connector: first pass at bridging guilds

For each guild specified in the config, create a space and bridge all contained text channels that the user has permissions to view. * Finally add a custom config struct where we accept a list of guild IDs to bridge. This is intended to be temporary as we flesh out the proper interfaces for managing which guilds to bridge. * Defined a custom meta type for portals that holds the containing guild ID of the channel (if any). * Transferred the responsibility of building a channel's ChatInfo and ChatMemberList to the DiscordChatResync event itself.
2026-01-09 17:08:33 -08:00
parent 86544bc7af
commit f5292e6a7d
9 changed files with 366 additions and 72 deletions
--- a/pkg/connector/client.go
+++ b/pkg/connector/client.go
@@ -33,6 +33,8 @@ import (
 	"maunium.net/go/mautrix/bridgev2/networkid"
 	"maunium.net/go/mautrix/bridgev2/status"

+	"go.mau.fi/util/ptr"
+
 	"go.mau.fi/mautrix-discord/pkg/discordid"
 )

@@ -202,6 +204,7 @@ func (cl *DiscordClient) BeginSyncingIfUserLoginPresent(ctx context.Context) {
 	}

 	go cl.syncPrivateChannels(ctx)
+	go cl.syncGuilds(ctx)
 }

 func (d *DiscordClient) syncPrivateChannels(ctx context.Context) {
@@ -212,13 +215,199 @@ func (d *DiscordClient) syncPrivateChannels(ctx context.Context) {
 		bts, _ := discordgo.SnowflakeTimestamp(b.LastMessageID)
 		return bts.Compare(ats)
 	})
+
 	// TODO(skip): This is startup_private_channel_create_limit. Support this in the config.
 	for _, dm := range dms[:10] {
 		zerolog.Ctx(ctx).Debug().Str("channel_id", dm.ID).Msg("Syncing private channel with recent activity")
-		d.syncChannel(ctx, dm, true)
+		d.syncChannel(ctx, dm)
 	}
 }

