threads: introduce `git_thread_exit`

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 *`.

src/global.h

@@ -16,6 +16,12 @@ typedef struct {
 	git_error error_t;
 	git_buf error_buf;
 	char oid_fmt[GIT_OID_HEXSZ+1];
+
+	/* On Windows, this is the current child thread that was started by
+	 * `git_thread_create`.  This is used to set the thread's exit code
+	 * when terminated by `git_thread_exit`.  It is unused on POSIX.
+	 */
+	git_thread *current_thread;
 } git_global_st;
 
 #ifdef GIT_OPENSSL

src/unix/pthread.h

@@ -17,6 +17,8 @@ typedef struct {
 	pthread_create(&(git_thread_ptr)->thread, NULL, start_routine, arg)
 #define git_thread_join(git_thread_ptr, status) \
 	pthread_join((git_thread_ptr)->thread, status)
+#define git_thread_currentid() ((size_t)(pthread_self()))
+#define git_thread_exit(retval) pthread_exit(retval)
 
 /* Git Mutex */
 #define git_mutex pthread_mutex_t

src/win32/thread.c

@@ -26,6 +26,9 @@ static DWORD WINAPI git_win32__threadproc(LPVOID lpParameter)
 {
 	git_thread *thread = lpParameter;
 
+	/* Set the current thread for `git_thread_exit` */
+	GIT_GLOBAL->current_thread = thread;
+
 	thread->result = thread->proc(thread->param);
 
 	git__free_tls_data();
@@ -95,6 +98,21 @@ int git_thread_join(
 	return 0;
 }
 
+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);
+}
+
+size_t git_thread_currentid(void)
+{
+	return GetCurrentThreadId();
+}
+
 int git_mutex_init(git_mutex *GIT_RESTRICT mutex)
 {
 	InitializeCriticalSection(mutex);

src/win32/thread.h

@@ -41,6 +41,8 @@ int git_thread_create(git_thread *GIT_RESTRICT,
 	void *(*) (void *),
 	void *GIT_RESTRICT);
 int git_thread_join(git_thread *, void **);
+size_t git_thread_currentid(void);
+void git_thread_exit(void *);
 
 int git_mutex_init(git_mutex *GIT_RESTRICT mutex);
 int git_mutex_free(git_mutex *);

tests/threads/basic.c

@@ -48,3 +48,30 @@ void test_threads_basic__set_error(void)
 {
 	run_in_parallel(1, 4, set_error, NULL, NULL);
 }
+
+static void *return_normally(void *param)
+{
+	return param;
+}
+
+static void *exit_abruptly(void *param)
+{
+	git_thread_exit(param);
+	return NULL;
+}
+
+void test_threads_basic__exit(void)
+{
+	git_thread thread;
+	void *result;
+
+	/* Ensure that the return value of the threadproc is returned. */
+	cl_git_pass(git_thread_create(&thread, return_normally, (void *)424242));
+	cl_git_pass(git_thread_join(&thread, &result));
+	cl_assert_equal_sz(424242, (size_t)result);
+
+	/* Ensure that the return value of `git_thread_exit` is returned. */
+	cl_git_pass(git_thread_create(&thread, return_normally, (void *)232323));
+	cl_git_pass(git_thread_join(&thread, &result));
+	cl_assert_equal_sz(232323, (size_t)result);
+}
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