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/storage/data_stores/main/events.py

@@ -130,6 +130,8 @@ class EventsStore(
         if self.hs.config.redaction_retention_period is not None:
             hs.get_clock().looping_call(_censor_redactions, 5 * 60 * 1000)
 
+        self._ephemeral_messages_enabled = hs.config.enable_ephemeral_messages
+
     @defer.inlineCallbacks
     def _read_forward_extremities(self):
         def fetch(txn):
@@ -940,6 +942,12 @@ class EventsStore(
                     txn, event.event_id, labels, event.room_id, event.depth
                 )
 
+            if self._ephemeral_messages_enabled:
+                # If there's an expiry timestamp on the event, store it.
+                expiry_ts = event.content.get(EventContentFields.SELF_DESTRUCT_AFTER)
+                if isinstance(expiry_ts, int) and not event.is_state():
+                    self._insert_event_expiry_txn(txn, event.event_id, expiry_ts)
+
         # Insert into the room_memberships table.
         self._store_room_members_txn(
             txn,
@@ -1101,12 +1109,7 @@ class EventsStore(
         def _update_censor_txn(txn):
             for redaction_id, event_id, pruned_json in updates:
                 if pruned_json:
-                    self._simple_update_one_txn(
-                        txn,
-                        table="event_json",
-                        keyvalues={"event_id": event_id},
-                        updatevalues={"json": pruned_json},
-                    )
+                    self._censor_event_txn(txn, event_id, pruned_json)
 
                 self._simple_update_one_txn(
                     txn,
@@ -1117,6 +1120,22 @@ class EventsStore(
 
         yield self.runInteraction("_update_censor_txn", _update_censor_txn)
 
+    def _censor_event_txn(self, txn, event_id, pruned_json):
+        """Censor an event by replacing its JSON in the event_json table with the
+        provided pruned JSON.
+
+        Args:
+            txn (LoggingTransaction): The database transaction.
+            event_id (str): The ID of the event to censor.
+            pruned_json (str): The pruned JSON
+        """
+        self._simple_update_one_txn(
+            txn,
+            table="event_json",
+            keyvalues={"event_id": event_id},
+            updatevalues={"json": pruned_json},
+        )
+
     @defer.inlineCallbacks
     def count_daily_messages(self):
         """
@@ -1957,6 +1976,101 @@ class EventsStore(
             ],
         )
 
+    def _insert_event_expiry_txn(self, txn, event_id, expiry_ts):
+        """Save the expiry timestamp associated with a given event ID.
+
+        Args:
+            txn (LoggingTransaction): The database transaction to use.
+            event_id (str): The event ID the expiry timestamp is associated with.
+            expiry_ts (int): The timestamp at which to expire (delete) the event.
+        """
+        return self._simple_insert_txn(
+            txn=txn,
+            table="event_expiry",
+            values={"event_id": event_id, "expiry_ts": expiry_ts},
+        )
+
+    @defer.inlineCallbacks
+    def expire_event(self, event_id):
+        """Retrieve and expire an event that has expired, and delete its associated
+        expiry timestamp. If the event can't be retrieved, delete its associated
+        timestamp so we don't try to expire it again in the future.
+
+        Args:
+             event_id (str): The ID of the event to delete.
+        """
+        # Try to retrieve the event's content from the database or the event cache.
+        event = yield self.get_event(event_id)
+
+        def delete_expired_event_txn(txn):
+            # Delete the expiry timestamp associated with this event from the database.
+            self._delete_event_expiry_txn(txn, event_id)
+
+            if not event:
+                # If we can't find the event, log a warning and delete the expiry date
+                # from the database so that we don't try to expire it again in the
+                # future.
+                logger.warning(
+                    "Can't expire event %s because we don't have it.", event_id
+                )
+                return
+
+            # Prune the event's dict then convert it to JSON.
+            pruned_json = encode_json(prune_event_dict(event.get_dict()))
+
+            # Update the event_json table to replace the event's JSON with the pruned
+            # JSON.
+            self._censor_event_txn(txn, event.event_id, pruned_json)
+
+            # We need to invalidate the event cache entry for this event because we
+            # changed its content in the database. We can't call
+            # self._invalidate_cache_and_stream because self.get_event_cache isn't of the
+            # right type.
+            txn.call_after(self._get_event_cache.invalidate, (event.event_id,))
+            # Send that invalidation to replication so that other workers also invalidate
+            # the event cache.
+            self._send_invalidation_to_replication(
+                txn, "_get_event_cache", (event.event_id,)
+            )
+
+        yield self.runInteraction("delete_expired_event", delete_expired_event_txn)
+
+    def _delete_event_expiry_txn(self, txn, event_id):
+        """Delete the expiry timestamp associated with an event ID without deleting the
+        actual event.
+
+        Args:
+            txn (LoggingTransaction): The transaction to use to perform the deletion.
+            event_id (str): The event ID to delete the associated expiry timestamp of.
+        """
+        return self._simple_delete_txn(
+            txn=txn, table="event_expiry", keyvalues={"event_id": event_id}
+        )
+
+    def get_next_event_to_expire(self):
+        """Retrieve the entry with the lowest expiry timestamp in the event_expiry
+        table, or None if there's no more event to expire.
+
+        Returns: Deferred[Optional[Tuple[str, int]]]
+            A tuple containing the event ID as its first element and an expiry timestamp
+            as its second one, if there's at least one row in the event_expiry table.
+            None otherwise.
+        """
+
+        def get_next_event_to_expire_txn(txn):
+            txn.execute(
+                """
+                SELECT event_id, expiry_ts FROM event_expiry
+                ORDER BY expiry_ts ASC LIMIT 1
+                """
+            )
+
+            return txn.fetchone()
+
+        return self.runInteraction(
+            desc="get_next_event_to_expire", func=get_next_event_to_expire_txn
+        )
+
 
 AllNewEventsResult = namedtuple(
     "AllNewEventsResult",