Feature Requests item #502236, was opened at 2002-01-11 03:46 Message generated for change (Comment added) made by akuchling You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=355470&aid=502236&group_id=5470
Please note that this message will contain a full copy of the comment thread, including the initial issue submission, for this request, not just the latest update. >Category: Extension Modules >Group: None Status: Open Resolution: None Priority: 5 Private: No Submitted By: paul rubin (phr) Assigned to: Nobody/Anonymous (nobody) >Summary: Asynchronous exceptions between threads Initial Comment: I may be missing something but there doesn't seem to be any easy, correct way to exit a Python application and shut down the interpreter. sys.exit() raises the SysExit exception in the current thread but other threads keep running, so the application doesn't stop. You can do something brutal like os.kill() your process, but then cleanup actions (finally clauses) don't get run. Or you can create some elaborate application-specific framework for propagating an exit flag from one thread to all the rest. That's incredibly lame. What's needed is a simple function (maybe sys.allexit) that raises SysExit in all the threads of the application (it might have to first stop all the threads using the GIL or something like that). I'm surprised there isn't already something like this, but there's been some c.l.py discussion about it and it really seems to not be there. Is there some problem with the idea? I don't see one but I can't be sure. ---------------------------------------------------------------------- >Comment By: A.M. Kuchling (akuchling) Date: 2006-12-21 09:52 Message: Logged In: YES user_id=11375 Originator: NO Changing summary to be clearer; reclassifying as 'Feature Request' type. ---------------------------------------------------------------------- Comment By: paul rubin (phr) Date: 2002-01-15 14:40 Message: Logged In: YES user_id=72053 I don't feel qualified to write such a PEP. You guys are more on top of this than I am. Maybe there's no portable way to do asynchronous exceptions between threads. I don't know what happens if you hit ctrl-C when running a multi- threaded Linux program. I don't think ctrl-C even works in Windows--all you can really do is bring up the task manager and blow away the whole process. But I think asynchronous exceptions are worth having even if they're OS specific and only work on some systems. As for how a thread can exit the whole program, I thought Guido's first message (saying raise some asynchronous exception in all the threads) sounded fine, if it's feasible. ---------------------------------------------------------------------- Comment By: Guido van Rossum (gvanrossum) Date: 2002-01-15 14:33 Message: Logged In: YES user_id=6380 (1) I think subclassing SystemExit can work. (2) But the difference is that killing the main thread without giving it a chance to clean up is worse than killing another thread. Does anybody want to write a PEP on thread cancellation? It sure looks like a hairy issue! ---------------------------------------------------------------------- Comment By: Martin v. Löwis (loewis) Date: 2002-01-14 03:40 Message: Logged In: YES user_id=21627 On (1): This is quite unfortunate, as I do think sys.exit and thread.exit should do different things. There would be approaches of further subclassing SystemExit; preserving the property that raising SystemExit in a thread exits the thread only - would such a solution be acceptable? On (2): This is no different from raising SystemExit in the main thread, which also does not invoke finally clauses in other threads. I don't think there can be a complete, reliable implementation of thread cancellation, only a best-effort approach. For POSIX, I'd suggest sending SIGUSR2 to each thread through pthread_kill. We might need to add a mechanism indicating whether a thread is ready to be cancelled, similar to the POSIX cancellation points. That would prevent the signal from arriving in an arbitrary library function, and defer thread termination until the library function completes. For blocking system calls, sending signals would need to be activated explicitly, to allow aborting them. ---------------------------------------------------------------------- Comment By: Guido van Rossum (gvanrossum) Date: 2002-01-12 15:03 Message: Logged In: YES user_id=6380 (1) It's too late to change the meaning of SystemExit -- people are relying on SystemExit exiting the thread only. (2) If we provide a way for a thread to exit the whole program, how will finally clauses in other threads (including the main thread) be executed? ---------------------------------------------------------------------- Comment By: Martin v. Löwis (loewis) Date: 2002-01-11 20:05 Message: Logged In: YES user_id=21627 Tim, I'm not surprised that raising SystemExit in a thread fails to exit Python; the reason simply is that threadmodule.c:t_bootstrap choses to ignore PyExc_SystemExit, when it instead should invoke PyErr_PrintEx instead. The bug is obviously that exit_thread also raises SystemExit, when people apparently really want sys.exit and thread.exit to be two different things. So I think thread.exit should raise ThreadExit, and raising SystemExit in a thread should cause Py_Exit to be invoked (along with all other necessary cleanup). In short, 2.12 of threadmodule.c was wrong, IMO. ---------------------------------------------------------------------- Comment By: Tim Peters (tim_one) Date: 2002-01-11 16:16 Message: Logged In: YES user_id=31435 Martin, change your program to do sys.exit(1) inside func() on the first iteration (or raise SystemExit similarly), and whether you're on Windows or Linux you should find that while the func() thread goes away, the main program keeps on running. If you do os._exit(1) instead, on Windows the main program does go away, but Guido said it doesn't on Linux (which didn't surprise me, given Linux's conflation of threads with processes). See also the "Killing Threads" thread in Python-Dev from late May of 2001, which said all the same things. The only thing I may <wink> have learned since then is that supposedly the *C* exit() function terminates all threads on Linux, via an atexit handler installed by libc, and so _exit() on Linux doesn't terminate all threads because _exit () doesn't call atexit functions. ---------------------------------------------------------------------- Comment By: Martin v. Löwis (loewis) Date: 2002-01-11 11:19 Message: Logged In: YES user_id=21627 Can you produce an example demonstrating the problem? Please see the attached thr.py. It terminates fine; I fail to see the problem. When SystemExit is raised, it will eventually invoke the C library's exit(). If that is not incredibly lame, it will terminate all threads. ---------------------------------------------------------------------- Comment By: Guido van Rossum (gvanrossum) Date: 2002-01-11 10:46 Message: Logged In: YES user_id=6380 I'm struggling with this in a threaded distributed app we're developing for Zope Corp, so I agree that something like this would be nice. I think for starters, there should be a way to raise an asynchronous exception in a specific thread; all threads is easy then. There are plenty of implementation problems with this, however: if a thread is blocked in I/O, there's no portable way to get it to bail out of that I/O operation. There are also plenty of application-level issues: if asynchronous exceptions are common, maybe it is necessary to provide a way to temporarily block such exceptions in order to provide safety in some situations. Etc., etc. So please spare us your surprise and help looking for a mechanism that can be implemented. ---------------------------------------------------------------------- You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=355470&aid=502236&group_id=5470 _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
