Newsfile

This fixes a bug where if a pusher gets told about a new event to
push it will ignore the backoff and immediately retry sending any
pending push.

changelog.d/18363.bugfix

@@ -0,0 +1 @@
+Fix longstanding bug where Synapse would immediately retry a failing push endpoint when a new event is received, ignoring any backoff timers.

synapse/push/httppusher.py

@@ -205,6 +205,12 @@ class HttpPusher(Pusher):
         if self._is_processing:
             return
 
+        # Check if we are trying, but failing, to contact the pusher. If so, we
+        # don't try and start processing immediately and instead wait for the
+        # retry loop to try again later (which is controlled by the timer).
+        if self.failing_since and self.timed_call and self.timed_call.active():
+            return
+
         run_as_background_process("httppush.process", self._process)
 
     async def _process(self) -> None:

tests/push/test_http.py

@@ -1167,3 +1167,81 @@ class HTTPPusherTests(HomeserverTestCase):
             self.assertEqual(
                 self.push_attempts[0][2]["notification"]["counts"]["unread"], 1
             )
+
+    def test_push_backoff(self) -> None:
+        """
+        The HTTP pusher will backoff correctly if it fails to contact the pusher.
+        """
+
+        # Register the user who gets notified
+        user_id = self.register_user("user", "pass")
+        access_token = self.login("user", "pass")
+
+        # Register the user who sends the message
+        other_user_id = self.register_user("otheruser", "pass")
+        other_access_token = self.login("otheruser", "pass")
+
+        # Register the pusher
+        user_tuple = self.get_success(
+            self.hs.get_datastores().main.get_user_by_access_token(access_token)
+        )
+        assert user_tuple is not None
+        device_id = user_tuple.device_id
+
+        self.get_success(
+            self.hs.get_pusherpool().add_or_update_pusher(
+                user_id=user_id,
+                device_id=device_id,
+                kind="http",
+                app_id="m.http",
+                app_display_name="HTTP Push Notifications",
+                device_display_name="pushy push",
+                pushkey="a@example.com",
+                lang=None,
+                data={"url": "http://example.com/_matrix/push/v1/notify"},
+            )
+        )
+
+        # Create a room with the other user
+        room = self.helper.create_room_as(user_id, tok=access_token)
+        self.helper.join(room=room, user=other_user_id, tok=other_access_token)
+
+        # The other user sends some messages
+        self.helper.send(room, body="Message 1", tok=other_access_token)
+
+        # One push was attempted to be sent
+        self.assertEqual(len(self.push_attempts), 1)
+        self.assertEqual(
+            self.push_attempts[0][1], "http://example.com/_matrix/push/v1/notify"
+        )
+        self.assertEqual(
+            self.push_attempts[0][2]["notification"]["content"]["body"], "Message 1"
+        )
+        self.push_attempts[0][0].callback({})
+        self.pump()
+
+        # Send another message, this time it fails
+        self.helper.send(room, body="Message 2", tok=other_access_token)
+        self.assertEqual(len(self.push_attempts), 2)
+        self.push_attempts[1][0].errback(Exception("couldn't connect"))
+        self.pump()
+
+        # Sending yet another message doesn't trigger a push immediately
+        self.helper.send(room, body="Message 3", tok=other_access_token)
+        self.pump()
+        self.assertEqual(len(self.push_attempts), 2)
+
+        # .. but waiting for a bit will cause more pushes
+        self.reactor.advance(10)
+        self.assertEqual(len(self.push_attempts), 3)
+        self.assertEqual(
+            self.push_attempts[2][2]["notification"]["content"]["body"], "Message 2"
+        )
+        self.push_attempts[2][0].callback({})
+        self.pump()
+
+        self.assertEqual(len(self.push_attempts), 4)
+        self.assertEqual(
+            self.push_attempts[3][2]["notification"]["content"]["body"], "Message 3"
+        )
+        self.push_attempts[3][0].callback({})