win32: Add implementation of pthread_once

This function will be required to make dav1d_init threadsafe.

Full erro checking support is included in this implementation,
for completeness, even though dav1d_init cannot fail, and thus
will ignore the return value of pthread_once.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

src/thread.h

@@ -32,8 +32,11 @@
 
 #include <windows.h>
 
+#define PTHREAD_ONCE_INIT INIT_ONCE_STATIC_INIT
+
 typedef CRITICAL_SECTION pthread_mutex_t;
 typedef CONDITION_VARIABLE pthread_cond_t;
+typedef INIT_ONCE pthread_once_t;
 typedef void *pthread_t;
 typedef void *pthread_mutexattr_t;
 typedef void *pthread_condattr_t;
@@ -54,6 +57,8 @@ int pthread_create(pthread_t* thread, const pthread_attr_t* attr,
                    void*(*proc)(void*), void* param);
 void pthread_join(pthread_t thread, void** res);
 
+int pthread_once(pthread_once_t *once_control, void (*init_routine)(void));
+
 #else
 
 #include <pthread.h>

src/win32/thread.c

@@ -125,4 +125,23 @@ void pthread_join(pthread_t thread, void** res)
     free(th);
 }
 
+int pthread_once(pthread_once_t *once_control, void (*init_routine)(void))
+{
+    BOOL fPending = FALSE;
+    BOOL fStatus;
+
+    fStatus = InitOnceBeginInitialize(once_control, 0, &fPending, NULL);
+    if (fStatus != TRUE)
+        return EINVAL;
+
+    if (fPending == TRUE)
+        init_routine();
+
+    fStatus = InitOnceComplete(once_control, 0, NULL);
+    if (!fStatus)
+        return EINVAL;
+
+    return 0;
+}
+
 #endif