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The error only occured once, and it was not reproduced when test_cmd_line was rerunning in verbose mode.
http://www.python.org/dev/buildbot/builders/x86 Tiger 3.x/builds/25/steps/test/logs/stdio
test test_cmd_line failed -- Traceback (most recent call last):
 File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/test/test_cmd_line.py", line 58, in test_q
 self.verify_valid_flag('-Qold')
 File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/test/test_cmd_line.py", line 49, in verify_valid_flag
 data = self.start_python(cmd_line)
 File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/test/test_cmd_line.py", line 34, in start_python
 p = spawn_python(*args)
 File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/test/script_helper.py", line 29, in spawn_python
 stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
 File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/subprocess.py", line 694, in __init__
 self.stdin = io.open(p2cwrite, 'wb', bufsize)
OSError: [Errno 9] Bad file descriptor
Re-running test test_cmd_line in verbose mode
test_directories (test.test_cmd_line.CmdLineTest) ... ok
test_large_PYTHONPATH (test.test_cmd_line.CmdLineTest) ... ok
test_optimize (test.test_cmd_line.CmdLineTest) ... ok
test_q (test.test_cmd_line.CmdLineTest) ... ok
test_run_code (test.test_cmd_line.CmdLineTest) ... ok
test_run_module (test.test_cmd_line.CmdLineTest) ... ok
test_run_module_bug1764407 (test.test_cmd_line.CmdLineTest) ... ok
test_site_flag (test.test_cmd_line.CmdLineTest) ... ok
test_unbuffered_input (test.test_cmd_line.CmdLineTest) ... ok
test_unbuffered_output (test.test_cmd_line.CmdLineTest) ... ok
test_usage (test.test_cmd_line.CmdLineTest) ... ok
test_verbose (test.test_cmd_line.CmdLineTest) ... ok
test_version (test.test_cmd_line.CmdLineTest) ... ok
----------------------------------------------------------------------
Ran 13 tests in 3.860s
OK

See also #8457.

I have also seen this test failure occasionally but only on OS X 10.4 systems, not 10.5 or 10.6.

With 3.2a4, OS X 10.4 Tiger buildbot failures reported:
======================================================================
ERROR: test_run_code (test.test_cmd_line.CmdLineTest)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/test/test_cmd_line.py", line 95, in test_run_code
 assert_python_failure('-c')
 File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/test/script_helper.py", line 55, in assert_python_failure
 return _assert_python(False, *args, **env_vars)
 File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/test/script_helper.py", line 29, in _assert_python
 env=env)
 File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/subprocess.py", line 683, in __init__
 self.stdin = io.open(p2cwrite, 'wb', bufsize)
OSError: [Errno 9] Bad file descriptor
======================================================================
ERROR: test_run_module (test.test_cmd_line.CmdLineTest)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/test/test_cmd_line.py", line 72, in test_run_module
 assert_python_failure('-m')
 File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/test/script_helper.py", line 55, in assert_python_failure
 return _assert_python(False, *args, **env_vars)
 File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/test/script_helper.py", line 29, in _assert_python
 env=env)
 File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/subprocess.py", line 683, in __init__
 self.stdin = io.open(p2cwrite, 'wb', bufsize)
OSError: [Errno 9] Bad file descriptor
======================================================================
ERROR: test_version (test.test_cmd_line.CmdLineTest)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/test/test_cmd_line.py", line 48, in test_version
 rc, out, err = assert_python_ok('-V')
 File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/test/script_helper.py", line 48, in assert_python_ok
 return _assert_python(True, *args, **env_vars)
 File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/test/script_helper.py", line 29, in _assert_python
 env=env)
 File "/Users/buildbot/buildarea/3.x.parc-tiger-1/build/Lib/subprocess.py", line 683, in __init__
 self.stdin = io.open(p2cwrite, 'wb', bufsize)
OSError: [Errno 9] Bad file descriptor
On my 10.4 system, it seems to be a race condition: sometimes I see all three of these failures, sometimes just one, sometimes none. Again, only on 10.4 (Tiger), not 10.5 or 10.6. But the 10.4 machine I'm using is by far the slowest of the three so it is possible that could be a factor. Perhaps there is a race condition with cleaning up the p2c pipe from a previous run?

See also msg124254.

> Perhaps there is a race condition with cleaning up the p2c pipe from a 
> previous run?
This would be rather strange, since it would have a different file descriptor. Or it means there's a bug somewgere (perhaps in subprocess, or perhaps even in io.open).
It someone can reproduce even intermittently on their home machine, it would be nice to dig a bit deeper.

The code surrounding the p2c file descriptors in subprocess has changed a bit recently since this issue was filed. someone still needs to come up with a way to reproduce it.

Well, it occurred today again :)
http://www.python.org/dev/buildbot/all/builders/x86%20Tiger%203.x/builds/1884/steps/test/logs/stdio
test test_cmd_line failed -- Traceback (most recent call last):
 File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/test/test_cmd_line.py", line 95, in test_run_code
 assert_python_failure('-c')
 File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/test/script_helper.py", line 57, in assert_python_failure
 return _assert_python(False, *args, **env_vars)
 File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/test/script_helper.py", line 30, in _assert_python
 env=env)
 File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/subprocess.py", line 721, in __init__
 self.stdin = io.open(p2cwrite, 'wb', bufsize)
OSError: [Errno 9] Bad file descriptor
But I would suggest an OS X bug instead. My random guess is that the child process exits very quickly (as you see, it runs "python -c" which is an immediate error), therefore the other end of the pipe gets closed and OS X then returns EBADF when we try to do something with our end of the pipe.

that race condition, if thats what this is, is likely not OS X specific. the child process could complete or close its fds before we reach that code on any OS. We either need to guard these io.open(p2c*...) lines against EBADF OSError's or we should create the io wrappers before _execute_child() is called.

> that race condition, if thats what this is, is likely not OS X
> specific. the child process could complete or close its fds before we
> reach that code on any OS. We either need to guard these
> io.open(p2c*...) lines against EBADF OSError's or we should create the
> io wrappers before _execute_child() is called.
Creating them before _execute_child() is the obvious and simple solution
(if my diagnosis is correct). Is there any downside to doing so?

Off the top of my head I don't _think_ so.. at least for posix. I'm not sure about the implications of the if mswindows: code in there though.

Well we could try a run of it on the buildbots, then :)

Trying it in r87744.

I'm closing it in the hope that it's fixed. If it reappears, we'll have to reopen.