Update sema priority compare to take aux optionally as priority of first sema

src/threads/synch.c

@@ -268,22 +268,49 @@ struct semaphore_elem
 /* Function that compares the two *semaphores* associated with the provided
    list_elem structures. [i.e., takes list_elem of semaphore_elem, and]
    Returns true if the thread associated with the semaphore associated with a_
-   has a higher priority than that of b_. */
+   has a higher priority than that of b_.
+
+    If aux is provided, then it is a pointer to an integer representing the
+    priority of the first semaphore. This is useful when the thread has not been
+    sema'd down yet. */
 static bool
 sema_priority_more(const struct list_elem *a_, const struct list_elem *b_,
-                   void *aux UNUSED)
+                   void *aux)
 {
-  struct semaphore_elem *a = list_entry(a_, struct semaphore_elem, elem);
+  int a_priority, b_priority;
+
+  /* If an aux is provided, then use it as the priority of the first semaphore.
+     Otherwise, get the priority of the first semaphore. */
+  if (aux != NULL)
+    a_priority = *(int *) aux;
+  else
+    {
+      struct semaphore_elem *a = list_entry(a_, struct semaphore_elem, elem);
+
+      /* If waiters list is empty, return false (i.e., a has lower priority) */
+      if (list_empty(&a->semaphore.waiters))
+        return false;
+
+      /* Otherwise, get the thread with the highest priority from the waiters
+         list. By design, this is the first one in the list (See sema_down). */
+      struct thread *a_thread =
+        list_entry(list_front(&a->semaphore.waiters), struct thread, elem);
+
+      a_priority = a_thread->priority;
+    }
+
   struct semaphore_elem *b = list_entry(b_, struct semaphore_elem, elem);
 
-  /* Get the highest priority thread from the waiters list of each semaphore. By
-     design, this is the first element in the list (See sema_down). */
-  struct thread *a_thread =
-    list_entry(list_front(&a->semaphore.waiters), struct thread, elem);
+  /* If waiters list is empty, return true (i.e., a has higher priority) */
+  if (list_empty(&b->semaphore.waiters))
+    return true;
+
   struct thread *b_thread =
     list_entry(list_front(&b->semaphore.waiters), struct thread, elem);
 
-  return a_thread->priority > b_thread->priority;
+  b_priority = b_thread->priority;
+
+  return a_priority > b_priority;
 }
 
 /* Initializes condition variable COND.  A condition variable