[WIP] Send introduction text and help after the bot joins a room

This appears to work, except that the join event is triggered twice,
and we send the (introduction + help) twice.

bot/sync.go

@@ -9,6 +9,16 @@ import (
 )
 
 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)
+		},
+	)
 	b.lp.OnEventType(
 		event.EventMessage,
 		func(_ mautrix.EventSource, evt *event.Event) {
@@ -21,6 +31,31 @@ func (b *Bot) initSync() {
 		})
 }
 
+func (b *Bot) onMembership(evt *event.Event) {
+	hub := sentry.CurrentHub().Clone()
+	hub.ConfigureScope(func(scope *sentry.Scope) {
+		scope.SetUser(sentry.User{ID: evt.Sender.String()})
+		scope.SetContext("event", map[string]string{
+			"id":     evt.ID.String(),
+			"room":   evt.RoomID.String(),
+			"sender": evt.Sender.String(),
+		})
+	})
+
+	if evt.Sender == b.lp.GetClient().UserID {
+		// Handle membership events related to our own (bot) user first
+
+		switch evt.Content.AsMember().Membership {
+		case event.MembershipJoin:
+			b.onBotJoin(evt, hub)
+		}
+
+		return
+	}
+
+	// Handle membership events related to other users
+}
+
 func (b *Bot) onMessage(evt *event.Event) {
 	// ignore own messages
 	if evt.Sender == b.lp.GetClient().UserID {
@@ -70,3 +105,13 @@ func (b *Bot) onEncryptedMessage(evt *event.Event) {
 
 	b.handle(span.Context(), decrypted)
 }
+
+// onBotJoin handles the "bot joined the room" event
+func (b *Bot) onBotJoin(evt *event.Event, hub *sentry.Hub) {
+	ctx := sentry.SetHubOnContext(context.Background(), hub)
+	span := sentry.StartSpan(ctx, "http.server", sentry.TransactionName("onBotJoin"))
+	defer span.Finish()
+
+	b.sendIntroduction(ctx, evt.RoomID)
+	b.sendHelp(ctx, evt.RoomID)
+}