bug#39343: 27.0.50; Thrad signal crashes Emacs with sgmentation fault

[Christian Johansson]

Hi

I understand, wish I had the skills to help with that but my C is weak and my 
macOS internals is even weaker.

Sometimes I get another thread issue on macOS. It also crashes Emacs but the 
error is something about try to clear a pointer that doesn’t point to anything 
anymore. Would it be helpful if I try to find a reproducable way of causing 
that and file another issue? It seems to occur non-determinstically so I 
haven’t been able to reproduce it yet. Could perhaps depend on garbage 
collector and maybe some kind of race-condition..

I have two plugins for Emacs that would improve significantly with stable 
thread support but at the moment it seems threads are unstable on macOS. I have 
not experienced any issues on Linux. Async.el works but it adds unecessary 
overhead by launching a separate Emacs process and it would be nice to not need 
it.

Kind regards
Christian

> 29 jan. 2020 kl. 21:42 skrev Alan Third <alan@idiocy.org>:
> 
>> On Wed, Jan 29, 2020 at 12:27:19PM +0100, Christian Johansson wrote:
>> Running following works on Linux but not on macOS.
>> 
>> (let ((loop t)
>>       (iterations 1))
>>   (while loop
>>     (make-thread
>>      (lambda()
>>        (message "Thread %d" iterations)
>>        (signal 'error (list "My error"))))
>>     (setq iterations (1+ iterations))))
>> 
>> Error in console: Fatal error 11: Segmentation faultAbort trap: 6
>> 
>> It seems to be some kind of issue with quitting threads on macOS which is
>> not present on Linux
> 
> It’s a stack overflow caused by the main thread getting stuck in a
> loop processing calls to ns_send_appdefined which must be coming from
> the 2000‐odd threads the script starts.
> 
> Presumably it’s the call to ns_run_loop_break in sys_cond_broadcast
> that’s doing it.
> 
> I don’t think there’s really any way round this other than the massive
> rewrite the NS port needs to make it handle threads correctly.
> 
> -- 
> Alan Third