Introduce `git_thread_exit`, which will allow threads to terminate at an
arbitrary time, returning a `void *`. On Windows, this means that we
need to store the current `git_thread` in TLS, so that we can set its
`return` value when terminating.
We cannot simply use `ExitThread`, since Win32 returns `DWORD`s from
threads; we return `void *`.
This ensures that when using OpenSSL a safe default set of ciphers
is selected. This is done so that the client communicates securely
and we don't accidentally enable unsafe ciphers like RC4, or even
worse some old export ciphers.
Implements the first part of https://github.com/libgit2/libgit2/issues/3682
Instead of allocating a brand new buffer for each error string we want
to store, we can use a per-thread buffer to store the error string and
re-use the underlying storage. We already use the buffer to format the
string, so this mostly makes that more direct.
The old `allocfmt` is of no use to callers, as they are not able to free
the returned buffer. Export a new API that returns a static string that
doesn't need to be freed.
Bring together all of the OpenSSL initialization to
git_threads_init() so it's together and doesn't need locks.
Moving it here also gives us libssh2 thread safety (when built against
openssl).
The OpenSSL init functions are not reentrant, which means that running
multiple fetches in parallel can cause us to crash.
Use a mutex to init OpenSSL, and since we're adding this extra checks,
init it only once.
Increasingly there are a number of components that want to do some
cleanup at global shutdown time (at least if there are not going
to be memory leaks). This creates a very simple system of shutdown
hooks that will be invoked by git_threads_shutdown. Right now, the
maximum number of hooks is hardcoded, but since adding a hook is
not a public API, it should be fine and I thought it was better to
start off with really simple code.
This removes the lock from the repository object and changes the
internals to use the new atomic git__compare_and_swap to update
the _odb, _config, _index, and _refdb variables in a threadsafe
manner.
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.
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()`.
