Fix bug in userprog-merge where file writes were denied in the wrong thread

src/threads/synch.c

@@ -260,6 +260,7 @@ lock_acquire (struct lock *lock)
   ASSERT (!lock_held_by_current_thread (lock));
 
   struct thread *t = thread_current ();
+  ASSERT (t->waiting_lock == NULL);
 
   enum intr_level old_level = intr_disable ();
   if (lock->holder != NULL)

src/userprog/process.c

@@ -128,7 +128,14 @@ start_process (void *proc_start_data)
   if_.eflags = FLAG_IF | FLAG_MBS;
   
   lock_acquire (&filesys_lock);
+  
+  /* Prevent writing to the file being executed. */
+  struct file *exec_file = filesys_open (data->file_name);
+  thread_current ()->exec_file = exec_file;
+  file_deny_write(exec_file);
+
   success = load (data->file_name, &if_.eip, &if_.esp);
+  
   lock_release (&filesys_lock);
 
   /* If load failed, quit. */