The here documented guarantee might simplify code using this mechanism
a lot, because it becomes unnecessary to invent a separate mechanism to
make the mp_dispatch_queue_process loop exit after processing a dispatch
callback. (Instead, the dispatch callback can set a flag, and the caller
of mp_dispatch_queue_process can check it.)
The wakeup is needed to make mp_dispatch_queue_process() return if
suspension is not needed anymore - which is only the case when the
request count reaches 0.
The assertion added with this commit always has/had to be true.
Note that this mechanism is similarly "unreliable" as for example
pthread_cond_timedwait(). Trying to target the exact wait time will just
make it more complex.
The main use case for this is for threads which either use the dispatch
centrally and want mp_dispatch_queue_process to do a blocking wait for
new work, or threads which have to implement timers. For the former,
anything is fine, as long as they don't have to do active waiting for
new works. For the former, callers are better off recalculating their
deadline after every call.
This is much simpler, leaves fairness isues etc. to the operating
system, and will work better with threading-related debugging tools.
The "trick" to this is that the lock can be acquired and held only while
the queue is in suspend mode. This way we don't need to make sure the
lock is held outside of mp_dispatch_queue_process, which would be quite
messy to get right, because it would have to be in locked state by
default.
mp_dispatch_queue_process() releases the queue->lock mutex while
processing a dispatch callback. But this allowed mp_dispatch_lock() to
grab the "logical" lock represented by queue->locked. Grabbing the
logical lock is not a problem in itself, but it can't be allowed to
happen while the callback is still running.
Fix this by claiming the logical lock while the dispatch callback is
processed. Also make sure that the thread calling mp_dispatch_lock() is
woken up properly.
Fortunately, this didn't matter, because the locking function is unused.
This was part of osdep/threads.c out of laziness. But it doesn't contain
anything OS dependent. Note that the rest of threads.c actually isn't
all that OS dependent either (just some minor ifdeffery to work around
the lack of clock_gettime() on OSX).
