python - 当 Popen 错误时，子进程的 Popen 关闭在另一个线程中使用的 stdout/stderr 文件描述符

Question

当我们从 Python 2.7.3 升级到 Python 2.7.5 时，大量使用 subprocess.Popen() 的内部库的自动化测试开始失败。该库用于线程环境。调试问题后，我能够创建一个简短的 Python 脚本来演示在失败的测试中看到的错误。

这是脚本(称为“threadedsubprocess.py”):

import time
import threading
import subprocess

def subprocesscall():
    p = subprocess.Popen(
        ['ls', '-l'],
        stdin=subprocess.PIPE,
        stdout=subprocess.PIPE,
        stderr=subprocess.PIPE,
        )
    time.sleep(2) # simulate the Popen call takes some time to complete.
    out, err = p.communicate()
    print 'succeeding command in thread:', threading.current_thread().ident

def failingsubprocesscall():
    try:
        p = subprocess.Popen(
            ['thiscommandsurelydoesnotexist'],
            stdin=subprocess.PIPE,
            stdout=subprocess.PIPE,
            stderr=subprocess.PIPE,
            )
    except Exception as e:
        print 'failing command:', e, 'in thread:', threading.current_thread().ident

print 'main thread is:', threading.current_thread().ident

subprocesscall_thread = threading.Thread(target=subprocesscall)
subprocesscall_thread.start()
failingsubprocesscall()
subprocesscall_thread.join()

注意:当从 Python 2.7.3 运行时，此脚本不会退出并出现 IOError。当从 Python 2.7.5 运行时(两者都在同一个 Ubuntu 12.04 64 位虚拟机上)，它确实至少有 50% 的时间失败。

在 Python 2.7.5 上引发的错误是这样的:

/opt/python/2.7.5/bin/python ./threadedsubprocess.py 
main thread is: 139899583563520
failing command: [Errno 2] No such file or directory 139899583563520
Exception in thread Thread-1:
Traceback (most recent call last):
  File "/opt/python/2.7.5/lib/python2.7/threading.py", line 808, in __bootstrap_inner
    self.run()
  File "/opt/python/2.7.5/lib/python2.7/threading.py", line 761, in run
    self.__target(*self.__args, **self.__kwargs)
  File "./threadedsubprocess.py", line 13, in subprocesscall
    out, err = p.communicate()
  File "/opt/python/2.7.5/lib/python2.7/subprocess.py", line 806, in communicate
    return self._communicate(input)
  File "/opt/python/2.7.5/lib/python2.7/subprocess.py", line 1379, in _communicate
    self.stdin.close()
IOError: [Errno 9] Bad file descriptor

close failed in file object destructor:
IOError: [Errno 9] Bad file descriptor

当比较 Python 2.7.3 和 Python 2.7.5 的子进程模块时，我看到 Popen() 的 __init__() 调用现在确实明确地关闭了 stdin、stdout 和 stderr 文件描述符，以防执行命令以某种方式失败。这似乎是在 Python 2.7.4 中应用的预期修复，以防止泄漏文件描述符 (http://hg.python.org/cpython/file/ab05e7dd2788/Misc/NEWS#l629)。

似乎与此问题相关的 Python 2.7.3 和 Python 2.7.5 之间的差异在 Popen __init__() 中:

@@ -671,12 +702,33 @@
          c2pread, c2pwrite,
          errread, errwrite) = self._get_handles(stdin, stdout, stderr)

-        self._execute_child(args, executable, preexec_fn, close_fds,
-                            cwd, env, universal_newlines,
-                            startupinfo, creationflags, shell,
-                            p2cread, p2cwrite,
-                            c2pread, c2pwrite,
-                            errread, errwrite)
+        try:
+            self._execute_child(args, executable, preexec_fn, close_fds,
+                                cwd, env, universal_newlines,
+                                startupinfo, creationflags, shell,
+                                p2cread, p2cwrite,
+                                c2pread, c2pwrite,
+                                errread, errwrite)
+        except Exception:
+            # Preserve original exception in case os.close raises.
+            exc_type, exc_value, exc_trace = sys.exc_info()
+
+            to_close = []
+            # Only close the pipes we created.
+            if stdin == PIPE:
+                to_close.extend((p2cread, p2cwrite))
+            if stdout == PIPE:
+                to_close.extend((c2pread, c2pwrite))
+            if stderr == PIPE:
+                to_close.extend((errread, errwrite))
+
+            for fd in to_close:
+                try:
+                    os.close(fd)
+                except EnvironmentError:
+                    pass
+
+            raise exc_type, exc_value, exc_trace

我想我有三个问题:

1) 在线程环境中是否应该主要使用 subprocess.Popen，以及用于 stdin、stdout 和 stderr 的 PIPE？

2) 当其中一个线程中的 Popen() 失败时，如何防止关闭 stdin、stdout 和 stderr 的文件描述符？

3) 我在这里做错了吗？

Answer 1

我想回答你的问题:

1. 是的。
2. 你不应该这样做。
3. 没有。

Python 2.7.4 确实也有这个错误。

我认为这是库代码中的错误。如果您在程序中添加锁并确保对 subprocess.Popen 的两次调用是原子执行的，则不会发生错误。

@@ -1,32 +1,40 @@
 import time
 import threading
 import subprocess

+lock = threading.Lock()
+
 def subprocesscall():
+    lock.acquire()
     p = subprocess.Popen(
         ['ls', '-l'],
         stdin=subprocess.PIPE,
         stdout=subprocess.PIPE,
         stderr=subprocess.PIPE,
         )
+    lock.release()
     time.sleep(2) # simulate the Popen call takes some time to complete.
     out, err = p.communicate()
     print 'succeeding command in thread:', threading.current_thread().ident

 def failingsubprocesscall():
     try:
+        lock.acquire()
         p = subprocess.Popen(
             ['thiscommandsurelydoesnotexist'],
             stdin=subprocess.PIPE,
             stdout=subprocess.PIPE,
             stderr=subprocess.PIPE,
             )
     except Exception as e:
         print 'failing command:', e, 'in thread:', threading.current_thread().ident
+    finally:
+        lock.release()
+

 print 'main thread is:', threading.current_thread().ident

 subprocesscall_thread = threading.Thread(target=subprocesscall)
 subprocesscall_thread.start()
 failingsubprocesscall()
 subprocesscall_thread.join()

这意味着它很可能是由于 Popen 的实现中的一些数据竞争造成的。我会冒险猜测:错误可能在 pipe_cloexec 的实现中，由 _get_handles 调用，它(在 2.7.4 中)是:

def pipe_cloexec(self):
    """Create a pipe with FDs set CLOEXEC."""
    # Pipes' FDs are set CLOEXEC by default because we don't want them
    # to be inherited by other subprocesses: the CLOEXEC flag is removed
    # from the child's FDs by _dup2(), between fork() and exec().
    # This is not atomic: we would need the pipe2() syscall for that.
    r, w = os.pipe()
    self._set_cloexec_flag(r)
    self._set_cloexec_flag(w)
    return r, w

评论明确警告它不是原子的...这肯定会导致数据竞争，但如果不进行实验，我不知道这是否是导致问题的原因。