add hg and python

sys/lib/python/mutex.py

@@ -0,0 +1,52 @@
+"""Mutual exclusion -- for use with module sched
+
+A mutex has two pieces of state -- a 'locked' bit and a queue.
+When the mutex is not locked, the queue is empty.
+Otherwise, the queue contains 0 or more (function, argument) pairs
+representing functions (or methods) waiting to acquire the lock.
+When the mutex is unlocked while the queue is not empty,
+the first queue entry is removed and its function(argument) pair called,
+implying it now has the lock.
+
+Of course, no multi-threading is implied -- hence the funny interface
+for lock, where a function is called once the lock is aquired.
+"""
+
+from collections import deque
+
+class mutex:
+    def __init__(self):
+        """Create a new mutex -- initially unlocked."""
+        self.locked = 0
+        self.queue = deque()
+
+    def test(self):
+        """Test the locked bit of the mutex."""
+        return self.locked
+
+    def testandset(self):
+        """Atomic test-and-set -- grab the lock if it is not set,
+        return True if it succeeded."""
+        if not self.locked:
+            self.locked = 1
+            return True
+        else:
+            return False
+
+    def lock(self, function, argument):
+        """Lock a mutex, call the function with supplied argument
+        when it is acquired.  If the mutex is already locked, place
+        function and argument in the queue."""
+        if self.testandset():
+            function(argument)
+        else:
+            self.queue.append((function, argument))
+
+    def unlock(self):
+        """Unlock a mutex.  If the queue is not empty, call the next
+        function with its argument."""
+        if self.queue:
+            function, argument = self.queue.popleft()
+            function(argument)
+        else:
+            self.locked = 0