Re: Releasing the thread global_lock from the module API

[Eli Zaretskii <eliz@gnu.org>]

> From: Spencer Baugh <sbaugh@janestreet.com>
> Date: Fri, 01 Mar 2024 16:21:03 -0500
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Where would you take the data for opening the socket? doesn't that
> > come from some Lisp program or from a Lisp variable?  And how would
> > you know what kind of request to send? doesn't that come from Lisp as
> > well?
> 
> Yes: I get those things as arguments from Lisp and then convert them
> into the native datastructures of my library, which can be used without
> further interacting with the Lisp machine.
> 
> Then I would release the lock and call into my library, which does some
> useful work which takes a while.

How is this different from starting your own native thread, then
releasing the lock?

> > And what would you do with the stuff you get in response? don't
> > you want to put this in some Lisp variable or provide as input for
> > some Lisp program?  Etc. etc.
> 
> Yes: After I finish the call into my library, I would take the lock
> again and call further Lisp functions to put the results back into the
> Lisp machine.

How is this different from doing the same when your native thread
finishes?

> >> Since it's useful for me, I'd like to write a patch which allow modules
> >> to do this; would it be considered?
> >
> > Once again: what cannot you do from a native thread started by the
> > module?  IOW, why would you need access to the global lock machinery
> > in the first place, if all you want to do is something that is
> > unrelated to Emacs and its global state?
> 
> See above: The call into my library, which takes a while, and is useful,
> does not touch the Lisp machine.  But other code around that call does
> touch the Lisp machine, and so needs to run with the lock.  The ability
> to release and re-acquire the lock means my module doesn't hold the lock
> when it doesn't need to.

You didn't answer my question about doing this from a native thread.

As for re-acquiring the lock: you cannot do that with how the Lisp
threads are currently implemented.  You can only wait for the lock to
be released, and try re-acquiring it; whether you succeed is anyone's
guess, if there are other threads competing for the lock.