[Zion] Add a semaphore primitive with related syscalls.

zion/object/semaphore.cpp

@@ -0,0 +1,29 @@
+#include "object/semaphore.h"
+
+#include "scheduler/scheduler.h"
+
+glcr::RefPtr<Semaphore> Semaphore::Create() {
+  return glcr::AdoptPtr<Semaphore>(new Semaphore());
+}
+
+// FIXME: We almost certainly have some race conditions
+// between this and unlock where we could end up with
+// a thread in the blocked_threads_ queue while noone is holding the lock.
+void Semaphore::Signal() {
+  __atomic_fetch_add(&lock_, 0x1, __ATOMIC_SEQ_CST);
+  if (blocked_threads_.size() > 0) {
+    auto thread = blocked_threads_.PopFront();
+    thread->SetState(Thread::RUNNABLE);
+    gScheduler->Enqueue(thread);
+  }
+}
+
+void Semaphore::Wait() {
+  while (lock_ == 0) {
+    auto thread = gScheduler->CurrentThread();
+    thread->SetState(Thread::BLOCKED);
+    blocked_threads_.PushBack(thread);
+    gScheduler->Yield();
+  }
+  __atomic_fetch_sub(&lock_, 0x1, __ATOMIC_SEQ_CST);
+}