Add ephemeral messages support (MSC2228) (#6409)

Implement part [MSC2228](https://github.com/matrix-org/matrix-doc/pull/2228). The parts that differ are:

* the feature is hidden behind a configuration flag (`enable_ephemeral_messages`)
* self-destruction doesn't happen for state events
* only implement support for the `m.self_destruct_after` field (not the `m.self_destruct` one)
* doesn't send synthetic redactions to clients because for this specific case we consider the clients to be able to destroy an event themselves, instead we just censor it (by pruning its JSON) in the database

synapse/handlers/message.py

@@ -15,6 +15,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
+from typing import Optional
 
 from six import iteritems, itervalues, string_types
 
@@ -22,9 +23,16 @@ from canonicaljson import encode_canonical_json, json
 
 from twisted.internet import defer
 from twisted.internet.defer import succeed
+from twisted.internet.interfaces import IDelayedCall
 
 from synapse import event_auth
-from synapse.api.constants import EventTypes, Membership, RelationTypes, UserTypes
+from synapse.api.constants import (
+    EventContentFields,
+    EventTypes,
+    Membership,
+    RelationTypes,
+    UserTypes,
+)
 from synapse.api.errors import (
     AuthError,
     Codes,
@@ -62,6 +70,17 @@ class MessageHandler(object):
         self.storage = hs.get_storage()
         self.state_store = self.storage.state
         self._event_serializer = hs.get_event_client_serializer()
+        self._ephemeral_events_enabled = hs.config.enable_ephemeral_messages
+        self._is_worker_app = bool(hs.config.worker_app)
+
+        # The scheduled call to self._expire_event. None if no call is currently
+        # scheduled.
+        self._scheduled_expiry = None  # type: Optional[IDelayedCall]
+
+        if not hs.config.worker_app:
+            run_as_background_process(
+                "_schedule_next_expiry", self._schedule_next_expiry
+            )
 
     @defer.inlineCallbacks
     def get_room_data(
@@ -225,6 +244,100 @@ class MessageHandler(object):
             for user_id, profile in iteritems(users_with_profile)
         }
 
+    def maybe_schedule_expiry(self, event):
+        """Schedule the expiry of an event if there's not already one scheduled,
+        or if the one running is for an event that will expire after the provided
+        timestamp.
+
+        This function needs to invalidate the event cache, which is only possible on
+        the master process, and therefore needs to be run on there.
+
+        Args:
+            event (EventBase): The event to schedule the expiry of.
+        """
+        assert not self._is_worker_app
+
+        expiry_ts = event.content.get(EventContentFields.SELF_DESTRUCT_AFTER)
+        if not isinstance(expiry_ts, int) or event.is_state():
+            return
+
+        # _schedule_expiry_for_event won't actually schedule anything if there's already
+        # a task scheduled for a timestamp that's sooner than the provided one.
+        self._schedule_expiry_for_event(event.event_id, expiry_ts)
+
+    @defer.inlineCallbacks
+    def _schedule_next_expiry(self):
+        """Retrieve the ID and the expiry timestamp of the next event to be expired,
+        and schedule an expiry task for it.
+
+        If there's no event left to expire, set _expiry_scheduled to None so that a
+        future call to save_expiry_ts can schedule a new expiry task.
+        """
+        # Try to get the expiry timestamp of the next event to expire.
+        res = yield self.store.get_next_event_to_expire()
+        if res:
+            event_id, expiry_ts = res
+            self._schedule_expiry_for_event(event_id, expiry_ts)
+
+    def _schedule_expiry_for_event(self, event_id, expiry_ts):
+        """Schedule an expiry task for the provided event if there's not already one
+        scheduled at a timestamp that's sooner than the provided one.
+
+        Args:
+            event_id (str): The ID of the event to expire.
+            expiry_ts (int): The timestamp at which to expire the event.
+        """
+        if self._scheduled_expiry:
+            # If the provided timestamp refers to a time before the scheduled time of the
+            # next expiry task, cancel that task and reschedule it for this timestamp.
+            next_scheduled_expiry_ts = self._scheduled_expiry.getTime() * 1000
+            if expiry_ts < next_scheduled_expiry_ts:
+                self._scheduled_expiry.cancel()
+            else:
+                return
+
+        # Figure out how many seconds we need to wait before expiring the event.
+        now_ms = self.clock.time_msec()
+        delay = (expiry_ts - now_ms) / 1000
+
+        # callLater doesn't support negative delays, so trim the delay to 0 if we're
+        # in that case.
+        if delay < 0:
+            delay = 0
+
+        logger.info("Scheduling expiry for event %s in %.3fs", event_id, delay)
+
+        self._scheduled_expiry = self.clock.call_later(
+            delay,
+            run_as_background_process,
+            "_expire_event",
+            self._expire_event,
+            event_id,
+        )
+
+    @defer.inlineCallbacks
+    def _expire_event(self, event_id):
+        """Retrieve and expire an event that needs to be expired from the database.
+
+        If the event doesn't exist in the database, log it and delete the expiry date
+        from the database (so that we don't try to expire it again).
+        """
+        assert self._ephemeral_events_enabled
+
+        self._scheduled_expiry = None
+
+        logger.info("Expiring event %s", event_id)
+
+        try:
+            # Expire the event if we know about it. This function also deletes the expiry
+            # date from the database in the same database transaction.
+            yield self.store.expire_event(event_id)
+        except Exception as e:
+            logger.error("Could not expire event %s: %r", event_id, e)
+
+        # Schedule the expiry of the next event to expire.
+        yield self._schedule_next_expiry()
+
 
 # The duration (in ms) after which rooms should be removed
 # `_rooms_to_exclude_from_dummy_event_insertion` (with the effect that we will try
@@ -295,6 +408,10 @@ class EventCreationHandler(object):
                 5 * 60 * 1000,
             )
 
+        self._message_handler = hs.get_message_handler()
+
+        self._ephemeral_events_enabled = hs.config.enable_ephemeral_messages
+
     @defer.inlineCallbacks
     def create_event(
         self,
@@ -877,6 +994,10 @@ class EventCreationHandler(object):
             event, context=context
         )
 
+        if self._ephemeral_events_enabled:
+            # If there's an expiry timestamp on the event, schedule its expiry.
+            self._message_handler.maybe_schedule_expiry(event)
+
         yield self.pusher_pool.on_new_notifications(event_stream_id, max_stream_id)
 
         def _notify():