[Wayland-bugs] [Bug 91767] wl_display_dispatch(_queue) documentation about thread safeness is probably wrong

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=91767

--- Comment #2 from Marek Chalupa <mchqwerty at gmail.com> ---
(In reply to Jonas Ådahl from comment #0)
> There is documentation about wl_display_dispatch() states one may not mix
> wl_display_dispatch(_queue) with wl_display_prepare_read() and friends, but
> this is probably a common misconception, because internally,
> wl_display_dispatch_queue() is implemented in the same way as one would use
> wl_display_prepare_read() and friends, just without using those functions.
> 
> I have patches in progress that changes wl_display_dispatch_queue() to
> simply use wl_display_prepare_read() and related API internally to make this
> more obvious.

I forgot not only the details :) But looking into the code...

Clearly you cannot do something like:
  wl_display_prepare_read()
  wl_display_dispatch()
in one thread, that would leak to dead-lock.

You're right that the wl_display_dispatch() works on the same basis as the
prepare+read API. I think that they should not be mixed for the same reason you
cannot call wl_display_dispatch_queue() from more threads. That is, there is
(probably) no dead-lock, but thread can get sleeping with events queued.
http://cgit.freedesktop.org/wayland/wayland/tree/src/wayland-client.c#n1454

If you call prepare + poll + read in thread A and wl_display_dispatch_queue()
in another thread B (dispatching two different queues), the fd could become
readable before wl_display_dispatch_queue() calls poll
(http://cgit.freedesktop.org/wayland/wayland/tree/src/wayland-client.c#n1574)
and whole prepare+read sequence could run before it, queing events into both
queues. After prepare+read queued events, thread B goes to sleep in poll with
events hanging in the queue.

Avoiding this behaviour was why wl_display_prepare_read() and
wl_display_read_events() API was added and why programs dispatching events from
more threads should use it, while programs with single dispatcher thread can
use the usual wl_display_dispatch_...()

Implementing wl_display_dispatch(_queue) using prepare+read should fix this
IMHO.

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