LibC: Wait for thread before returning from pthread_create

We now make sure the thread starts up and sets its id. There was a race
condition where a process could create a thread and immediately try to
join it. If the thread had not initialized itself the join would fail
with EINVAL as we passed -1 as its id.

userspace/libraries/LibC/pthread.cpp

@@ -99,7 +99,7 @@ extern "C" void _pthread_trampoline_cpp(void* arg)
 {
 	auto info = *static_cast<pthread_trampoline_info_t*>(arg);
 
-	info.uthread->id = syscall(SYS_THREAD_GETID);
+	BAN::atomic_store(info.uthread->id, syscall(SYS_THREAD_GETID));
 
 #if defined(__x86_64__)
 	syscall(SYS_SET_FSBASE, info.uthread);
@@ -500,6 +500,10 @@ int pthread_create(pthread_t* __restrict thread, const pthread_attr_t* __restric
 	if (syscall(SYS_THREAD_CREATE, _pthread_trampoline, info, info->uthread->attr.stackaddr, info->uthread->attr.stacksize) == -1)
 		goto pthread_create_error;
 
+	// wait for the thread to initialize its id
+	while (BAN::atomic_load(result->id) == -1)
+		sched_yield();
+
 	if (thread)
 		*thread = result;
 	return 0;