[Wayland-bugs] [Bug 91766] There is no way to cancel wl_display_dispatch(_pending) call without a server roundtrip

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=91766

            Bug ID: 91766
           Summary: There is no way to cancel
                    wl_display_dispatch(_pending) call without a server
                    roundtrip
           Product: Wayland
           Version: unspecified
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: wayland
          Assignee: wayland-bugs at lists.freedesktop.org
          Reporter: jadahl at gmail.com

Assuming a client is using a multi threaded UI architecture, dispatching on
more than one thread, either using the example API usage
(wl_display_prepare_read(), poll(fds, 1, -1), ...) or the API helpers
wl_display_dispatch() and wl_display_dispatch_queue(), there is currently now
good way to make the dispatch call return so that the client can disconnect.

Calling wl_display_disconnect() simply closes the fd and frees the wl_display
structure, causing the poll() call to stall and likely a segmentation fault if
it leaves it.

Calling pthread_kill() does not work as is, since wl_display_dispatch_queue()
will simply re-poll on EINTR.

What one can do is to set a variable causing the dispatcher thread not to
continue dispatching and then call wl_display_sync(). This would make the
server write data to the fd, causing the dispatcher thread to dispatch and
return. This should IMHO only be considered a temporary work-around.

Potentially better solutions could be:

1) Make sure wl_display_disconnect() waits for all registered readers to
finish, while making the readers using the helpers cancel using some fd
signaling technique like eventfd. A problem with this is that the
wl_display_dispatch(_queue) functions would need to return an error (maybe
ECANCELED is a reasonable one) This has the side effect that existing clients
doing poll(1, -1) and calling wl_display_disconnect() would suddenly start to
dead lock. The dead lock could be avoided however by only waiting for
wl_display_dispatch_pending() callers, meaning letting the other ones simply
continue to deadlock and/or crash in their dispatcher thread as they would
before.

2) Introduce new API for canceling wl_display_dispatch(_queue), letting
wl_display_disconnect() stay thread unsafe, using some method similar to above.
Similar issues as above, but without changing the behavior of
wl_display_disconnect().

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