This is obnoxiously complicated. If anyone cares about playing_changed or
meta_changed, something similar will probably have to be done.

This is a pretty bizarre two-step system to inform the extension manager
that the input has changed, but it's necessary to avoid a series of
possible deadlocks and other issues. Here are other possible approaches
that don't work:

- Just call into the extension manager in -PlaylistItemChanged on the
main thread. This can pretty easily cause a deadlock if we call
-PlaylistItemChanged twice in quick succession. The first call will poke
the condvar the extension is waiting on, causing the extension thread to
wake up and run extension code; many parts of it -- including the dialog
code -- must be run on the main thread. The extension thread goes back to
sleep while blocking on the main thread to become available, while
holding the extension lock. Meanwhile the main thread goes into the
second call of -PlaylistItemChanged, attempts to lock the extension, and
that's a deadlock.

- Restructure the dialog manager to never block on the main thread while
holding the extension lock. This should work, but as it turns out doesn't
because the main thread will attempt to lock the same lock twice. What
happens is that -performSelectorOnMainThread works by injecting an event
into the main event loop of the main thread. For some unknown reason, as
part of its processing, when creating an NSAttributedString with HTML, it
runs the main event loop, which means we can end up executing one
-performSelectoOnMainThread as part of another. Since the dialog manager
uses attributed strings with HTML (since dialogs are HTML), we deadlock
here too. This seems strictly like a flaw in NSAttributedString and/or
in -performSelectorOnMainThread and is documented elsewhere:
  http://mrrsoftware.com/blog/tag/nsattributedstring/
  https://www.bluestatic.org/blog/2010/05/31/nsattributedstring-spins-a-nested-run-loop/

- Change around this bit of code to not force it to run on the main
thread. This would probably work, but, as a newcomer to VLC, I don't
quite know the implications of doing this, particularly since a lot of
code here seems to serailize on the main thread as a way of thread
safety; it would likely require some somewhat intricate restructuring
and adding of locks.

- Let the extension manager deal with listening for events the same way
that we do here. That would work, but would require duplicating a
nontrivial amount of code from here to deal with tracking the current
input.

- So, instead, we just serialized all calls to -PlaylistItemChanged (so
we make sure to process them in order, with no one trampling
p_input_changed), do most of the work on the main thread as before, and
then actually inform the extension manager out here where we don't block
the main thread. It seems likely that there are other pre-existing
deadlock possibilities here -- the main thread can't lock an extension!
-- but it at least tends to work in my testing.
Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>