Add a sleep to the Limiter to fix stack overflows.

Fixes #3570

synapse/util/async.py

@@ -248,11 +248,15 @@ class Limiter(object):
             # do some work.
 
     """
-    def __init__(self, max_count):
+    def __init__(self, max_count, clock=None):
         """
         Args:
             max_count(int): The maximum number of concurrent access
         """
+        if not clock:
+            from twisted.internet import reactor
+            clock = Clock(reactor)
+        self._clock = clock
         self.max_count = max_count
 
         # key_to_defer is a map from the key to a 2 element list where
@@ -277,10 +281,23 @@ class Limiter(object):
             with PreserveLoggingContext():
                 yield new_defer
             logger.info("Acquired limiter lock for key %r", key)
+            entry[0] += 1
+
+            # if the code holding the lock completes synchronously, then it
+            # will recursively run the next claimant on the list. That can
+            # relatively rapidly lead to stack exhaustion. This is essentially
+            # the same problem as http://twistedmatrix.com/trac/ticket/9304.
+            #
+            # In order to break the cycle, we add a cheeky sleep(0) here to
+            # ensure that we fall back to the reactor between each iteration.
+            #
+            # (This needs to happen while we hold the lock, and the context manager's exit
+            # code must be synchronous, so this is the only sensible place.)
+            yield self._clock.sleep(0)
+
         else:
             logger.info("Acquired uncontended limiter lock for key %r", key)
-
-        entry[0] += 1
+            entry[0] += 1
 
         @contextmanager
         def _ctx_manager():

tests/util/test_limiter.py

@@ -48,21 +48,21 @@ class LimiterTestCase(unittest.TestCase):
             self.assertFalse(d4.called)
             self.assertFalse(d5.called)
 
-        self.assertTrue(d4.called)
+        cm4 = yield d4
         self.assertFalse(d5.called)
 
         with cm3:
             self.assertFalse(d5.called)
 
-        self.assertTrue(d5.called)
+        cm5 = yield d5
 
         with cm2:
             pass
 
-        with (yield d4):
+        with cm4:
             pass
 
-        with (yield d5):
+        with cm5:
             pass
 
         d6 = limiter.queue(key)