From 236a128129f594657478e3868430871dc75f60fa Mon Sep 17 00:00:00 2001
From: Slavi Pantaleev <slavi@devture.com>
Date: Thu, 25 Aug 2022 20:10:43 +0300
Subject: [PATCH] Fix double membership=join event handling

This fixes the problem mentioned in 8e1aa5b11c3cf0db.

Turns out that it's a long-standing Synapse bug:
https://github.com/matrix-org/synapse/issues/9768
---
 bot/bot.go  | 17 +++++++++--------
 bot/sync.go | 12 ++++++++----
 2 files changed, 17 insertions(+), 12 deletions(-)

diff --git a/bot/bot.go b/bot/bot.go
index 946d1f3..c9b37f5 100644
--- a/bot/bot.go
+++ b/bot/bot.go
@@ -19,14 +19,15 @@ import (
 
 // Bot represents matrix bot
 type Bot struct {
-	noowner    bool
-	federation bool
-	prefix     string
-	domain     string
-	rooms      map[string]id.RoomID
-	roomsmu    *sync.Mutex
-	log        *logger.Logger
-	lp         *linkpearl.Linkpearl
+	noowner       bool
+	federation    bool
+	prefix        string
+	domain        string
+	rooms         map[string]id.RoomID
+	roomsmu       *sync.Mutex
+	log           *logger.Logger
+	lp            *linkpearl.Linkpearl
+	handledEvents sync.Map
 }
 
 // New creates a new matrix bot
diff --git a/bot/sync.go b/bot/sync.go
index 4cc7af6..35efec0 100644
--- a/bot/sync.go
+++ b/bot/sync.go
@@ -12,10 +12,6 @@ func (b *Bot) initSync() {
 	b.lp.OnEventType(
 		event.StateMember,
 		func(_ mautrix.EventSource, evt *event.Event) {
-			// Trying to debug the membership=join event being handled twice here.
-			eventJSON, _ := evt.MarshalJSON()
-			b.log.Debug(string(eventJSON))
-
 			go b.onMembership(evt)
 		},
 	)
@@ -108,6 +104,14 @@ func (b *Bot) onEncryptedMessage(evt *event.Event) {
 
 // onBotJoin handles the "bot joined the room" event
 func (b *Bot) onBotJoin(evt *event.Event, hub *sentry.Hub) {
+	// Workaround for membership=join events which are delivered to us twice,
+	// as described in this bug report: https://github.com/matrix-org/synapse/issues/9768
+	_, exists := b.handledEvents.LoadOrStore(evt.ID, true)
+	if exists {
+		b.log.Info("Suppressing already handled event %s", evt.ID)
+		return
+	}
+
 	ctx := sentry.SetHubOnContext(context.Background(), hub)
 	span := sentry.StartSpan(ctx, "http.server", sentry.TransactionName("onBotJoin"))
 	defer span.Finish()