Do not process /sync responses for rooms the bot has just joined

Else we can process !commands multiple times and/or process !commands from before
we were in the room (if `history_visibility` allows it).

src/github.com/matrix-org/go-neb/matrix/matrix.go

@@ -252,9 +252,7 @@ func (cli *Client) Sync() {
 			return
 		}
 
-		isFirstSync := nextToken == ""
-
-		// Update client state
+		processResponse := cli.shouldProcessResponse(nextToken, &syncResponse)
 		nextToken = syncResponse.NextBatch
 		logger.WithField("next_batch", nextToken).Print("Received sync response")
 
@@ -263,12 +261,54 @@ func (cli *Client) Sync() {
 		// a malformed/buggy event which keeps making us panic.
 		cli.NextBatchStorer.Save(cli.UserID, nextToken)
 
-		if !isFirstSync {
+		if processResponse {
+			// Update client state
 			channel <- syncResponse
 		}
 	}
 }
 
+// shouldProcessResponse returns true if the response should be processed. May modify the response to remove
+// stuff that shouldn't be processed.
+func (cli *Client) shouldProcessResponse(tokenOnSync string, syncResponse *syncHTTPResponse) bool {
+	if tokenOnSync == "" {
+		return false
+	}
+	// This is a horrible hack because /sync will return the most recent messages for a room
+	// as soon as you /join it. We do NOT want to process those events in that particular room
+	// because they may have already been processed (if you toggle the bot in/out of the room).
+	//
+	// Work around this by inspecting each room's timeline and seeing if an m.room.member event for us
+	// exists and is "join" and then discard processing that room entirely if so.
+	for roomID, roomData := range syncResponse.Rooms.Join {
+		for i := len(roomData.Timeline.Events) - 1; i >= 0; i-- {
+			e := roomData.Timeline.Events[i]
+			if e.Type == "m.room.member" && e.StateKey == cli.UserID {
+				m := e.Content["membership"]
+				mship, ok := m.(string)
+				if !ok {
+					continue
+				}
+				if mship == "join" {
+					log.WithFields(log.Fields{
+						"room_id":     roomID,
+						"user_id":     cli.UserID,
+						"start_token": tokenOnSync,
+					}).Info("Discarding /sync events in room: just joined it.")
+					_, ok := syncResponse.Rooms.Join[roomID]
+					if !ok {
+						panic("room " + roomID + " does not exist in Join?!")
+					}
+					delete(syncResponse.Rooms.Join, roomID)   // don't re-process !commands
+					delete(syncResponse.Rooms.Invite, roomID) // don't re-process invites
+					break
+				}
+			}
+		}
+	}
+	return true
+}
+
 func (cli *Client) incrementSyncingID() uint32 {
 	cli.syncingMutex.Lock()
 	defer cli.syncingMutex.Unlock()