Don't use __thread, which requires heavy runtime in some cases (such as
MinGW-w64, at least under some configurations, forcing you to link to
its pthread runtime DLL).
The pthread_t struct was needed over a simple thread ID, because
pthread_join() needed to access some sort of context from pthread_t.
Further, pthread_exit() and pthread_detach() need the context of the
current thread, for which we relied on TLS.
Replace these uses by a global thread array. This includes all threads
created by the thread wrapper. Hopefully the number of threads created
by mpv is low (say, below 20), and threads are not that often created or
destroyed. So just keeping them in an array with linear search lookup
should be reasonable.
Windows, as of the Creators Update, finally has a sane API for giving a
name to a thread that can be used by debuggers. This is similar to
pthread_setname_np on Linux and some Unixes. Expose it in the pthread
wrapper and use it for mpthread_set_name().
The license text refers a "above copyright notice", so I guess it'd be
good to actually provide such a notice.
Add the license to some files that were missing it (since in theory, our
Copyright file says that such files are LGPL by default).
Remove the questionable remarks about the license in the client API.
SRW locks are available since Windows Vista. They work essentially like
Linux futexes. In particular, they can be statically initialized, and do
not require deinitialization. This makes them ideal for implementing
PTHREAD_MUTEX_INITIALIZER.
We still need CRITICAL_SECTION for recursive mutexes.
We used double-checked locking on pthread_mutex_t.requires_init in order
to lazily initialize static mutexes (since CRITICAL_SECTION has no
native way to do this). This was kind of unclean: we relied on MSVC
semantics for volatile (which apparently means all accesses are weakly
atomic), which is not such a good idea since mpv can't even be compiled
with MSVC.
Since it's too much of a pain to get weak atomics, just use INIT_ONCE
for initializing the CRITICAL_SECTION. Microsoft most likely implemented
this in an extremely efficient way. Essentially, it provides a mechanism
for correct double-checked locking without having to deal with the
tricky details. We still use an extra flag to avoid calling it at all
for normal locks.
(To get weak atomics, we could have used stdatomic.h, which modern MinGW
provides just fine. But I don't want this wrapper depend on MinGW
specifics if possible.)
