From a14bacc9c13309593c6eab2d19b70bcd315d193d Mon Sep 17 00:00:00 2001
From: Alex Auvolat <alex@adnab.me>
Date: Fri, 21 Feb 2020 19:36:16 +0100
Subject: [PATCH] Keep in cache the Id of the last seen post for backlogging
 purposes

---
 appservice/account.go              | 14 ++++++++++++++
 connector/connector.go             |  7 +++++++
 connector/mattermost/mattermost.go | 12 ++++++++++--
 3 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/appservice/account.go b/appservice/account.go
index e2f2ee9..e17c425 100644
--- a/appservice/account.go
+++ b/appservice/account.go
@@ -315,3 +315,17 @@ func (a *Account) eventInternal(event *Event) error {
 		return nil
 	}
 }
+
+// ----
+
+func (a *Account) CacheGet(key string) string {
+	cache_key := fmt.Sprintf("%s/account/%s/%s/%s",
+		a.Protocol, a.MatrixUser, a.AccountName, key)
+	return dbCacheGet(cache_key)
+}
+
+func (a *Account) CachePut(key string, value string) {
+	cache_key := fmt.Sprintf("%s/account/%s/%s/%s",
+		a.Protocol, a.MatrixUser, a.AccountName, key)
+	dbCachePut(cache_key, value)
+}
diff --git a/connector/connector.go b/connector/connector.go
index 30dfea4..3a2e20c 100644
--- a/connector/connector.go
+++ b/connector/connector.go
@@ -84,6 +84,13 @@ type Handler interface {
 	// Called when an event occurs in a room
 	// This must not be called for events authored by the user of the connection
 	Event(event *Event)
+
+	// These two functions enable the connector to access a simple key/value
+	// database to cache some information in order not to generate useless events.
+	// The connector should function when they are not implemented,
+	// in which case CacheGet always returns ""
+	CachePut(key string, value string)
+	CacheGet(key string) string
 }
 
 type EventType int
diff --git a/connector/mattermost/mattermost.go b/connector/mattermost/mattermost.go
index 117286b..ab612ba 100644
--- a/connector/mattermost/mattermost.go
+++ b/connector/mattermost/mattermost.go
@@ -337,6 +337,7 @@ func (mm *Mattermost) handleConnected() {
 		mm.handler.RoomInfoUpdated(id, UserID(""), room_info)
 
 		// Update member list
+		// TODO (when this will be slow, i.e. hundreds of members): do only a diff
 		members, resp := mm.conn.Client.GetChannelMembers(ch.Id, 0, 1000, "")
 		if resp.Error == nil {
 			for _, mem := range *members {
@@ -356,8 +357,14 @@ func (mm *Mattermost) handleConnected() {
 		}
 
 		// Read backlog
-		// TODO: remember what was the last seen post in this channel
-		backlog, resp := mm.conn.Client.GetPostsForChannel(ch.Id, 0, 1000, "")
+		var backlog *model.PostList
+		last_seen_post := mm.handler.CacheGet(fmt.Sprintf("last_seen_%s", ch.Id))
+		if last_seen_post != "" {
+			backlog, resp = mm.conn.Client.GetPostsAfter(ch.Id, last_seen_post, 0, 1000, "")
+			// TODO: if there are more than 1000, loop around
+		} else {
+			backlog, resp = mm.conn.Client.GetPostsForChannel(ch.Id, 0, 1000, "")
+		}
 		if resp.Error == nil {
 			for i := 0; i < len(backlog.Order); i++ {
 				post_id := backlog.Order[len(backlog.Order)-i-1]
@@ -519,6 +526,7 @@ func (mm *Mattermost) handlePost(channel_name string, post *model.Post, only_mes
 		} else if post.Type == "" || post.Type == "me" {
 			msg_ev.Room = roomId
 			mm.handler.Event(msg_ev)
+			mm.handler.CachePut(fmt.Sprintf("last_seen_%s", post.ChannelId), post.Id)
 		} else {
 			return fmt.Errorf("Unhandled post type: %s", post.Type)
 		}