[Python-3000] threading, part 2

Tim Peters tim.peters at gmail.com
Wed Aug 9 21:48:58 CEST 2006

[Nick Coghlan]
>> That check is already there:
>> int PyThreadState_SetAsyncExc(  long id, PyObject *exc)
>>      Asynchronously raise an exception in a thread. The id argument is the
>> thread id of the target thread; exc is the exception object to be
raised. This
>> function does not steal any references to exc. To prevent naive misuse, you
>> must write your own C extension to call this. Must be called with the GIL
>> held. Returns the number of thread states modified; if it returns a number
>> greater than one, you're in trouble, and you should call it again
with exc set
>> to NULL to revert the effect. This raises no exceptions. New in version 2.3.

Guido, do you have any idea now what the "number greater than one"
business is about?  That would happen if and only if we found more
than one thread state with the given thread id in the interpreter's
list of thread states, but we're counting those with both the GIL and
the global head_mutex lock held.  My impression has been that it would
be an internal logic error if we ever saw this count exceed 1.

While I'm at it, I expect:

		p->async_exc = exc;

is better written:

		p->async_exc = exc;

for the same reason one should always incref B before decrefing A in

    A = B


>> All Tober is really asking for is a method on threading.Thread objects that
>> uses this existing API to set a builtin ThreadExit exception. The thread
>> module would consider a thread finishing with ThreadExit to be
>> non-exceptional, so you could easily do:
>>    th.terminate() # Raise ThreadExit in th's thread of control
>>    th.join() # Should finish up pretty quickly
>> Proper resource cleanup would be reliant on correct use of
try/finally or with
>> statements, but that's the case regardless of whether or not asynchronous
>> exceptions are allowed.

> I'm +0 on this.

Me too, although it won't stay that simple, and I'm clear as mud on
how implementations other than CPython could implement this.

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