Preserve threading between Slack instances (#529)

* Opportunistically preserve Slack threading when parent thread in cache. [#529]

* Removed slack-specific processing from gateway.

* Added docs.

* Add option to enable threading, with default to off.

* Did cleanup on @42wim's comments.

* Update gateway/gateway.go

Co-Authored-By: patcon <patrick.c.connolly@gmail.com>

* Suggestion from @42wim :)

* Suggestions from @42wim.

* More suggestions.

6 files changed, 66 insertions, 11 deletions

diff --git a/README.md b/README.md
index 537d18e0..1bcbf974 100644
--- a/README.md
+++ b/README.md
@@ -37,6 +37,7 @@ Minecraft server chat support via [MatterLink](https://github.com/elytra/MatterL
 * [Support bridging between any protocols](https://github.com/42wim/matterbridge/wiki/Features#support-bridging-between-any-protocols)
 * [Support multiple gateways(bridges) for your protocols](https://github.com/42wim/matterbridge/wiki/Features#support-multiple-gatewaysbridges-for-your-protocols)
 * [Message edits and deletes](https://github.com/42wim/matterbridge/wiki/Features#message-edits-and-deletes)
+* Preserves threading when possible
 * [Attachment / files handling](https://github.com/42wim/matterbridge/wiki/Features#attachment--files-handling)
 * [Username and avatar spoofing](https://github.com/42wim/matterbridge/wiki/Features#username-and-avatar-spoofing)
 * [Private groups](https://github.com/42wim/matterbridge/wiki/Features#private-groups)
diff --git a/bridge/config/config.go b/bridge/config/config.go
index 0e36d960..5ccf6041 100644
--- a/bridge/config/config.go
+++ b/bridge/config/config.go
@@ -35,6 +35,7 @@ type Message struct {
 	Event     string    `json:"event"`
 	Protocol  string    `json:"protocol"`
 	Gateway   string    `json:"gateway"`
+	ParentID  string    `json:"parent_id"`
 	Timestamp time.Time `json:"timestamp"`
 	ID        string    `json:"id"`
 	Extra     map[string][]interface{}
@@ -98,6 +99,7 @@ type Protocol struct {
 	NoTLS                  bool       // mattermost
 	Password               string     // IRC,mattermost,XMPP,matrix
 	PrefixMessagesWithNick bool       // mattemost, slack
+	PreserveThreading      bool       // slack
 	Protocol               string     // all protocols
 	QuoteDisable           bool       // telegram
 	QuoteFormat            string     // telegram
diff --git a/bridge/slack/handlers.go b/bridge/slack/handlers.go
index b6400a80..1428cba2 100644
--- a/bridge/slack/handlers.go
+++ b/bridge/slack/handlers.go
@@ -171,8 +171,10 @@ func (b *Bslack) handleMessageEvent(ev *slack.MessageEvent) (*config.Message, er
 		Account: b.Account,
 		ID:      "slack " + ev.Timestamp,
 		Extra:   map[string][]interface{}{},
+		ParentID: ev.ThreadTimestamp,
 	}
 
+
 	if b.useChannelID {
 		rmsg.Channel = "ID:" + channelInfo.ID
 	}
diff --git a/bridge/slack/slack.go b/bridge/slack/slack.go
index 2022ebce..d2d118b2 100644
--- a/bridge/slack/slack.go
+++ b/bridge/slack/slack.go
@@ -310,6 +310,10 @@ func (b *Bslack) prepareMessageParameters(msg *config.Message) *slack.PostMessag
 	params.Username = msg.Username
 	params.LinkNames = 1 // replace mentions
 	params.IconURL = config.GetIconURL(msg, b.GetString(iconURLConfig))
+	msgFields := strings.Fields(msg.ParentID)
+	if len(msgFields) >= 2 {
+		params.ThreadTimestamp = msgFields[1]
+	}
 	if msg.Avatar != "" {
 		params.IconURL = msg.Avatar
 	}
diff --git a/gateway/gateway.go b/gateway/gateway.go
index 5bfb00e2..c1905a9b 100644
--- a/gateway/gateway.go
+++ b/gateway/gateway.go
@@ -83,6 +83,25 @@ func New(cfg config.Gateway, r *Router) *Gateway {
 	return gw
 }
 
