Make Distributor run its processes as a background process

This is more involved than it might otherwise be, because the current
implementation just drops its logcontexts and runs everything in the sentinel
context.

It turns out that we aren't actually using a bunch of the functionality here
(notably suppress_failures and the fact that Distributor.fire returns a
deferred), so the easiest way to fix this is actually by simplifying a bunch of
code.

synapse/util/distributor.py

@@ -17,20 +17,18 @@
 
 from twisted.internet import defer
 
-from synapse.util import unwrapFirstError
-from synapse.util.logcontext import PreserveLoggingContext
+from synapse.metrics.background_process_metrics import run_as_background_process
+from synapse.util.logcontext import make_deferred_yieldable, run_in_background
 
 logger = logging.getLogger(__name__)
 
 
 def user_left_room(distributor, user, room_id):
-    with PreserveLoggingContext():
-        distributor.fire("user_left_room", user=user, room_id=room_id)
+    distributor.fire("user_left_room", user=user, room_id=room_id)
 
 
 def user_joined_room(distributor, user, room_id):
-    with PreserveLoggingContext():
-        distributor.fire("user_joined_room", user=user, room_id=room_id)
+    distributor.fire("user_joined_room", user=user, room_id=room_id)
 
 
 class Distributor(object):
@@ -44,9 +42,7 @@ class Distributor(object):
       model will do for today.
     """
 
-    def __init__(self, suppress_failures=True):
-        self.suppress_failures = suppress_failures
-
+    def __init__(self):
         self.signals = {}
         self.pre_registration = {}
 
@@ -56,7 +52,6 @@ def declare(self, name):
 
         self.signals[name] = Signal(
             name,
-            suppress_failures=self.suppress_failures,
         )
 
         if name in self.pre_registration:
@@ -82,7 +77,11 @@ def fire(self, name, *args, **kwargs):
         if name not in self.signals:
             raise KeyError("%r does not have a signal named %s" % (self, name))
 
-        return self.signals[name].fire(*args, **kwargs)
+        run_as_background_process(
+            name,
+            self.signals[name].fire,
+            *args, **kwargs
+        )
 
 
 class Signal(object):
@@ -95,9 +94,8 @@ class Signal(object):
     method into all of the observers.
     """
 
-    def __init__(self, name, suppress_failures):
+    def __init__(self, name):
         self.name = name
-        self.suppress_failures = suppress_failures
         self.observers = []
 
     def observe(self, observer):
@@ -107,7 +105,6 @@ def observe(self, observer):
         Each observer callable may return a Deferred."""
         self.observers.append(observer)
 
-    @defer.inlineCallbacks
     def fire(self, *args, **kwargs):
         """Invokes every callable in the observer list, passing in the args and
         kwargs. Exceptions thrown by observers are logged but ignored. It is
@@ -125,22 +122,17 @@ def eb(failure):
                         failure.type,
                         failure.value,
                         failure.getTracebackObject()))
-                if not self.suppress_failures:
-                    return failure
 
             return defer.maybeDeferred(observer, *args, **kwargs).addErrback(eb)
 
-        with PreserveLoggingContext():
-            deferreds = [
-                do(observer)
-                for observer in self.observers
-            ]
-
-            res = yield defer.gatherResults(
-                deferreds, consumeErrors=True
-            ).addErrback(unwrapFirstError)
+        deferreds = [
+            run_in_background(do, o)
+            for o in self.observers
+        ]
 
-        defer.returnValue(res)
+        return make_deferred_yieldable(defer.gatherResults(
+            deferreds, consumeErrors=True,
+        ))
 
     def __repr__(self):
         return "<Signal name=%r>" % (self.name,)

tests/test_distributor.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.
@@ -15,8 +16,6 @@
 
 from mock import Mock, patch
 
-from twisted.internet import defer
-
 from synapse.util.distributor import Distributor
 
 from . import unittest
@@ -27,38 +26,15 @@ class DistributorTestCase(unittest.TestCase):
     def setUp(self):
         self.dist = Distributor()
 
-    @defer.inlineCallbacks
     def test_signal_dispatch(self):
         self.dist.declare("alert")
 
         observer = Mock()
         self.dist.observe("alert", observer)
 
-        d = self.dist.fire("alert", 1, 2, 3)
-        yield d
-        self.assertTrue(d.called)
+        self.dist.fire("alert", 1, 2, 3)
         observer.assert_called_with(1, 2, 3)
 
-    @defer.inlineCallbacks
-    def test_signal_dispatch_deferred(self):
-        self.dist.declare("whine")
-
-        d_inner = defer.Deferred()
-
-        def observer():
-            return d_inner
-
-        self.dist.observe("whine", observer)
-
-        d_outer = self.dist.fire("whine")
-
-        self.assertFalse(d_outer.called)
-
-        d_inner.callback(None)
-        yield d_outer
-        self.assertTrue(d_outer.called)
-
-    @defer.inlineCallbacks
     def test_signal_catch(self):
         self.dist.declare("alarm")
 
@@ -71,9 +47,7 @@ def test_signal_catch(self):
         with patch(
             "synapse.util.distributor.logger", spec=["warning"]
         ) as mock_logger:
-            d = self.dist.fire("alarm", "Go")
-            yield d
-            self.assertTrue(d.called)
+            self.dist.fire("alarm", "Go")
 
             observers[0].assert_called_once_with("Go")
             observers[1].assert_called_once_with("Go")
@@ -83,34 +57,12 @@ def test_signal_catch(self):
                 mock_logger.warning.call_args[0][0], str
             )
 
-    @defer.inlineCallbacks
-    def test_signal_catch_no_suppress(self):
-        # Gut-wrenching
-        self.dist.suppress_failures = False
-
-        self.dist.declare("whail")
-
-        class MyException(Exception):
-            pass
-
-        @defer.inlineCallbacks
-        def observer():
-            raise MyException("Oopsie")
-
-        self.dist.observe("whail", observer)
-
-        d = self.dist.fire("whail")
-
-        yield self.assertFailure(d, MyException)
-        self.dist.suppress_failures = True
-
-    @defer.inlineCallbacks
     def test_signal_prereg(self):
         observer = Mock()
         self.dist.observe("flare", observer)
 
         self.dist.declare("flare")
-        yield self.dist.fire("flare", 4, 5)
+        self.dist.fire("flare", 4, 5)
 
         observer.assert_called_with(4, 5)