bug#72689: 31.0.50; Proposal to improve string-pixel-width

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 20 Aug 2024 17:12:45 +0200
> Cc: 72689@debbugs.gnu.org
> From: David Ponce <da_vid@orange.fr>
> 
> On 18/08/2024 11:15 AM, Eli Zaretskii wrote:
> [...]
> >>> Thanks.  The idea SGTM, but I think the implementation needs to cater
> >>> for the case where more than one execution thread performs this job
> >>> "in parallel" (however improbable this could sound), so we need to be
> >>> able to detect when this buffer is "busy".  The simplest way is to use
> >>> some boolean buffer-local variable, which will be set non-nil when the
> >>> function starts using the buffer and reset to nil when the function is
> >>> done with its job.
> 
> I've been thinking more about the parallelism issue when a function
> reuses a temporary buffer for its activity, and I wonder if we could
> use a simple API like the one below to safely get an exclusive working
> buffer without having to create a new one on each call?

Thanks, but using a mutex is overkill: there could be no race between
two or more threads in this case in accessing the buffer-local
variable, because only one Lisp thread can be running at any given
time.  So the simpler method of testing the "busy" flag should be
sufficient.

> Compared to my previous proposal the quick benchmark above shows
> similar results for both performance and memory usage, but the new
> implementation is simpler, and the API might be useful in other
> similar cases.

Simpler implementation is OK, but I think it will be simpler yet if
you remove the mutex, which is not needed.