Re: [PATCH 5/4] run-command: implement abort_async for pthreads

[Erik Faye-Lund <kusmabite@gmail.com>]

On Fri, Apr 1, 2011 at 9:42 PM, Johannes Sixt <j6t@kdbg.org> wrote:
> On Freitag, 1. April 2011, Erik Faye-Lund wrote:
>> On Fri, Apr 1, 2011 at 7:27 PM, Johannes Sixt <j6t@kdbg.org> wrote:
>> > On Freitag, 1. April 2011, Erik Faye-Lund wrote:
>> >> OK, I've read up on thread-cancellation, and this code seems correct.
>> >> pthread_cancel doesn't kill the thread right away, it just signals a
>> >> cancellation-event, which is checked for at certain
>> >> cancellation-points. A lot of the CRT functions are defined as
>> >> cancellation points, so it'll be a matter for us Win32-guys to
>> >> implement pthread_testcancel() and inject that into the
>> >> function-wrappers of the CRT functions that are marked as
>> >> cancellation-points.
>> >
>> > That's not going to happen. We cannot implement pthread_cancel() on
>> > Windows because it would have to be able to interrupt blocking system
>> > calls. (TerminateThread() is a no-no, given all the caveats about leaking
>> > system resources that are mentioned in the manual.)
>>
>> Did you read my suggestion?
>
> Yes, I did.
>
>> I was talking about implementing
>> cancellation-points, just like on other platforms. This should not
>> lead to TerminateThread, but instead a conditional ExitThread from the
>> thread in question.
>>
>> Something like this (I've only added a cancellation-point at close,
>> just to illustrate what I mean):
>
> But this does not help the case at hand in any way. How would you interrupt a
> thread that is blocked in ReadFile()? The point of pthread_cancel() is that
> it interrupts blocked system calls

There is no mention of such a guarantee in POSIX (section 2.9.5 Thread
Cancellation), so relying on that is undefined behavior.