From 810dd7842ce1ee4ac79edfaced959959eb756f29 Mon Sep 17 00:00:00 2001
From: Eric Eastwood <contact@ericeastwood.com>
Date: Mon, 28 Dec 2020 16:12:04 -0600
Subject: [PATCH] Filter out events after the leave event

---
 syncapi/storage/shared/syncserver.go | 88 ++++++++++++++++++++++++----
 1 file changed, 77 insertions(+), 11 deletions(-)

diff --git a/syncapi/storage/shared/syncserver.go b/syncapi/storage/shared/syncserver.go
index e65ee7c6..0f8e09cd 100644
--- a/syncapi/storage/shared/syncserver.go
+++ b/syncapi/storage/shared/syncserver.go
@@ -937,6 +937,7 @@ func (d *Database) getJoinResponseForCompleteSync(
 	// "Can sync a room with a message with a transaction id" - which does a complete sync to check.
 	recentEvents := d.StreamEventsToEvents(&device, recentStreamEvents)
 	stateEvents = removeDuplicates(stateEvents, recentEvents)
+
 	jr = types.NewJoinResponse()
 	jr.Timeline.PrevBatch = prevBatch
 	jr.Timeline.Events = gomatrixserverlib.HeaderedToClientEvents(recentEvents, gomatrixserverlib.FormatSync)
@@ -952,23 +953,88 @@ func (d *Database) getLeaveResponseForCompleteSync(
 	filter *gomatrixserverlib.Filter,
 	device userapi.Device,
 ) (lr *types.LeaveResponse, err error) {
-	jr, err := d.getJoinResponseForCompleteSync(
-		ctx,
-		txn,
-		roomID,
-		r,
-		filter,
-		device,
+	var stateEvents []*gomatrixserverlib.HeaderedEvent
+	stateEvents, err = d.CurrentRoomState.SelectCurrentState(ctx, txn, roomID, &filter.Room.State)
+	if err != nil {
+		return
+	}
+
+	numRecentEventsPerRoom := filter.Room.Timeline.Limit
+
+	// TODO: When filters are added, we may need to call this multiple times to get enough events.
+	//       See: https://github.com/matrix-org/synapse/blob/v0.19.3/synapse/handlers/sync.py#L316
+	var recentStreamEvents []types.StreamEvent
+	var limited bool
+	recentStreamEvents, limited, err = d.OutputEvents.SelectRecentEvents(
+		ctx, txn, roomID, r, numRecentEventsPerRoom, true, true,
 	)
 	if err != nil {
 		return
 	}
 
+	// TODO FIXME: We don't fully implement history visibility yet. To avoid leaking events which the
+	// user shouldn't see, we check the recent events and remove any prior to the join event of the user
+	// which is equiv to history_visibility: joined
+	joinEventIndex := -1
+	leaveEventIndex := -1
+	for i := len(recentStreamEvents) - 1; i >= 0; i-- {
+		ev := recentStreamEvents[i]
+		if ev.Type() == gomatrixserverlib.MRoomMember && ev.StateKeyEquals(device.UserID) {
+			membership, _ := ev.Membership()
+			if membership == gomatrixserverlib.Join {
+				joinEventIndex = i
+				if i > 0 {
+					// the create event happens before the first join, so we should cut it at that point instead
+					if recentStreamEvents[i-1].Type() == gomatrixserverlib.MRoomCreate && recentStreamEvents[i-1].StateKeyEquals("") {
+						joinEventIndex = i - 1
+					}
+				}
+				break
+			} else if membership == gomatrixserverlib.Leave {
+				leaveEventIndex = i - 1
+			}
+
+			if joinEventIndex != -1 && leaveEventIndex != -1 {
+				break
+			}
+		}
+	}
+	// We can assume that if the person joined, they also left and will only show the events between (inclusive)
+	if joinEventIndex != -1 && leaveEventIndex != -1 {
+		// cut all events earlier than the join (but not the join itself)
+		// and cut all events after the person left
+		recentStreamEvents = recentStreamEvents[joinEventIndex:leaveEventIndex]
+		limited = false // so clients know not to try to backpaginate
+	}
+
+	// Retrieve the backward topology position, i.e. the position of the
+	// oldest event in the room's topology.
+	var prevBatchStr string
+	if len(recentStreamEvents) > 0 {
+		var backwardTopologyPos, backwardStreamPos types.StreamPosition
+		backwardTopologyPos, backwardStreamPos, err = d.Topology.SelectPositionInTopology(ctx, txn, recentStreamEvents[0].EventID())
+		if err != nil {
+			return
+		}
+		prevBatch := types.TopologyToken{
+			Depth:       backwardTopologyPos,
+			PDUPosition: backwardStreamPos,
+		}
+		prevBatch.Decrement()
+		prevBatchStr = prevBatch.String()
+	}
+
+	// We don't include a device here as we don't need to send down
+	// transaction IDs for complete syncs, but we do it anyway because Sytest demands it for:
+	// "Can sync a room with a message with a transaction id" - which does a complete sync to check.
+	recentEvents := d.StreamEventsToEvents(&device, recentStreamEvents)
+	stateEvents = removeDuplicates(stateEvents, recentEvents)
+
 	lr = types.NewLeaveResponse()
-	lr.Timeline.PrevBatch = jr.Timeline.PrevBatch
-	lr.Timeline.Events = jr.Timeline.Events
-	lr.Timeline.Limited = jr.Timeline.Limited
-	lr.State.Events = jr.State.Events
+	lr.Timeline.PrevBatch = prevBatchStr
+	lr.Timeline.Events = gomatrixserverlib.HeaderedToClientEvents(recentEvents, gomatrixserverlib.FormatSync)
+	lr.Timeline.Limited = limited
+	lr.State.Events = gomatrixserverlib.HeaderedToClientEvents(stateEvents, gomatrixserverlib.FormatSync)
 	return lr, nil
 }