+// Find the canonical ID that the message is keyed under in cache
+func (gw *Gateway) FindCanonicalMsgID(mID string) string {
+	if gw.Messages.Contains(mID) {
+		return mID
+	}
+
+	// If not keyed, iterate through cache for downstream, and infer upstream.
+	for _, mid := range gw.Messages.Keys() {
+		v, _ := gw.Messages.Peek(mid)
+		ids := v.([]*BrMsgID)
+		for _, downstreamMsgObj := range ids {
+			if mID == downstreamMsgObj.ID {
+				return mid.(string)
+			}
+		}
+	}
+	return ""
+}
+
 func (gw *Gateway) AddBridge(cfg *config.Bridge) error {
 	br := gw.Router.getBridge(cfg.Account)
 	if br == nil {
@@ -206,6 +225,20 @@ func (gw *Gateway) getDestChannel(msg *config.Message, dest bridge.Bridge) []con
 	return channels
 }
 
+func (gw *Gateway) getDestMsgID(msgID string, dest *bridge.Bridge, channel config.ChannelInfo) string {
+	if res, ok := gw.Messages.Get(msgID); ok {
+		IDs := res.([]*BrMsgID)
+		for _, id := range IDs {
+			// check protocol, bridge name and channelname
+			// for people that reuse the same bridge multiple times. see #342
+			if dest.Protocol == id.br.Protocol && dest.Name == id.br.Name && channel.ID == id.ChannelID {
+				return id.ID
+			}
+		}
+	}
+	return ""
+}
+
 func (gw *Gateway) handleMessage(msg config.Message, dest *bridge.Bridge) []*BrMsgID {
 	var brMsgIDs []*BrMsgID
 
@@ -242,6 +275,13 @@ func (gw *Gateway) handleMessage(msg config.Message, dest *bridge.Bridge) []*BrM
 		return brMsgIDs
 	}
 
+	// Get the ID of the parent message in thread
+	var canonicalParentMsgID string
+	if msg.ParentID != "" && (gw.Config.General.PreserveThreading || dest.GetBool("PreserveThreading")) {
+		thisParentMsgID := dest.Protocol + " " + msg.ParentID
+		canonicalParentMsgID = gw.FindCanonicalMsgID(thisParentMsgID)
+	}
+
 	originchannel := msg.Channel
 	origmsg := msg
 	channels := gw.getDestChannel(&msg, *dest)
@@ -258,28 +298,28 @@ func (gw *Gateway) handleMessage(msg config.Message, dest *bridge.Bridge) []*BrM
 			}
 		}
 		flog.Debugf("=> Sending %#v from %s (%s) to %s (%s)", msg, msg.Account, originchannel, dest.Account, channel.Name)
+
 		msg.Channel = channel.Name
 		msg.Avatar = gw.modifyAvatar(origmsg, dest)
 		msg.Username = gw.modifyUsername(origmsg, dest)
-		msg.ID = ""
-		if res, ok := gw.Messages.Get(origmsg.ID); ok {
-			IDs := res.([]*BrMsgID)
-			for _, id := range IDs {
-				// check protocol, bridge name and channelname
-				// for people that reuse the same bridge multiple times. see #342
-				if dest.Protocol == id.br.Protocol && dest.Name == id.br.Name && channel.ID == id.ChannelID {
-					msg.ID = id.ID
-				}
-			}
-		}
+
+		msg.ID = gw.getDestMsgID(origmsg.ID, dest, channel)
+
 		// for api we need originchannel as channel
 		if dest.Protocol == "api" {
 			msg.Channel = originchannel
 		}
+
+		msg.ParentID =  gw.getDestMsgID(canonicalParentMsgID, dest, channel)
+		if msg.ParentID == "" {
+		   msg.ParentID = canonicalParentMsgID
+		}
+
 		mID, err := dest.Send(msg)
 		if err != nil {
 			flog.Error(err)
 		}
+
 		// append the message ID (mID) from this bridge (dest) to our brMsgIDs slice
 		if mID != "" {
 			flog.Debugf("mID %s: %s", dest.Account, mID)
diff --git a/matterbridge.toml.sample b/matterbridge.toml.sample
index 76875aca..441a333c 100644
--- a/matterbridge.toml.sample
+++ b/matterbridge.toml.sample
@@ -659,6 +659,12 @@ StripNick=false
 #OPTIONAL (default false)
 ShowTopicChange=false
 
+#Opportunistically preserve threaded replies between Slack channels.
+#This only works if the parent message is still in the cache.
+#Cache is flushed between restarts.
+#OPTIONAL (default false)
+PreserveThreading=false
+
 ###################################################################
 #discord section
 ###################################################################