This could also use PTHREAD_MUTEX_INITIALIZER, but a dynamic initializer seems like a more portable concept, and we won't need another #define on top of git_mutex_init()
Up to now, the idea was that the user would do all the operations for
one repository in the same thread. Thus we could have the
memory-mapped window information thread-local and avoid any locking.
This is not practical in a few environments, such as Apple's GCD which
allocates threads arbitrarily or the .NET CLR, where the OS-level
thread can change at any moment.
Make the control structure global and protect it with a mutex so we
don't depend on the thread currently executing the code.
This adds a `git_pool` object that can do simple paged memory
allocation with free for the entire pool at once. Using this,
you can replace many small allocations with large blocks that
can then cheaply be doled out in small pieces. This is best
used when you plan to free the small blocks all at once - for
example, if they represent the parsed state from a file or data
stream that are either all kept or all discarded.
There are two real patterns of usage for `git_pools`: either
for "string" allocation, where the item size is a single byte
and you end up just packing the allocations in together, or for
"fixed size" allocation where you are allocating a large object
(e.g. a `git_oid`) and you generally just allocation single
objects that can be tightly packed. Of course, you can use it
for other things, but those two cases are the easiest.
See `global.c` for a description of what we're doing.
When libgit2 is built with GIT_THREADS support, the threading system
must be explicitly initialized with `git_threads_init()`.
