diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 63c56a4a32..7711cded01 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -122,6 +122,28 @@ class FederationHandler(BaseHandler):
self.room_queues[pdu.room_id].append((pdu, origin))
return
+ # If we're no longer in the room just ditch the event entirely. This
+ # is probably an old server that has come back and thinks we're still
+ # in the room (or we've been rejoined to the room by a state reset).
+ #
+ # If we were never in the room then maybe our database got vaped and
+ # we should check if we *are* in fact in the room. If we are then we
+ # can magically rejoin the room.
+ is_in_room = yield self.auth.check_host_in_room(
+ pdu.room_id,
+ self.server_name
+ )
+ if not is_in_room:
+ was_in_room = yield self.store.was_host_joined(
+ pdu.room_id, self.server_name,
+ )
+ if was_in_room:
+ logger.info(
+ "Ignoring PDU %s for room %s from %s as we've left the room!",
+ pdu.event_id, pdu.room_id, origin,
+ )
+ return
+
state = None
auth_chain = []
diff --git a/synapse/storage/events.py b/synapse/storage/events.py
index 7002b3752e..4f0b43c36d 100644
--- a/synapse/storage/events.py
+++ b/synapse/storage/events.py
@@ -784,6 +784,9 @@ class EventsStore(SQLBaseStore):
self._invalidate_cache_and_stream(
txn, self.is_host_joined, (room_id, host)
)
+ self._invalidate_cache_and_stream(
+ txn, self.was_host_joined, (room_id, host)
+ )
self._invalidate_cache_and_stream(
txn, self.get_users_in_room, (room_id,)
diff --git a/synapse/storage/roommember.py b/synapse/storage/roommember.py
index 457ca288d0..a0fc9a6867 100644
--- a/synapse/storage/roommember.py
+++ b/synapse/storage/roommember.py
@@ -533,6 +533,46 @@ class RoomMemberStore(SQLBaseStore):
defer.returnValue(True)
+ @cachedInlineCallbacks()
+ def was_host_joined(self, room_id, host):
+ """Check whether the server is or ever was in the room.
+
+ Args:
+ room_id (str)
+ host (str)
+
+ Returns:
+ Deferred: Resolves to True if the host is/was in the room, otherwise
+ False.
+ """
+ if '%' in host or '_' in host:
+ raise Exception("Invalid host name")
+
+ sql = """
+ SELECT user_id FROM room_memberships
+ WHERE room_id = ?
+ AND user_id LIKE ?
+ AND membership = 'join'
+ LIMIT 1
+ """
+
+ # We do need to be careful to ensure that host doesn't have any wild cards
+ # in it, but we checked above for known ones and we'll check below that
+ # the returned user actually has the correct domain.
+ like_clause = "%:" + host
+
+ rows = yield self._execute("was_host_joined", None, sql, room_id, like_clause)
+
+ if not rows:
+ defer.returnValue(False)
+
+ user_id = rows[0][0]
+ if get_domain_from_id(user_id) != host:
+ # This can only happen if the host name has something funky in it
+ raise Exception("Invalid host name")
+
+ defer.returnValue(True)
+
def get_joined_hosts(self, room_id, state_entry):
state_group = state_entry.state_group
if not state_group:
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