From 8942e23a6941dc740f6f703f7e353f273874f104 Mon Sep 17 00:00:00 2001
From: 14mRh4X0r <14mRh4X0r@gmail.com>
Date: Mon, 7 Jun 2021 16:42:05 +0200
Subject: [PATCH] Always update AS last_pos, even on no events (#10107)

Fixes #1834.

`get_new_events_for_appservice` internally calls `get_events_as_list`, which will filter out any rejected events. If all returned events are filtered out, `_notify_interested_services` will return without updating the last handled stream position. If there are 100 consecutive such events, processing will halt altogether.

Breaking the loop is now done by checking whether we're up-to-date with `current_max` in the loop condition, instead of relying on an empty `events` list.


Signed-off-by: Willem Mulder <14mRh4X0r@gmail.com>
---
 changelog.d/10107.bugfix          |  1 +
 synapse/handlers/appservice.py    | 25 ++++++++++++-------------
 tests/handlers/test_appservice.py |  6 ++----
 3 files changed, 15 insertions(+), 17 deletions(-)
 create mode 100644 changelog.d/10107.bugfix

diff --git a/changelog.d/10107.bugfix b/changelog.d/10107.bugfix
new file mode 100644
index 000000000..80030efab
--- /dev/null
+++ b/changelog.d/10107.bugfix
@@ -0,0 +1 @@
+Fixed a bug that could cause Synapse to stop notifying application services. Contributed by Willem Mulder.
diff --git a/synapse/handlers/appservice.py b/synapse/handlers/appservice.py
index 177310f0b..862638cc4 100644
--- a/synapse/handlers/appservice.py
+++ b/synapse/handlers/appservice.py
@@ -87,7 +87,8 @@ class ApplicationServicesHandler:
             self.is_processing = True
             try:
                 limit = 100
-                while True:
+                upper_bound = -1
+                while upper_bound < self.current_max:
                     (
                         upper_bound,
                         events,
@@ -95,9 +96,6 @@ class ApplicationServicesHandler:
                         self.current_max, limit
                     )
 
-                    if not events:
-                        break
-
                     events_by_room = {}  # type: Dict[str, List[EventBase]]
                     for event in events:
                         events_by_room.setdefault(event.room_id, []).append(event)
@@ -153,9 +151,6 @@ class ApplicationServicesHandler:
 
                     await self.store.set_appservice_last_pos(upper_bound)
 
-                    now = self.clock.time_msec()
-                    ts = await self.store.get_received_ts(events[-1].event_id)
-
                     synapse.metrics.event_processing_positions.labels(
                         "appservice_sender"
                     ).set(upper_bound)
@@ -168,12 +163,16 @@ class ApplicationServicesHandler:
 
                     event_processing_loop_counter.labels("appservice_sender").inc()
 
-                    synapse.metrics.event_processing_lag.labels(
-                        "appservice_sender"
-                    ).set(now - ts)
-                    synapse.metrics.event_processing_last_ts.labels(
-                        "appservice_sender"
-                    ).set(ts)
+                    if events:
+                        now = self.clock.time_msec()
+                        ts = await self.store.get_received_ts(events[-1].event_id)
+
+                        synapse.metrics.event_processing_lag.labels(
+                            "appservice_sender"
+                        ).set(now - ts)
+                        synapse.metrics.event_processing_last_ts.labels(
+                            "appservice_sender"
+                        ).set(ts)
             finally:
                 self.is_processing = False
 
diff --git a/tests/handlers/test_appservice.py b/tests/handlers/test_appservice.py
index b037b12a0..5d6cc2885 100644
--- a/tests/handlers/test_appservice.py
+++ b/tests/handlers/test_appservice.py
@@ -57,10 +57,10 @@ class AppServiceHandlerTestCase(unittest.TestCase):
             sender="@someone:anywhere", type="m.room.message", room_id="!foo:bar"
         )
         self.mock_store.get_new_events_for_appservice.side_effect = [
-            make_awaitable((0, [event])),
             make_awaitable((0, [])),
+            make_awaitable((1, [event])),
         ]
-        self.handler.notify_interested_services(RoomStreamToken(None, 0))
+        self.handler.notify_interested_services(RoomStreamToken(None, 1))
 
         self.mock_scheduler.submit_event_for_as.assert_called_once_with(
             interested_service, event
@@ -77,7 +77,6 @@ class AppServiceHandlerTestCase(unittest.TestCase):
         self.mock_as_api.query_user.return_value = make_awaitable(True)
         self.mock_store.get_new_events_for_appservice.side_effect = [
             make_awaitable((0, [event])),
-            make_awaitable((0, [])),
         ]
 
         self.handler.notify_interested_services(RoomStreamToken(None, 0))
@@ -95,7 +94,6 @@ class AppServiceHandlerTestCase(unittest.TestCase):
         self.mock_as_api.query_user.return_value = make_awaitable(True)
         self.mock_store.get_new_events_for_appservice.side_effect = [
             make_awaitable((0, [event])),
-            make_awaitable((0, [])),
         ]
 
         self.handler.notify_interested_services(RoomStreamToken(None, 0))