A set of improvements to the Limiter

changelog.d/3570.bugfix

@@ -0,0 +1 @@
+Fix potential stack overflow and deadlock under heavy load

changelog.d/3571.misc

@@ -0,0 +1 @@
+Merge Linearizer and Limiter

synapse/handlers/message.py

@@ -33,7 +33,7 @@
 from synapse.events.validator import EventValidator
 from synapse.replication.http.send_event import send_event_to_master
 from synapse.types import RoomAlias, RoomStreamToken, UserID
-from synapse.util.async import Limiter, ReadWriteLock
+from synapse.util.async import Linearizer, ReadWriteLock
 from synapse.util.frozenutils import frozendict_json_encoder
 from synapse.util.logcontext import run_in_background
 from synapse.util.metrics import measure_func
@@ -427,7 +427,7 @@ def __init__(self, hs):
 
         # We arbitrarily limit concurrent event creation for a room to 5.
         # This is to stop us from diverging history *too* much.
-        self.limiter = Limiter(max_count=5)
+        self.limiter = Linearizer(max_count=5, name="room_event_creation_limit")
 
         self.action_generator = hs.get_action_generator()
 

synapse/util/async.py

@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 # Copyright 2014-2016 OpenMarket Ltd
+# Copyright 2018 New Vector Ltd.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -12,7 +13,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-
+import collections
 import logging
 from contextlib import contextmanager
 
@@ -156,54 +157,56 @@ def _concurrently_execute_inner():
 
 
 class Linearizer(object):
-    """Linearizes access to resources based on a key. Useful to ensure only one
-    thing is happening at a time on a given resource.
+    """Limits concurrent access to resources based on a key. Useful to ensure
+    only a few things happen at a time on a given resource.
 
     Example:
 
-        with (yield linearizer.queue("test_key")):
+        with (yield limiter.queue("test_key")):
             # do some work.
 
     """
-    def __init__(self, name=None, clock=None):
+    def __init__(self, name=None, max_count=1, clock=None):
+        """
+        Args:
+            max_count(int): The maximum number of concurrent accesses
+        """
         if name is None:
             self.name = id(self)
         else:
             self.name = name
-        self.key_to_defer = {}
 
         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
+        # the first element is the number of things executing, and
+        # the second element is a deque of deferreds for the things blocked from
+        # executing.
+        self.key_to_defer = {}
 
     @defer.inlineCallbacks
     def queue(self, key):
-        # If there is already a deferred in the queue, we pull it out so that
-        # we can wait on it later.
-        # Then we replace it with a deferred that we resolve *after* the
-        # context manager has exited.
-        # We only return the context manager after the previous deferred has
-        # resolved.
-        # This all has the net effect of creating a chain of deferreds that
-        # wait for the previous deferred before starting their work.
-        current_defer = self.key_to_defer.get(key)
+        entry = self.key_to_defer.setdefault(key, [0, collections.deque()])
 
-        new_defer = defer.Deferred()
-        self.key_to_defer[key] = new_defer
+        # If the number of things executing is greater than the maximum
+        # then add a deferred to the list of blocked items
+        # When on of the things currently executing finishes it will callback
+        # this item so that it can continue executing.
+        if entry[0] >= self.max_count:
+            new_defer = defer.Deferred()
+            entry[1].append(new_defer)
 
-        if current_defer:
             logger.info(
-                "Waiting to acquire linearizer lock %r for key %r", self.name, key
+                "Waiting to acquire linearizer lock %r for key %r", self.name, key,
             )
-            try:
-                with PreserveLoggingContext():
-                    yield current_defer
-            except Exception:
-                logger.exception("Unexpected exception in Linearizer")
+            yield make_deferred_yieldable(new_defer)
 
-            logger.info("Acquired linearizer lock %r for key %r", self.name,
-                        key)
+            logger.info("Acquired linearizer lock %r for key %r", self.name, 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
@@ -213,89 +216,31 @@ def queue(self, key):
             # 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.
             #
-            # (There's no particular need for it to happen before we return
-            # the context manager, but it needs to happen while we hold the
-            # lock, and the context manager's exit code must be synchronous,
-            # so actually this is the only sensible place.
+            # (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 linearizer lock %r for key %r",
-                        self.name, key)
+            logger.info(
+                "Acquired uncontended linearizer lock %r for key %r", self.name, key,
+            )
+            entry[0] += 1
 
         @contextmanager
         def _ctx_manager():
             try:
                 yield
             finally:
                 logger.info("Releasing linearizer lock %r for key %r", self.name, key)
-                with PreserveLoggingContext():
-                    new_defer.callback(None)
-                current_d = self.key_to_defer.get(key)
-                if current_d is new_defer:
-                    self.key_to_defer.pop(key, None)
-
-        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
-
-        # key_to_defer is a map from the key to a 2 element list where
-        # the first element is the number of things executing
-        # the second element is a list of deferreds for the things blocked from
-        # executing.
-        self.key_to_defer = {}
-
-    @defer.inlineCallbacks
-    def queue(self, key):
-        entry = self.key_to_defer.setdefault(key, [0, []])
-
-        # If the number of things executing is greater than the maximum
-        # then add a deferred to the list of blocked items
-        # When on of the things currently executing finishes it will callback
-        # this item so that it can continue executing.
-        if entry[0] >= self.max_count:
-            new_defer = defer.Deferred()
-            entry[1].append(new_defer)
-
-            logger.info("Waiting to acquire limiter lock for key %r", key)
-            with PreserveLoggingContext():
-                yield new_defer
-            logger.info("Acquired limiter lock for key %r", key)
-        else:
-            logger.info("Acquired uncontended limiter lock for key %r", key)
-
-        entry[0] += 1
-
-        @contextmanager
-        def _ctx_manager():
-            try:
-                yield
-            finally:
-                logger.info("Releasing limiter lock for key %r", key)
 
                 # We've finished executing so check if there are any things
                 # blocked waiting to execute and start one of them
                 entry[0] -= 1
 
                 if entry[1]:
-                    next_def = entry[1].pop(0)
+                    next_def = entry[1].popleft()
 
+                    # we need to run the next thing in the sentinel context.
                     with PreserveLoggingContext():
                         next_def.callback(None)
                 elif entry[0] == 0:

tests/util/test_linearizer.py

@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 # Copyright 2016 OpenMarket Ltd
+# Copyright 2018 New Vector Ltd.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -65,3 +66,49 @@ def func(i, sleep=False):
             func(i)
 
         return func(1000)
+
+    @defer.inlineCallbacks
+    def test_multiple_entries(self):
+        limiter = Linearizer(max_count=3)
+
+        key = object()
+
+        d1 = limiter.queue(key)
+        cm1 = yield d1
+
+        d2 = limiter.queue(key)
+        cm2 = yield d2
+
+        d3 = limiter.queue(key)
+        cm3 = yield d3
+
+        d4 = limiter.queue(key)
+        self.assertFalse(d4.called)
+
+        d5 = limiter.queue(key)
+        self.assertFalse(d5.called)
+
+        with cm1:
+            self.assertFalse(d4.called)
+            self.assertFalse(d5.called)
+
+        cm4 = yield d4
+        self.assertFalse(d5.called)
+
+        with cm3:
+            self.assertFalse(d5.called)
+
+        cm5 = yield d5
+
+        with cm2:
+            pass
+
+        with cm4:
+            pass
+
+        with cm5:
+            pass
+
+        d6 = limiter.queue(key)
+        with (yield d6):
+            pass