的人:正如pooryorick 在评论中所说，如果写入 p1.stdin 的数据大于缓冲区，这也可能会阻塞.在这种情况下，您应该将数据分成更小的部分，并使用 select.select() 来了解何时读取/写入.问题中的代码应该提示如何实现它.

PS: For people with a similar problem reading this answer: As pooryorick said in a comment, that also could block if data written to p1.stdin is bigger than the buffers. In that case you should chunk the data into smaller pieces, and use select.select() to know when to read/write. The code in the question should give a hint on how to implement that.

在pooryorick 的更多帮助下找到了另一种解决方案 - 而不是使用 close_fds=True 并关闭 ALL fds，可以关闭 fds，在执行第二个时，它将起作用.关闭必须在子进程中完成，因此 Popen 中的 preexec_fn 函数非常方便地做到这一点.在执行 p2 时，您可以执行以下操作:

p2 = Popen(cmd2, stdin=p1.stdout, stdout=PIPE, stderr=devnull, preexec_fn=p1.stdin.close)