Eryk Sun added the comment:
Due to a race condition, the Popen call in the second process is inadvertently
inheriting the handle for the write end of the pipe that's created for the
first process. Thus stdout.readline() in the first thread doesn't see EOF until
that handle is closed.
For the 2nd process, since you don't need to inherit standard handles, you can
pass close_fds=True. In general if you do need to inherit standard handles in
processes that are created concurrently, you can synchronize on a lock to
ensure Popen properly closes inheritable handles. For example:
_execute_lock = threading.Lock()
def __init__(self, *args, **kwds):
super(Popen, self).__init__(*args, **kwds)
In Python 3 this should be addressed by implementing the suggestion in issue
19764 to use PROC_THREAD_ATTRIBUTE_HANDLE_LIST.
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