Handle missing previous read marker event. (#15464)

If the previous read marker is pointing to an event that no longer exists
(e.g. due to retention) then assume that the newly given read marker
is newer.

4 files changed, 162 insertions, 10 deletions

diff --git a/changelog.d/15464.bugfix b/changelog.d/15464.bugfix
new file mode 100644
index 0000000000..3c655989b3
--- /dev/null
+++ b/changelog.d/15464.bugfix
@@ -0,0 +1 @@
+Fix a long-standing bug where setting the read marker could fail when using message retention. Contributed by Nick @ Beeper (@fizzadar).
diff --git a/synapse/handlers/read_marker.py b/synapse/handlers/read_marker.py
index 6d35e61880..49a497a860 100644
--- a/synapse/handlers/read_marker.py
+++ b/synapse/handlers/read_marker.py
@@ -16,6 +16,7 @@ import logging
 from typing import TYPE_CHECKING
 
 from synapse.api.constants import ReceiptTypes
+from synapse.api.errors import SynapseError
 from synapse.util.async_helpers import Linearizer
 
 if TYPE_CHECKING:
@@ -47,12 +48,21 @@ class ReadMarkerHandler:
             )
 
             should_update = True
+            # Get event ordering, this also ensures we know about the event
+            event_ordering = await self.store.get_event_ordering(event_id)
 
             if existing_read_marker:
-                # Only update if the new marker is ahead in the stream
-                should_update = await self.store.is_event_after(
-                    event_id, existing_read_marker["event_id"]
-                )
+                try:
+                    old_event_ordering = await self.store.get_event_ordering(
+                        existing_read_marker["event_id"]
+                    )
+                except SynapseError:
+                    # Old event no longer exists, assume new is ahead. This may
+                    # happen if the old event was removed due to retention.
+                    pass
+                else:
+                    # Only update if the new marker is ahead in the stream
+                    should_update = event_ordering > old_event_ordering
 
             if should_update:
                 content = {"event_id": event_id}
diff --git a/synapse/storage/databases/main/events_worker.py b/synapse/storage/databases/main/events_worker.py
index 53aa5933d5..a39bc90974 100644
--- a/synapse/storage/databases/main/events_worker.py
+++ b/synapse/storage/databases/main/events_worker.py
@@ -1973,12 +1973,6 @@ class EventsWorkerStore(SQLBaseStore):
 
         return rows, to_token, True
 
-    async def is_event_after(self, event_id1: str, event_id2: str) -> bool:
-        """Returns True if event_id1 is after event_id2 in the stream"""
-        to_1, so_1 = await self.get_event_ordering(event_id1)
-        to_2, so_2 = await self.get_event_ordering(event_id2)
-        return (to_1, so_1) > (to_2, so_2)
-
     @cached(max_entries=5000)
     async def get_event_ordering(self, event_id: str) -> Tuple[int, int]:
         res = await self.db_pool.simple_select_one(
diff --git a/tests/rest/client/test_read_marker.py b/tests/rest/client/test_read_marker.py
new file mode 100644
index 0000000000..0eedcdb476
--- /dev/null
+++ b/tests/rest/client/test_read_marker.py
@@ -0,0 +1,147 @@
+# Copyright 2023 Beeper
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# 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 twisted.test.proto_helpers import MemoryReactor
+
+import synapse.rest.admin
+from synapse.api.constants import EventTypes
+from synapse.rest import admin
+from synapse.rest.client import login, read_marker, register, room
+from synapse.server import HomeServer
+from synapse.util import Clock
+
+from tests import unittest
+
+ONE_HOUR_MS = 3600000
+ONE_DAY_MS = ONE_HOUR_MS * 24
+
+
+class ReadMarkerTestCase(unittest.HomeserverTestCase):
+    servlets = [
+        login.register_servlets,
+        register.register_servlets,
+        read_marker.register_servlets,
+        room.register_servlets,
+        synapse.rest.admin.register_servlets,
+        admin.register_servlets,
+    ]
+
+    def make_homeserver(self, reactor: MemoryReactor, clock: Clock) -> HomeServer:
+        config = self.default_config()
+
+        # merge this default retention config with anything that was specified in
+        # @override_config
+        retention_config = {
+            "enabled": True,
+            "allowed_lifetime_min": ONE_DAY_MS,
+            "allowed_lifetime_max": ONE_DAY_MS * 3,
+        }
+        retention_config.update(config.get("retention", {}))
+        config["retention"] = retention_config
+
+        self.hs = self.setup_test_homeserver(config=config)
+
+        return self.hs
+
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
+        self.owner = self.register_user("owner", "pass")
+        self.owner_tok = self.login("owner", "pass")
+        self.store = self.hs.get_datastores().main
+        self.clock = self.hs.get_clock()
+
+    def test_send_read_marker(self) -> None:
+        room_id = self.helper.create_room_as(self.owner, tok=self.owner_tok)
+
+        def send_message() -> str:
+            res = self.helper.send(room_id=room_id, body="1", tok=self.owner_tok)
+            return res["event_id"]
+
+        # Test setting the read marker on the room
+        event_id_1 = send_message()
+
+        channel = self.make_request(
+            "POST",
+            "/rooms/!abc:beep/read_markers",
+            content={
+                "m.fully_read": event_id_1,
+            },
+            access_token=self.owner_tok,
+        )
+        self.assertEqual(channel.code, 200, channel.result)
+
+        # Test moving the read marker to a newer event
+        event_id_2 = send_message()
+        channel = self.make_request(
+            "POST",
+            "/rooms/!abc:beep/read_markers",
+            content={
+                "m.fully_read": event_id_2,
+            },
+            access_token=self.owner_tok,
+        )
+        self.assertEqual(channel.code, 200, channel.result)
+
+    def test_send_read_marker_missing_previous_event(self) -> None:
+        """
+        Test moving a read marker from an event that previously existed but was
+        later removed due to retention rules.
+        """
+
+        room_id = self.helper.create_room_as(self.owner, tok=self.owner_tok)
+
+        # Set retention rule on the room so we remove old events to test this case
+        self.helper.send_state(
+            room_id=room_id,
+            event_type=EventTypes.Retention,
+            body={"max_lifetime": ONE_DAY_MS},
+            tok=self.owner_tok,
+        )
+
+        def send_message() -> str:
+            res = self.helper.send(room_id=room_id, body="1", tok=self.owner_tok)
+            return res["event_id"]
+
+        # Test setting the read marker on the room
+        event_id_1 = send_message()
+
+        channel = self.make_request(
+            "POST",
+            "/rooms/!abc:beep/read_markers",
+            content={
+                "m.fully_read": event_id_1,
+            },
+            access_token=self.owner_tok,
+        )
+
+        # Send a second message (retention will not remove the latest event ever)
+        send_message()
+        # And then advance so retention rules remove the first event (where the marker is)
+        self.reactor.advance(ONE_DAY_MS * 2 / 1000)
+
+        event = self.get_success(self.store.get_event(event_id_1, allow_none=True))
+        assert event is None
+
+        # TODO See https://github.com/matrix-org/synapse/issues/13476
+        self.store.get_event_ordering.invalidate_all()
+
+        # Test moving the read marker to a newer event
+        event_id_2 = send_message()
+        channel = self.make_request(
+            "POST",
+            "/rooms/!abc:beep/read_markers",
+            content={
+                "m.fully_read": event_id_2,
+            },
+            access_token=self.owner_tok,
+        )
+        self.assertEqual(channel.code, 200, channel.result)