diff --git a/synapse/visibility.py b/synapse/visibility.py
new file mode 100644
index 0000000000..948ad51772
--- /dev/null
+++ b/synapse/visibility.py
@@ -0,0 +1,210 @@
+# -*- coding: utf-8 -*-
+# Copyright 2014 - 2016 OpenMarket Ltd
+#
+# 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.internet import defer
+
+from synapse.api.constants import Membership, EventTypes
+
+from synapse.util.logcontext import preserve_fn
+
+import logging
+
+
+logger = logging.getLogger(__name__)
+
+
+VISIBILITY_PRIORITY = (
+ "world_readable",
+ "shared",
+ "invited",
+ "joined",
+)
+
+
+MEMBERSHIP_PRIORITY = (
+ Membership.JOIN,
+ Membership.INVITE,
+ Membership.KNOCK,
+ Membership.LEAVE,
+ Membership.BAN,
+)
+
+
+@defer.inlineCallbacks
+def filter_events_for_clients(store, user_tuples, events, event_id_to_state):
+ """ Returns dict of user_id -> list of events that user is allowed to
+ see.
+
+ Args:
+ user_tuples (str, bool): (user id, is_peeking) for each user to be
+ checked. is_peeking should be true if:
+ * the user is not currently a member of the room, and:
+ * the user has not been a member of the room since the
+ given events
+ events ([synapse.events.EventBase]): list of events to filter
+ """
+ forgotten = yield defer.gatherResults([
+ preserve_fn(store.who_forgot_in_room)(
+ room_id,
+ )
+ for room_id in frozenset(e.room_id for e in events)
+ ], consumeErrors=True)
+
+ # Set of membership event_ids that have been forgotten
+ event_id_forgotten = frozenset(
+ row["event_id"] for rows in forgotten for row in rows
+ )
+
+ ignore_dict_content = yield store.get_global_account_data_by_type_for_users(
+ "m.ignored_user_list", user_ids=[user_id for user_id, _ in user_tuples]
+ )
+
+ # FIXME: This will explode if people upload something incorrect.
+ ignore_dict = {
+ user_id: frozenset(
+ content.get("ignored_users", {}).keys() if content else []
+ )
+ for user_id, content in ignore_dict_content.items()
+ }
+
+ def allowed(event, user_id, is_peeking, ignore_list):
+ """
+ Args:
+ event (synapse.events.EventBase): event to check
+ user_id (str)
+ is_peeking (bool)
+ ignore_list (list): list of users to ignore
+ """
+ if not event.is_state() and event.sender in ignore_list:
+ return False
+
+ state = event_id_to_state[event.event_id]
+
+ # get the room_visibility at the time of the event.
+ visibility_event = state.get((EventTypes.RoomHistoryVisibility, ""), None)
+ if visibility_event:
+ visibility = visibility_event.content.get("history_visibility", "shared")
+ else:
+ visibility = "shared"
+
+ if visibility not in VISIBILITY_PRIORITY:
+ visibility = "shared"
+
+ # if it was world_readable, it's easy: everyone can read it
+ if visibility == "world_readable":
+ return True
+
+ # Always allow history visibility events on boundaries. This is done
+ # by setting the effective visibility to the least restrictive
+ # of the old vs new.
+ if event.type == EventTypes.RoomHistoryVisibility:
+ prev_content = event.unsigned.get("prev_content", {})
+ prev_visibility = prev_content.get("history_visibility", None)
+
+ if prev_visibility not in VISIBILITY_PRIORITY:
+ prev_visibility = "shared"
+
+ new_priority = VISIBILITY_PRIORITY.index(visibility)
+ old_priority = VISIBILITY_PRIORITY.index(prev_visibility)
+ if old_priority < new_priority:
+ visibility = prev_visibility
+
+ # likewise, if the event is the user's own membership event, use
+ # the 'most joined' membership
+ membership = None
+ if event.type == EventTypes.Member and event.state_key == user_id:
+ membership = event.content.get("membership", None)
+ if membership not in MEMBERSHIP_PRIORITY:
+ membership = "leave"
+
+ prev_content = event.unsigned.get("prev_content", {})
+ prev_membership = prev_content.get("membership", None)
+ if prev_membership not in MEMBERSHIP_PRIORITY:
+ prev_membership = "leave"
+
+ new_priority = MEMBERSHIP_PRIORITY.index(membership)
+ old_priority = MEMBERSHIP_PRIORITY.index(prev_membership)
+ if old_priority < new_priority:
+ membership = prev_membership
+
+ # otherwise, get the user's membership at the time of the event.
+ if membership is None:
+ membership_event = state.get((EventTypes.Member, user_id), None)
+ if membership_event:
+ if membership_event.event_id not in event_id_forgotten:
+ membership = membership_event.membership
+
+ # if the user was a member of the room at the time of the event,
+ # they can see it.
+ if membership == Membership.JOIN:
+ return True
+
+ if visibility == "joined":
+ # we weren't a member at the time of the event, so we can't
+ # see this event.
+ return False
+
+ elif visibility == "invited":
+ # user can also see the event if they were *invited* at the time
+ # of the event.
+ return membership == Membership.INVITE
+
+ else:
+ # visibility is shared: user can also see the event if they have
+ # become a member since the event
+ #
+ # XXX: if the user has subsequently joined and then left again,
+ # ideally we would share history up to the point they left. But
+ # we don't know when they left.
+ return not is_peeking
+
+ defer.returnValue({
+ user_id: [
+ event
+ for event in events
+ if allowed(event, user_id, is_peeking, ignore_dict.get(user_id, []))
+ ]
+ for user_id, is_peeking in user_tuples
+ })
+
+
+@defer.inlineCallbacks
+def filter_events_for_client(store, user_id, events, is_peeking=False):
+ """
+ Check which events a user is allowed to see
+
+ Args:
+ user_id(str): user id to be checked
+ events([synapse.events.EventBase]): list of events to be checked
+ is_peeking(bool): should be True if:
+ * the user is not currently a member of the room, and:
+ * the user has not been a member of the room since the given
+ events
+
+ Returns:
+ [synapse.events.EventBase]
+ """
+ types = (
+ (EventTypes.RoomHistoryVisibility, ""),
+ (EventTypes.Member, user_id),
+ )
+ event_id_to_state = yield store.get_state_for_events(
+ frozenset(e.event_id for e in events),
+ types=types
+ )
+ res = yield filter_events_for_clients(
+ store, [(user_id, is_peeking)], events, event_id_to_state
+ )
+ defer.returnValue(res.get(user_id, []))
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