Re: Callbacks from modules

[Eli Zaretskii <eliz@gnu.org>]

> From: Philipp Stephani <p.stephani2@gmail.com>
> Date: Thu, 26 Nov 2015 18:16:13 +0000
> Cc: Eli Zaretskii <eliz@gnu.org>, Daniel Colascione <dancol@dancol.org>
> 
> There are two major issues with allowing callbacks at arbitrary times, which
> need to be discussed and resolved first. The first is that such a facility
> would change the execution model of Emacs in a quite fundamental way. Right now
> all user-defined code inside the Emacs process gets run through a path under
> the control of Emacs, either as a response to an input event, or a timer, or a
> process filter etc. Adding a facility to run module code from arbitrary
> callbacks would remove this invariant, which can have global effects on Emacs.

I don't even understand what is meant by "arbitrary callbacks".  If
they are called in the context of a separate thread, we will not allow
them to run.

IOW, such arbitrary callbacks need to run in the C land, and
communicate with Lisp via timers.  In the future, we might add
facilities that would allow a module register a file descriptor
through which events could flow into the Emacs event queue.  But that
is not yet available, and I'm not sure we'd want to allow it.