global: convert to fiber-local storage to fix exit races

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.

src/global.c

@@ -141,14 +141,21 @@ static void shutdown_common(void)
  */
 #if defined(GIT_THREADS) && defined(GIT_WIN32)
 
-static DWORD _tls_index;
+static DWORD _fls_index;
 static volatile LONG _mutex = 0;
 
+static void WINAPI fls_free(void *st)
+{
+	git__global_state_cleanup(st);
+	git__free(st);
+}
+
 static int synchronized_threads_init(void)
 {
 	int error;
 
-	_tls_index = TlsAlloc();
+	if ((_fls_index = FlsAlloc(fls_free)) == FLS_OUT_OF_INDEXES)
+		return -1;
 
 	git_threads_init();
 
@@ -190,9 +197,7 @@ int git_libgit2_shutdown(void)
 	if ((ret = git_atomic_dec(&git__n_inits)) == 0) {
 		shutdown_common();
 
-		git__free_tls_data();
-
-		TlsFree(_tls_index);
+		FlsFree(_fls_index);
 		git_mutex_free(&git__mwindow_mutex);
 
 #if defined(GIT_MSVC_CRTDBG)
@@ -213,7 +218,7 @@ git_global_st *git__global_state(void)
 
 	assert(git_atomic_get(&git__n_inits) > 0);
 
-	if ((ptr = TlsGetValue(_tls_index)) != NULL)
+	if ((ptr = FlsGetValue(_fls_index)) != NULL)
 		return ptr;
 
 	ptr = git__calloc(1, sizeof(git_global_st));
@@ -222,43 +227,10 @@ git_global_st *git__global_state(void)
 
 	git_buf_init(&ptr->error_buf, 0);
 
-	TlsSetValue(_tls_index, ptr);
+	FlsSetValue(_fls_index, ptr);
 	return ptr;
 }
 
-/**
- * Free the TLS data associated with this thread.
- * This should only be used by the thread as it
- * is exiting.
- */
-void git__free_tls_data(void)
-{
-	void *ptr = TlsGetValue(_tls_index);
-	if (!ptr)
-		return;
-
-	git__global_state_cleanup(ptr);
-	git__free(ptr);
-	TlsSetValue(_tls_index, NULL);
-}
-
-BOOL WINAPI DllMain(HINSTANCE hInstDll, DWORD fdwReason, LPVOID lpvReserved)
-{
-	GIT_UNUSED(hInstDll);
-	GIT_UNUSED(lpvReserved);
-
-	/* This is how Windows lets us know our thread is being shut down */
-	if (fdwReason == DLL_THREAD_DETACH) {
-		git__free_tls_data();
-	}
-
-	/*
-	 * Windows pays attention to this during library loading. We don't do anything
-	 * so we trivially succeed.
-	 */
-	return TRUE;
-}
-
 #elif defined(GIT_THREADS) && defined(_POSIX_THREADS)
 
 static pthread_key_t _tls_key;

src/global.h

@@ -35,8 +35,6 @@ typedef void (*git_global_shutdown_fn)(void);
 
 extern void git__on_shutdown(git_global_shutdown_fn callback);
 
-extern void git__free_tls_data(void);
-
 extern const char *git_libgit2__user_agent(void);
 extern const char *git_libgit2__ssl_ciphers(void);
 

src/win32/thread.c

@@ -32,8 +32,6 @@ static DWORD WINAPI git_win32__threadproc(LPVOID lpParameter)
 
 	thread->result = thread->proc(thread->param);
 
-	git__free_tls_data();
-
 	return CLEAN_THREAD_EXIT;
 }
 
@@ -103,9 +101,6 @@ void git_thread_exit(void *value)
 {
 	assert(GIT_GLOBAL->current_thread);
 	GIT_GLOBAL->current_thread->result = value;
-
-	git__free_tls_data();
-
 	ExitThread(CLEAN_THREAD_EXIT);
 }