[PATCH v2 1/2] compat/win32/pthread: add pthread_once()

[Matheus Tavares <matheus.bernardino@usp.br>]

Add pthread_once() emulation for Windows. This function provides an easy
way to do thread-safe one-time initializations in multithreaded code. It
will be used in the next commit to make hash_to_hex_algop()
thread-safe.

The pthread_once() implementation added comes from libav
(https://git.libav.org/?p=libav.git), commit b22693b ("w32pthreads: Add
pthread_once emulation", 2015-10-07). The code is licensed under
LGPLv2.1 which is compatible with GPLv2. Only the section for support on
Windows Vista+ has been ported, as that's the minimum version required
to build Git for Windows.  Also, the code was modified to (1) check and
return error codes; and (2) do not call InitOnceComplete() again after a
successful initialization, as that results in an error.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
---
 compat/win32/pthread.c | 18 ++++++++++++++++++
 compat/win32/pthread.h |  5 +++++
 thread-utils.c         | 11 +++++++++++
 thread-utils.h         |  7 +++++++
 4 files changed, 41 insertions(+)

diff --git a/compat/win32/pthread.c b/compat/win32/pthread.c
index 2e7eead42c..cb32f8c504 100644
--- a/compat/win32/pthread.c
+++ b/compat/win32/pthread.c
@@ -56,3 +56,21 @@ pthread_t pthread_self(void)
 	t.tid = GetCurrentThreadId();
 	return t;
 }
+
+/* Adapted from libav's compat/w32pthreads.h. */
+int pthread_once(pthread_once_t *once_control, void (*init_routine)(void))
+{
+	BOOL pending = FALSE;
+	int ret = 0;
+
+	if (!InitOnceBeginInitialize(once_control, 0, &pending, NULL)) {
+		ret = err_win_to_posix(GetLastError());
+	} else if (pending) {
+		init_routine();
+		if (!InitOnceComplete(once_control, 0, NULL))
+			ret = err_win_to_posix(GetLastError());
+	}
+
+	/* POSIX doesn't allow pthread_once() to return EINTR */
+	return ret == EINTR ? EIO : ret;
+}
diff --git a/compat/win32/pthread.h b/compat/win32/pthread.h
index 737983d00b..c50f1e89c7 100644
--- a/compat/win32/pthread.h
+++ b/compat/win32/pthread.h
@@ -40,6 +40,11 @@ typedef int pthread_mutexattr_t;
 #define pthread_cond_signal WakeConditionVariable
 #define pthread_cond_broadcast WakeAllConditionVariable
 
+#define pthread_once_t INIT_ONCE
+
+#define PTHREAD_ONCE_INIT INIT_ONCE_STATIC_INIT
+int pthread_once(pthread_once_t *once_control, void (*init_routine)(void));
+
 /*
  * Simple thread creation implementation using pthread API
  */
diff --git a/thread-utils.c b/thread-utils.c
index 5329845691..937deb3f2e 100644
--- a/thread-utils.c
+++ b/thread-utils.c
@@ -122,4 +122,15 @@ int dummy_pthread_join(pthread_t pthread, void **retval)
 	return ENOSYS;
 }
 
+int nothreads_pthread_once(pthread_once_t *once_control,
+			   void (*init_routine)(void))
+{
+	if (*once_control == 1)
+		return 0;
+
+	init_routine();
+	*once_control = 1;
+	return 0;
+}
+
 #endif
diff --git a/thread-utils.h b/thread-utils.h
index 4961487ed9..8f9786217a 100644
--- a/thread-utils.h
+++ b/thread-utils.h
@@ -19,6 +19,7 @@
 #define pthread_mutex_t int
 #define pthread_cond_t int
 #define pthread_key_t int
+#define pthread_once_t int
 
 #define pthread_mutex_init(mutex, attr) dummy_pthread_init(mutex)
 #define pthread_mutex_lock(mutex)
@@ -48,6 +49,12 @@ int dummy_pthread_join(pthread_t pthread, void **retval);
 
 int dummy_pthread_init(void *);
 
+#define PTHREAD_ONCE_INIT 0
+#define pthread_once(once, routine) nothreads_pthread_once(once, routine)
+
+int nothreads_pthread_once(pthread_once_t *once_control,
+			   void (*init_routine)(void));
+
 #endif
 
 int online_cpus(void);
-- 
2.26.2