子进程（在后台启动 ssh 进程）如果启用 stderr，则通信挂起

Question

我有下一个代码，接下来会做：

1. ssh -f -M

   的子进程让 ssh 在后台启动共享套接字
2. 如上是在后台，所以对于第二个 ssh 连接，我们可以重复使用套接字

   /tmp/control-channel

   来连接 ssh 服务器而无需密码。

测试.py：

import subprocess
import os
import sys
import stat

ssh_user = "my_user"       # change to your account
ssh_passwd = "my_password" # change to your password

try:
    os.remove("/tmp/control-channel")
except:
    pass

# prepare passwd file
file = open("./passwd","w")
passwd_content = f"#!/bin/sh\necho {ssh_passwd}"
file.write(passwd_content)
file.close()
os.chmod("./passwd", stat.S_IRWXU)

# setup shared ssh socket, put it in background
env = {'SSH_ASKPASS': "./passwd", 'DISPLAY':'', 'SSH_ASKPASS_REQUIRE':'force'}
args = ['ssh', '-f', '-o', 'LogLevel=ERROR', '-x', '-o', 'ConnectTimeout=30', '-o', 'ControlPersist=300', '-o', 'UserKnownHostsFile=/dev/null', '-o', 'StrictHostKeyChecking=no', '-o', 'ServerAliveInterval=15', '-MN', '-S', '/tmp/control-channel', '-p', '22', '-l', ssh_user, 'localhost']
process = subprocess.Popen(args, env=env,
        stdout=subprocess.PIPE,
#        stderr=subprocess.STDOUT,   # uncomment this line to enable stderr will make subprocess hang
        stdin=subprocess.DEVNULL,
        start_new_session=True)
sout, serr = process.communicate()
print(sout)
print(serr)

# use shared socket
args2 = ['ssh', '-o', 'LogLevel=ERROR', '-o', 'ControlPath=/tmp/control-channel', '-p', '22', '-l', ssh_user, 'localhost', 'uname -a']
process2 = subprocess.Popen(args2,
        stdout=subprocess.PIPE,
        stderr=subprocess.STDOUT,
        stdin=subprocess.DEVNULL)
content, _ = process2.communicate()
print(content)

执行：

$ python3 test.py
b''
None
b'Linux shmachine 4.19.0-21-amd64 #1 SMP Debian 4.19.249-2 (2022-06-30) x86_64 GNU/Linux\n'

到目前为止一切顺利，只要我在第一个子进程中取消注释

stderr=subprocess.STDOUT

，它就会挂起：

$ python3 test.py
^CTraceback (most recent call last):
  File "test.py", line 29, in <module>
    sout, serr = process.communicate()
  File "/usr/lib/python3.7/subprocess.py", line 926, in communicate
    stdout = self.stdout.read()
KeyboardInterrupt

我想知道这里出了什么问题？

我的环境：

$ python3 --version
Python 3.7.3
$ ssh -V
OpenSSH_7.9p1 Debian-10+deb10u2, OpenSSL 1.1.1n  15 Mar 2022
$ cat /etc/issue
Debian GNU/Linux 10 \n \l

Answer 1

答案实际上隐藏在Python文档subprocess.Popen中，并且在

Popen.communicate

的文档中，读取以下内容：

Note: The data read is buffered in memory, so do not use this method if the data size is large or unlimited.

因此，一个（可能）可行的解决方案（因为我实际上没有可用的 SSH 来测试），就是在

timeout

调用中添加

communicate

。