Add Limiter: limit concurrent access to resource

synapse/util/async.py

@@ -197,6 +197,51 @@ class Linearizer(object):
         defer.returnValue(_ctx_manager())
 
 
+class Limiter(object):
+    """Limits concurrent access to resources based on a key. Useful to ensure
+    only a few thing happen at a time on a given resource.
+
+    Example:
+
+        with (yield limiter.queue("test_key")):
+            # do some work.
+
+    """
+    def __init__(self, max_count):
+        """
+        Args:
+            max_count(int): The maximum number of concurrent access
+        """
+        self.max_count = max_count
+        self.key_to_defer = {}
+
+    @defer.inlineCallbacks
+    def queue(self, key):
+        entry = self.key_to_defer.setdefault(key, [0, []])
+
+        if entry[0] >= self.max_count:
+            new_defer = defer.Deferred()
+            entry[1].append(new_defer)
+            with PreserveLoggingContext():
+                yield new_defer
+
+        entry[0] += 1
+
+        @contextmanager
+        def _ctx_manager():
+            try:
+                yield
+            finally:
+                entry[0] -= 1
+                try:
+                    entry[1].pop(0).callback(None)
+                except IndexError:
+                    if entry[0] == 0:
+                        self.key_to_defer.pop(key, None)
+
+        defer.returnValue(_ctx_manager())
+
+
 class ReadWriteLock(object):
     """A deferred style read write lock.