[Python-Dev] Status of thread cancellation (original) (raw)

"Martin v. Löwis" martin at v.loewis.de
Thu Mar 15 17:24:31 CET 2007

Jean-Paul Calderone schrieb:

I inferred from Martin's proposal that he expected the thread to be able to catch the exception. Perhaps he can elaborate on what cleanup actions the dying thread will be allowed to perform. Perhaps he can. Hopefully, he can specifically address these points: 1. A thread can throw a ThreadDeath exception almost anywhere. All synchronized methods and blocks would have to be studied in great detail, with this in mind. 2. A thread can throw a second ThreadDeath exception while cleaning up from the first (in the catch or finally clause). Cleanup would have to repeated till it succeeded. The code to ensure this would be quite complex.

Clearly, a thread need to have its finally blocks performed in response to a cancellation request. These issues are real, however, they apply to any asynchronous exception, not just to thread cancellation.

In Python, we already have an asynchronous exception: KeyboardInterrupt. This suffers from the same problems: a KeyboadInterrupt also can occur at any point, interrupting code in the middle of its finally-blocks. The other exception that is nearly-asynchronous is OutOfMemoryError, which can occur at nearly any point (but of course, never occurs in practice).

So yes, it would be good if Python's exception handling supported asynchronous exceptions in a sensible way. I have to research somewhat more, but I think the standard solution to the problem in operating system (i.e. disabling interrupts at certain points, explicitly due to code or implicitly as a result of entering the interrupt handler) may apply.

Regards, Martin

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