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On Windows platforms, we automatically clean up the thread-local storage
upon detaching a thread via `DllMain()`. The thing is that this happens
for every thread of applications that link against the libgit2 DLL, even
those that don't have anything to do with libgit2 itself. As a result,
we cannot assume that these unsuspecting threads make use of our
`git_libgit2_init()` and `git_libgit2_shutdow()` reference counting,
which may lead to racy situations:
Thread 1 Thread 2
git_libgit2_shutdown()
DllMain(DETACH_THREAD)
git__free_tls_data()
git_atomic_dec() == 0
git__free_tls_data()
TlsFree(_tls_index)
TlsGetValue(_tls_index)
Due to the second thread never having executed `git_libgit2_init()`, the
first thread will clean up TLS data and as a result also free the
`_tls_index` variable. When detaching the second thread, we
unconditionally access the now-free'd `_tls_index` variable, which is
obviously not going to work out well.
Fix the issue by converting the code to use fiber-local storage instead
of thread-local storage. While FLS will behave the exact same as TLS if
no fibers are in use, it does allow us to specify a destructor similar
to the one that is accepted by pthread_key_create(3P). Like this, we do
not have to manually free indices anymore, but will let the FLS handle
calling the destructor. This allows us to get rid of `DllMain()`
completely, as we only used it to keep track of when threads were
exiting and results in an overall simplification of TLS cleanup.
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GetProcAddress is prototyped to return a `FARPROC`, which is meant to be
a generic function pointer. It's literally `int (FAR WINAPI * FARPROC)()`
which gcc complains if you attempt to cast to a `void (*)(GIT_SRWLOCK *)`.
Cast to a `void *` before casting to avoid warnings about the arguments.
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Next to including several files, our "common.h" header also declares
various macros which are then used throughout the project. As such, we
have to make sure to always include this file first in all
implementation files. Otherwise, we might encounter problems or even
silent behavioural differences due to macros or defines not being
defined as they should be. So in fact, our header and implementation
files should make sure to always include "common.h" first.
This commit does so by establishing a common include pattern. Header
files inside of "src" will now always include "common.h" as its first
other file, separated by a newline from all the other includes to make
it stand out as special. There are two cases for the implementation
files. If they do have a matching header file, they will always include
this one first, leading to "common.h" being transitively included as
first file. If they do not have a matching header file, they instead
include "common.h" as first file themselves.
This fixes the outlined problems and will become our standard practice
for header and source files inside of the "src/" from now on.
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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 *`.
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The old pthread-file did re-implement the pthreads API with exact symbol
matching. As the thread-abstraction has now been split up between Unix- and
Windows-specific files within the `git_` namespace to avoid symbol-clashes
between libgit2 and pthreads, the rewritten wrappers have nothing to do with
pthreads anymore.
Rename the Windows-specific pthread-files to honor this change.
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