[ python-Bugs-892939 ] Race condition in popen2

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Bugs item #892939, was opened at 2004-02-08 09:31
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Category: Python Library
Group: Python 2.3
Status: Open
Resolution: None
Priority: 5
Submitted By: Ken McNeil (kenmcneil)
Assigned to: Nobody/Anonymous (nobody)
Summary: Race condition in popen2

Initial Comment:
The "fix" for bug #761888 created a race condition in
Popen3  . The interaction between wait and _cleanup is
the root of the problem.

def wait(self):
  """Wait for and return the exit status of the child
process."""
  if self.sts < 0:
    pid, sts = os.waitpid(self.pid, 0)
    if pid == self.pid:
      self.sts = sts
  return self.sts

def _cleanup():
    for inst in _active[:]:
        inst.poll()

In wait, between the check of self.sts and the call to
os.waitpid a new Popen3 object can be created in
another thread which will trigger a call to _cleanup.
Since the call to _cleanup polls the process, when the
thread running wait starts back up again it will try to
poll the process using os.waitpid, which will throw an
OSError because os.waitpid has already been called for
the PID indirectly in _cleanup.

A work around is for the caller of wait to catch the
OSError and check the sts field, and if sts is
non-negative then the OSError is most likely because of
this problem and can be ignored. However, sts is
undocumented and should probably stay that way.

My suggestion is that the patch that added _active, 
_cleanup, and all  be removed and a more suitable
mechanism for fixing bug #761888 be found. As it has
been said in the discussion of bug #761888, magically
closing FDs is not a "good thing". It seems to me that
surrounding the call to os.fork with a try/except, and
closing the pipes in the except, would be suitable but
I don't know how this would interact with a failed call
to fork, therefore I wont provide a patch.

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