Re: master 4ac4cec652f: Prevent freezes on macOS (bug#69561)

["Gerd Möllmann" <gerd.moellmann@gmail.com>]

Po Lu <luangruo@yahoo.com> writes:

> Gerd Moellmann <gerd@gnu.org> writes:
>
>>    if (NSApp == nil
>>        || ![NSThread isMainThread]
>>        || (timeout && timeout->tv_sec == 0 && timeout->tv_nsec == 0))
>> -    return thread_select (pselect, nfds, readfds, writefds,
>> -			  exceptfds, timeout, sigmask);
>> +    thread_select (pselect, nfds, readfds, writefds,
>> +		   exceptfds, timeout, sigmask);
>
> Far be it from me to offer advice on code I know very little about, but
> on the surface this appears to force non-main threads to enter [NSApp
> run] whenever they enter select.  Is this really the desired behavior?
>
> I should think that this function, being provided by the toolkit for the
> purpose of running its event loop, must be called by strictly one thread
> at a time.

Well, let me first make the usual disclaimer that I don't know much
about about Cocoa. (And don't really want to, I must admit ;-)).

I couldn't find much advice which I'd call definite on using Cocoa from
multiple threads, except

  https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/Multithreading/ThreadSafetySummary/ThreadSafetySummary.html#:~:text=Guidelines%20for%20using%20Cocoa%20from,are%20generally%20not%20thread%2Dsafe.

I interpret this as meaning that it's possible to do this, considering
that Emacs' threads run one at a time only, AFAIU. If that's no longer
the case, we have a problem, I agree.