From 439f5d10336e3233f8fd4aec369700a48c549651 Mon Sep 17 00:00:00 2001
From: Tulir Asokan <tulir@maunium.net>
Date: Tue, 8 Aug 2023 17:05:56 +0300
Subject: [PATCH] Delay missed backfill if messages are recent

---
 backfill.go | 32 ++++++++++++++++++++++++++++----
 portal.go   |  5 +++--
 user.go     |  2 +-
 3 files changed, 32 insertions(+), 7 deletions(-)

diff --git a/backfill.go b/backfill.go
index cebc8d7..86dff9a 100644
--- a/backfill.go
+++ b/backfill.go
@@ -43,17 +43,41 @@ func (portal *Portal) initialForwardBackfill(user *User, markRead bool) {
 	portal.forwardBackfill(ctx, user, time.Time{}, portal.bridge.Config.Bridge.Backfill.InitialLimit, markRead)
 }
 
+const recentBackfillDelay = 15 * time.Second
+
+type pendingBackfill struct {
+	cancel        context.CancelFunc
+	lastMessageID string
+	lastMessageTS time.Time
+}
+
 func (portal *Portal) missedForwardBackfill(user *User, lastMessageTS time.Time, lastMessageID string, markRead bool) bool {
 	if portal.bridge.Config.Bridge.Backfill.MissedLimit == 0 {
 		return false
 	}
+	log := portal.zlog.With().
+		Str("action", "missed forward backfill").
+		Str("latest_message_id", lastMessageID).
+		Logger()
+	ctx := log.WithContext(context.TODO())
+	if time.Since(lastMessageTS) < 5*time.Minute {
+		var cancel context.CancelFunc
+		ctx, cancel = context.WithCancel(ctx)
+		prev := portal.pendingRecentBackfill.Swap(&pendingBackfill{cancel: cancel, lastMessageID: lastMessageID, lastMessageTS: lastMessageTS})
+		if prev != nil {
+			prev.cancel()
+		}
+		log.Debug().Msg("Delaying missed forward backfill as latest message is new")
+		select {
+		case <-time.After(recentBackfillDelay):
+		case <-ctx.Done():
+			log.Debug().Msg("Backfill was cancelled by a newer backfill")
+			return false
+		}
+	}
 
 	portal.forwardBackfillLock.Lock()
 	defer portal.forwardBackfillLock.Unlock()
-	log := portal.zlog.With().
-		Str("action", "missed forward backfill").
-		Logger()
-	ctx := log.WithContext(context.TODO())
 	if !lastMessageTS.IsZero() {
 		if portal.lastMessageTS.IsZero() {
 			lastMsg, err := portal.bridge.DB.Message.GetLastInChat(ctx, portal.Key)
diff --git a/portal.go b/portal.go
index 5e19249..9089daa 100644
--- a/portal.go
+++ b/portal.go
@@ -25,6 +25,7 @@ import (
 	_ "image/gif"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/gabriel-vasile/mimetype"
@@ -239,6 +240,8 @@ type Portal struct {
 	forwardBackfillLock sync.Mutex
 	lastMessageTS       time.Time
 
+	pendingRecentBackfill atomic.Pointer[pendingBackfill]
+
 	recentlyHandled      [recentlyHandledLength]string
 	recentlyHandledLock  sync.Mutex
 	recentlyHandledIndex uint8
@@ -270,8 +273,6 @@ func (portal *Portal) handleMessageLoopItem(msg PortalMessage) {
 	switch {
 	case msg.evt != nil:
 		portal.handleMessage(msg.source, msg.evt)
-	//case msg.receipt != nil:
-	//	portal.handleReceipt(msg.receipt, msg.source)
 	default:
 		portal.zlog.Warn().Interface("portal_message", msg).Msg("Unexpected PortalMessage with no message")
 	}
diff --git a/user.go b/user.go
index 8bb1a54..f461c12 100644
--- a/user.go
+++ b/user.go
@@ -831,8 +831,8 @@ func (user *User) syncConversation(v *gmproto.Conversation, source string) {
 			}
 			log.Debug().Msg("Syncing existing portal")
 			portal.UpdateMetadata(user, v)
-			didBackfill := portal.missedForwardBackfill(user, time.UnixMicro(v.LastMessageTimestamp), v.LatestMessageID, !v.GetUnread())
 			user.syncChatDoublePuppetDetails(portal, v, false)
+			didBackfill := portal.missedForwardBackfill(user, time.UnixMicro(v.LastMessageTimestamp), v.LatestMessageID, !v.GetUnread())
 			if !didBackfill && !v.GetUnread() && v.LatestMessageID != "" && user.DoublePuppetIntent != nil {
 				// TODO this would spam a lot of read receipts on startup
 				//user.markSelfReadFull(portal, v.LatestMessageID)