Amrith Kumar added the comment:
After some debugging and reading code in python's subprocess.py (v2.7), here's
what I'm seeing.
(a) the error has nothing to do with eventlet and monkey-patching.
(b) the code in _communicate_with_select() and potentially
_communicate_with_poll() are the problem.
What's the problem?
-------------------
The code in _communicate_with_select() correctly sets up to handle the read and
write calls without blocking on any of them. It does this by establishing the
two (read and write) lists of descriptors and calls select without no timeout
specified.
When select returns, and indicates that a socket is in (for example) the read
list, what that means is that an attempt to read() will not block. However, it
is possible on some systems (Linux for sure) that
(a) a socket is non-blocking, and
(b) a call to select indicates that the socket is ready to read, and
(c) a call to read the socket returns an error EAGAIN (aka EWOULDBLOCK).
Callers of read() and write() on non-blocking sockets should be prepared to
handle this situation.
The python code in _communicate_with_select() is not.
It assumes that if select() returns that a read fd is ready for read, that a
call to read it will produce 0 or more bytes. The calls to read() for stdout
and stderr are not guarded with exception handlers. However, if a socket is
setup as non-blocking, any call can produce EWOULDBLOCK, EAGAIN, ...
Adding some debugging code it was possible to recreate the problem and show
that the backtrace was (in this case it happened with python 2.6)
Traceback (most recent call last):
[...]
File "trove/openstack/common/processutils.py", line 186, in execute
result = obj.communicate()
File "/usr/lib64/python2.6/subprocess.py", line 732, in communicate
stdout, stderr = self._communicate(input, endtime)
File "/usr/lib64/python2.6/subprocess.py", line 1318, in _communicate
stdout, stderr = self._communicate_with_select(input, endtime)
File "/usr/lib64/python2.6/subprocess.py", line 1483, in
_communicate_with_select
data = os.read(self.stdout.fileno(), 1024)
OSError: [Errno 11] Resource temporarily unavailable
The correct fix for this is to make _communicate_with_select() and maybe
_communicate_with_poll() properly handle the read() and write() calls and be
better prepared to handle a thrown condition of EAGAIN from os.read or os.write.
----------
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue22114>
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