In article <b3ebf4ec-e0bc-4bfc-8c29-368fee488...@l18g2000yqm.googlegroups.com>, Philip Winston <pwins...@gmail.com> wrote:
> We have a multiprocess Python program that uses Queue to communicate > between processes. Recently we've seen some errors while blocked > waiting on Queue.get: > > IOError: [Errno 4] Interrupted system call > > What causes the exception? Unix divides system calls up into "slow" and "fast". The difference is how the react to signals. Fast calls are things which are expected to return quickly. A canonical example would get getuid(), which just returns a number it looks up in a kernel data structure. Fast syscalls cannot be interrupted by signals. If a signal arrives while a fast syscall is running, delivery of the signal is delayed until after the call returns. Slow calls are things which may take an indeterminate amount of time to return. An example would be a read on a network socket; it will block until a message arrives, which may be forever. Slow syscalls get interrupted by signals. If a signal arrives while a slow syscall is blocking, the call returns EINTR. This lets your code have a chance to do whatever is appropriate, which might be clean up in preparation for process shutdown, or maybe just ignore the interrupt and re-issue the system call. Here's a short python program which shows how this works (tested on MacOS-10.6, but should be portable to just about any posix box): ----------------------------------------------------- #!/usr/bin/env python import socket import signal import os def handler(sig_num, stack_frame): return print "my pid is", os.getpid() signal.signal(signal.SIGUSR1, handler) s = socket.socket(type=socket.SOCK_DGRAM) s.bind(("127.0.0.1", 0)) s.recv(1024) ----------------------------------------------------- Run this in one window. It should print out its process number, then block on the recv() call. In another window, send it a SIGUSR1. You should get something like: play$ ./intr.py my pid is 6969 Traceback (most recent call last): File "./intr.py", line 14, in <module> s.recv(1024) socket.error: [Errno 4] Interrupted system call > Is it necessary to catch this exception > and manually retry the Queue operation? Thanks. That's a deeper question which I can't answer. My guess is the interrupted system call is the Queue trying to acquire a lock, but there's no predicting what the signal is. I'm tempted to say that it's a bug in Queue that it doesn't catch this exception internally, but people who know more about the Queue implementation than I do should chime in. > We have some Python 2.5 and 2.6 machines that have run this program > for many 1,000 hours with no errors. But we have one 2.5 machine and > one 2.7 machine that seem to get the error very often. Yup, that's the nature of signal delivery race conditions in multithreaded programs. Every machine will behave a little bit differently, with no rhyme or reason. Google "undefined behavior" for more details :-) The whole posix signal delivery mechanism dates back to the earliest Unix implementations, long before there were threads or networks. At this point, it's got many layers of duct tape. -- http://mail.python.org/mailman/listinfo/python-list