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Re: Emacs design and architecture. How about copy-on-write?

From: Ihor Radchenko
Subject: Re: Emacs design and architecture. How about copy-on-write?
Date: Thu, 21 Sep 2023 10:08:21 +0000

Eli Zaretskii <eliz@gnu.org> writes:

>> We will have a window whose point does not reflect its contents on the
>> display.  A supervening redisplay within the main thread will give due
>> consideration to its new value when it does transpire.
> I don't understand why only the main thread is allowed to do that.
> What is special in the main thread wrt redisplay that other threads
> are forbidden from doing that?  Would it be okay to have a separate
> non-main redisplay thread, for example? if not, why not?

Apart from toolkit considerations, consider two asynchronous threads
requesting to display something in the echo area. We cannot allow one
thread "cancel" displaying its message by another thread - this will
lose one of the messages that should be displayed to user. So, there at
least should be some kind of queue for threads trying to display
something in the same window.

Another scenario is one thread displaying multiline message while
another thread displaying a window adjacent to echo area. Let's say that
the thread redisplaying window started running first, considered its
window geometry, and started calculating the glyph matrix according to
the known number of lines fitting that window height. Then, "message"
thread wants to resize echo area to fit its multiline message. What
should the "window" thread do? Should it cancel its redisplay? Restart
it? Wait for the "message" thread to finish? It is not very clear and
likely depend on the specific scenario at hand.

Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
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