Fix assertion race condition in debug mode

Commit 21d206f3 added mutex assertions.

However, the "locker" variable to trace the locker thread id was read
and written by several threads without any protection, so it was racy.

Reported by TSAN.

app/src/util/thread.c

@@ -31,7 +31,7 @@ sc_mutex_init(sc_mutex *mutex) {
 
     mutex->mutex = sdl_mutex;
 #ifndef NDEBUG
-    mutex->locker = 0;
+    atomic_init(&mutex->locker, 0);
 #endif
     return true;
 }
@@ -52,7 +52,8 @@ sc_mutex_lock(sc_mutex *mutex) {
         abort();
     }
 
-    mutex->locker = sc_thread_get_id();
+    atomic_store_explicit(&mutex->locker, sc_thread_get_id(),
+                          memory_order_relaxed);
 #else
     (void) r;
 #endif
@@ -62,7 +63,7 @@ void
 sc_mutex_unlock(sc_mutex *mutex) {
 #ifndef NDEBUG
     assert(sc_mutex_held(mutex));
-    mutex->locker = 0;
+    atomic_store_explicit(&mutex->locker, 0, memory_order_relaxed);
 #endif
     int r = SDL_UnlockMutex(mutex->mutex);
 #ifndef NDEBUG
@@ -83,7 +84,9 @@ sc_thread_get_id(void) {
 #ifndef NDEBUG
 bool
 sc_mutex_held(struct sc_mutex *mutex) {
-    return mutex->locker == sc_thread_get_id();
+    sc_thread_id locker_id =
+        atomic_load_explicit(&mutex->locker, memory_order_relaxed);
+    return locker_id == sc_thread_get_id();
 }
 #endif
 
@@ -112,7 +115,8 @@ sc_cond_wait(sc_cond *cond, sc_mutex *mutex) {
         abort();
     }
 
-    mutex->locker = sc_thread_get_id();
+    atomic_store_explicit(&mutex->locker, sc_thread_get_id(),
+                          memory_order_relaxed);
 #else
     (void) r;
 #endif
@@ -127,7 +131,8 @@ sc_cond_timedwait(sc_cond *cond, sc_mutex *mutex, uint32_t ms) {
         abort();
     }
 
-    mutex->locker = sc_thread_get_id();
+    atomic_store_explicit(&mutex->locker, sc_thread_get_id(),
+                          memory_order_relaxed);
 #endif
     assert(r == 0 || r == SDL_MUTEX_TIMEDOUT);
     return r == 0;

app/src/util/thread.h

@@ -3,6 +3,7 @@
 
 #include "common.h"
 
+#include <stdatomic.h>
 #include <stdbool.h>
 #include <stdint.h>
 
@@ -12,7 +13,8 @@ typedef struct SDL_mutex SDL_mutex;
 typedef struct SDL_cond SDL_cond;
 
 typedef int sc_thread_fn(void *);
-typedef unsigned int sc_thread_id;
+typedef unsigned sc_thread_id;
+typedef atomic_uint sc_atomic_thread_id;
 
 typedef struct sc_thread {
     SDL_Thread *thread;
@@ -21,7 +23,7 @@ typedef struct sc_thread {
 typedef struct sc_mutex {
     SDL_mutex *mutex;
 #ifndef NDEBUG
-    sc_thread_id locker;
+    sc_atomic_thread_id locker;
 #endif
 } sc_mutex;