Fix stackoverflow and logcontexts from linearizer

1. make it not blow out the stack when there are more than 50 things waiting
   for a lock. Fixes https://github.com/matrix-org/synapse/issues/2505.

2. Make it not mess up the log contexts.

tests/util/test_linearizer.py

@@ -12,8 +12,7 @@
 # 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.
-
-
+from synapse.util import async, logcontext
 from tests import unittest
 
 from twisted.internet import defer
@@ -38,7 +37,28 @@ class LinearizerTestCase(unittest.TestCase):
         with cm1:
             self.assertFalse(d2.called)
 
-        self.assertTrue(d2.called)
-
         with (yield d2):
             pass
+
+    def test_lots_of_queued_things(self):
+        # we have one slow thing, and lots of fast things queued up behind it.
+        # it should *not* explode the stack.
+        linearizer = Linearizer()
+
+        @defer.inlineCallbacks
+        def func(i, sleep=False):
+            with logcontext.LoggingContext("func(%s)" % i) as lc:
+                with (yield linearizer.queue("")):
+                    self.assertEqual(
+                        logcontext.LoggingContext.current_context(), lc)
+                    if sleep:
+                        yield async.sleep(0)
+
+                self.assertEqual(
+                    logcontext.LoggingContext.current_context(), lc)
+
+        func(0, sleep=True)
+        for i in xrange(1, 100):
+            func(i)
+
+        return func(1000)