1 files changed, 59 insertions, 0 deletions
diff --git a/Lib/mutex.py b/Lib/mutex.py
new file mode 100644
index 0000000..38983fc
--- /dev/null
+++ b/Lib/mutex.py
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+# 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.
+#
+class mutex():
+	#
+	# Create a new mutex -- initially unlocked
+	#
+	def init(self):
+		self.locked = 0
+		self.queue = []
+		return self
+	#
+	# Test the locked bit of the mutex
+	#
+	def test(self):
+		return self.locked
+	#
+	# Atomic test-and-set -- grab the lock if it is not set,
+	# return true if it succeeded
+	#
+	def testandset(self):
+		if not self.locked:
+			self.locked = 1
+			return 1
+		else:
+			return 0
+	#
+	# 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.
+	#
+	def lock(self, (function, argument)):
+		if self.testandset():
+			function(argument)
+		else:
+			self.queue.append(function, argument)
+	#
+	# Unlock a mutex.  If the queue is not empty, call the next
+	# function with its argument.
+	#
+	def unlock(self):
+		if self.queue:
+			function, argument = self.queue[0]
+			del self.queue[0]
+			function(argument)
+		else:
+			self.locked = 0
+	#