From: Ken Raeburn <raeburn@raeburn.org>
To: Stefan Monnier <monnier@iro.umontreal.ca>
Cc: Tom Tromey <tromey@redhat.com>,
Emacs development discussions <emacs-devel@gnu.org>
Subject: Re: advice needed for multi-threading patch
Date: Mon, 28 Sep 2009 22:26:56 -0400 [thread overview]
Message-ID: <F887A742-D6A5-4AA7-92DC-F18D866BC9FC@raeburn.org> (raw)
In-Reply-To: <jwvocou8z5c.fsf-monnier+emacs@gnu.org>
On Sep 28, 2009, at 20:27, Stefan Monnier wrote:
>> In my view, a let-binding should always be thread-local, and I think
>> buffer-localness shouldn't affect this. This is enough of a rule to
>> answer your questions -- but it is just one choice, we could make
>> others, I suppose.
>
> So what should happen in the following case:
> Thread A, running in buffer B1, runs the following code:
>
> (let ((default-directory "/foo"))
> (with-current-buffer B2 default-directory))
>
> You seem to say that it should return "/foo", but currently it returns
> something else. Maybe we should try to come up with a run-time test
> to
> catch such cases in current Lisp code.
I suspect that any sensible approach we come up with is going to
involve a change in behavior that may affect some existing code. So
coming up with ways to detect code that changes meaning is probably
going to be a good idea.
> A related case is when a process
> filter or a sentinel is run via accept-process-output: we'd need to be
> careful to make sure the code is run in the same thread as the code
> that
> called accept-process-output.
Not necessarily; two threads could each be running helper programs in
subprocesses (or using network connections), setting up let-bound
variables, and calling accept-process-output, expecting the process
filters to have access to the let-bound variables. We might want to,
by default, bind the filters to run only in the thread that originally
created the process, so long as it exists. It could be less
efficient, but I think it might easily preserve more of the current
behavior of packages written for single-threaded Emacs, even if they
get run concurrently (but independently) in multiple threads. And we
could make it easy for code to allow other threads to process the
output too.
>> There are some oddities implied by making buffer-local let-bindings
>> also be thread-specific. For example, some buffer-locals affect
>> redisplay, so what the user sees will depend on the thread in which
>> redisplay is run.
>
> Redisplay should be run in a completely separate thread (at least
> conceptually).
I think it's less likely, but a package might also let-bind some
variables that affect the display (e.g., truncate-lines, selective-
display, show-trailing-whitespace, all of which are buffer-local, or
anything referenced via mode-line) and then call y-or-n-p or read-file-
name or some other routine that will trigger redisplay and prompt the
user for some info that may depend on what's displayed.... Be that as
it may, I think making it conceptually a separate thread is probably
the right approach. And this minor incompatibility is something we
can test for now, for the most part -- if redisplay examines a Lisp
variable, it should check for the presence of that variable in specpdl
and cause a warning to be issued later if it's found. (Uses of
":eval" in mode-line-format are probably too expensive to process.)
The other approach that came to my mind was, use the context of the
thread that's currently taking the keyboard input if there is one, to
try to approximate the "right thing" for any code already doing what I
described above. (If one thread is prompting the user for input, and
then another thread triggers redisplay but doesn't prompt for input, I
suspect seeing what's intended by the prompting thread is probably
more important.) Of course that would mean that when switching which
thread "owns" the current keyboard input, you might need to redisplay
everything because of the change of context, or at least redisplay the
affected buffers, which could be many of them, depending on the
behavior of such code as:
(let ((default-directory foo))
(with-current buffer buf1
(let ((default-directory bar))
(with-current-buffer buf2
...repeat ad nauseam...
So... yeah, I think I like the conceptually-separate-thread approach
better, even if it could break existing code. I'm not certain we
couldn't do better in terms of backwards compatibility, but it's
simple and clean.
Ken
next prev parent reply other threads:[~2009-09-29 2:26 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-08-26 4:38 advice needed for multi-threading patch Tom Tromey
2009-08-26 7:06 ` Ken Raeburn
2009-08-26 14:52 ` Stefan Monnier
2009-08-26 18:50 ` Ken Raeburn
2009-08-27 3:12 ` Stefan Monnier
2009-08-27 6:28 ` Ken Raeburn
2009-08-27 17:02 ` Stefan Monnier
2009-08-26 16:08 ` Tom Tromey
2009-08-26 18:50 ` Ken Raeburn
2009-08-27 5:07 ` Miles Bader
2009-08-27 6:39 ` Ken Raeburn
2009-08-27 6:50 ` Miles Bader
2009-08-28 20:48 ` Juri Linkov
2009-08-28 22:15 ` Miles Bader
2009-08-28 23:27 ` Juri Linkov
2009-08-28 23:54 ` Miles Bader
2009-08-29 20:21 ` Richard Stallman
2009-08-26 15:02 ` Stefan Monnier
2009-08-26 15:31 ` Tom Tromey
2009-08-26 19:18 ` Stefan Monnier
2009-09-18 22:59 ` Tom Tromey
2009-09-19 0:09 ` Stephen J. Turnbull
2009-09-19 0:32 ` Chong Yidong
2009-09-21 21:19 ` Stefan Monnier
2009-09-21 21:50 ` Tom Tromey
2009-09-22 14:24 ` Stefan Monnier
2009-09-22 23:59 ` Ken Raeburn
2009-09-23 3:11 ` Stefan Monnier
2009-09-23 15:53 ` Chong Yidong
2009-09-23 3:16 ` Tom Tromey
2009-09-24 17:25 ` Stefan Monnier
2009-09-24 17:57 ` Tom Tromey
2009-09-27 20:59 ` Tom Tromey
2009-09-27 23:05 ` Stefan Monnier
2009-09-28 4:27 ` Tom Tromey
2009-09-29 0:27 ` Stefan Monnier
2009-09-29 2:26 ` Ken Raeburn [this message]
2009-09-29 3:20 ` Stefan Monnier
2009-09-29 3:57 ` Ken Raeburn
2009-09-29 3:33 ` Tom Tromey
2009-09-29 4:07 ` Ken Raeburn
2009-09-29 2:30 ` Tom Tromey
2009-09-23 18:43 ` Giuseppe Scrivano
2009-09-24 17:29 ` Stefan Monnier
2009-09-24 18:53 ` Giuseppe Scrivano
2009-09-24 20:04 ` Tom Tromey
2009-09-24 21:59 ` Stefan Monnier
2009-09-24 22:23 ` Giuseppe Scrivano
2009-09-24 22:47 ` Ken Raeburn
2009-09-28 14:52 ` Ted Zlatanov
2009-10-05 6:02 ` joakim
2009-09-28 7:44 ` Lynbech Christian
2009-08-29 0:28 ` Giuseppe Scrivano
2009-08-29 4:57 ` Tom Tromey
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