+func (d *DiscordClient) canSeeGuildChannel(ctx context.Context, ch *discordgo.Channel) bool {
+	log := zerolog.Ctx(ctx).With().
+		Str("channel_id", ch.ID).
+		Int("channel_type", int(ch.Type)).
+		Str("action", "determine guild channel visbility").Logger()
+
+	sess := d.Session
+	myDiscordUserID := d.Session.State.User.ID
+
+	// To calculate guild channel visibility we need to know our effective permission
+	// bitmask, which can only be truly determined when we know which roles we have
+	// in the guild.
+	//
+	// To this end, make sure we have detailed information about ourselves in the
+	// cache ("state").
+
+	_, err := sess.State.Member(ch.GuildID, myDiscordUserID)
+	if errors.Is(err, discordgo.ErrStateNotFound) {
+		log.Debug().Msg("Fetching own membership in guild to check roles")
+
+		member, err := sess.GuildMember(ch.GuildID, myDiscordUserID)
+		if err != nil {
+			log.Warn().Err(err).Msg("Failed to get own membership in guild from server")
+		} else {
+			err = sess.State.MemberAdd(member)
+			if err != nil {
+				log.Warn().Err(err).Msg("Failed to add own membership in guild to cache")
+			}
+		}
+	} else if err != nil {
+		log.Warn().Err(err).Msg("Failed to get own membership in guild from cache")
+	}
+
+	err = sess.State.ChannelAdd(ch)
+	if err != nil {
+		log.Warn().Err(err).Msg("Failed to add channel to cache")
+	}
+
+	perms, err := sess.State.UserChannelPermissions(myDiscordUserID, ch.ID)
+	if err != nil {
+		log.Warn().Err(err).Msg("Failed to get permissions in channel to determine if it's bridgeable")
+		return true
+	}
+
+	canView := perms&discordgo.PermissionViewChannel > 0
+	log.Debug().
+		Int64("permissions", perms).
+		Bool("channel_visible", canView).
+		Msg("Computed visibility of guild channel")
+	return canView
+}
+
+// The string prepended to [networkid.PortalKey]s identifying spaces that
+// bridge Discord guilds.
+//
+// Every Discord guild created before August 2017 contained an channel
+// having _the same ID as the guild itself_. This channel also functioned as
+// the "default channel" in that incoming members would view this channel by
+// default. It was also impossible to delete.
+//
+// After this date, these "default channels" became deletable, and fresh guilds
+// were no longer created with a channel that exactly corresponded to the guild
+// ID.
+//
+// To accommodate Discord guilds created before this API change that have also
+// never deleted the default channel, we need a way to distinguish between the
+// guild and the default channel, as we wouldn't be able to bridge the guild
+// as a space otherwise.
+//
+// "*" was chosen as the asterisk character is used to filter by guilds in
+// the quick switcher (in Discord's first-party clients).
+//
+// For more information, see: https://discord.com/developers/docs/change-log#breaking-change-default-channels:~:text=New%20guilds%20will%20no%20longer.
+const guildPortalKeySigil = "*"
+
+func (d *DiscordClient) guildPortalKeyFromID(guildID string) networkid.PortalKey {
+	// TODO: Support configuring `split_portals`.
+	return networkid.PortalKey{
+		ID:       networkid.PortalID(guildPortalKeySigil + guildID),
+		Receiver: d.UserLogin.ID,
+	}
+}
+
+func (d *DiscordClient) makeAvatarForGuild(guild *discordgo.Guild) *bridgev2.Avatar {
+	return &bridgev2.Avatar{
+		ID: networkid.AvatarID(guild.Icon),
+		Get: func(ctx context.Context) ([]byte, error) {
+			url := discordgo.EndpointGuildIcon(guild.ID, guild.Icon)
+			return simpleDownload(ctx, url, "group dm icon")
+		},
+		Remove: guild.Icon == "",
+	}
+}
+
+func (d *DiscordClient) syncGuildSpace(ctx context.Context, guild *discordgo.Guild) error {
+	prt, err := d.connector.Bridge.GetPortalByKey(ctx, d.guildPortalKeyFromID(guild.ID))
+	if err != nil {
+		return fmt.Errorf("couldn't get/create portal corresponding to guild: %w", err)
+	}
+
+	selfEvtSender := d.selfEventSender()
+	info := &bridgev2.ChatInfo{
+		Name:  &guild.Name,
+		Topic: nil,
+		Members: &bridgev2.ChatMemberList{
+			MemberMap: map[networkid.UserID]bridgev2.ChatMember{selfEvtSender.Sender: {EventSender: selfEvtSender}},
+
+			// As recommended by the spec, prohibit normal events by setting
+			// `events_default` to a suitably high number.
+			PowerLevels: &bridgev2.PowerLevelOverrides{EventsDefault: ptr.Ptr(100)},
+		},
+		Avatar: d.makeAvatarForGuild(guild),
+		Type:   ptr.Ptr(database.RoomTypeSpace),
+	}
+
+	if prt.MXID == "" {
+		err := prt.CreateMatrixRoom(ctx, d.UserLogin, info)
+
+		if err != nil {
+			return fmt.Errorf("couldn't create room in order to materialize guild portal: %w", err)
+		}
+	} else {
+		prt.UpdateInfo(ctx, info, d.UserLogin, nil, time.Time{})
+	}
+
+	return nil
+}
+
+func (d *DiscordClient) syncGuilds(ctx context.Context) {
+	guildIDs := d.connector.Config.Guilds.BridgingGuildIDs
+
+	for _, guildID := range guildIDs {
+		log := zerolog.Ctx(ctx).With().
+			Str("guild_id", guildID).
+			Str("action", "sync guild").
+			Logger()
+
+		guild, err := d.Session.State.Guild(guildID)
+		if errors.Is(err, discordgo.ErrStateNotFound) || guild == nil {
+			log.Err(err).Msg("Couldn't find guild, user isn't a member?")
+			continue
+		}
+
+		err = d.syncGuildSpace(ctx, guild)
+		if err != nil {
+			log.Err(err).Msg("Couldn't sync guild space portal")
+			continue
+		}
+
+		for _, guildCh := range guild.Channels {
+			if guildCh.Type != discordgo.ChannelTypeGuildText {
+				// TODO implement categories (spaces) and news channels
+				log.Trace().
+					Str("channel_id", guildCh.ID).
+					Int("channel_type", int(guildCh.Type)).
+					Msg("Not bridging guild channel due to type")
+				continue
+			}
+
+			if !d.canSeeGuildChannel(ctx, guildCh) {
+				log.Trace().
+					Str("channel_id", guildCh.ID).
+					Int("channel_type", int(guildCh.Type)).
+					Msg("Not bridging guild channel that the user doesn't have permission to view")
+
+				continue
+			}
+
+			d.syncChannel(ctx, guildCh)
+		}
+
+		log.Debug().Msg("Subscribing to guild after bridging")
+		err = d.Session.SubscribeGuild(discordgo.GuildSubscribeData{
+			GuildID:    guild.ID,
+			Typing:     true,
+			Activities: true,
+			Threads:    true,
+		})
+		if err != nil {
+			log.Warn().Err(err).Msg("Failed to subscribe to guild")
+		}
+	}
+
+}
+
 func simpleDownload(ctx context.Context, url, thing string) ([]byte, error) {
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
 	if err != nil {
@@ -238,17 +427,6 @@ func simpleDownload(ctx context.Context, url, thing string) ([]byte, error) {
 	return data, nil
 }

-func makeChannelAvatar(ch *discordgo.Channel) *bridgev2.Avatar {
-	return &bridgev2.Avatar{
-		ID: networkid.AvatarID(ch.Icon),
-		Get: func(ctx context.Context) ([]byte, error) {
-			url := discordgo.EndpointGroupIcon(ch.ID, ch.Icon)
-			return simpleDownload(ctx, url, "group dm icon")
-		},
-		Remove: ch.Icon == "",
-	}
-}
-
 func (d *DiscordClient) makeEventSenderWithID(userID string) bridgev2.EventSender {
 	return bridgev2.EventSender{
 		IsFromMe:    userID == d.Session.State.User.ID,
@@ -257,49 +435,18 @@ func (d *DiscordClient) makeEventSenderWithID(userID string) bridgev2.EventSende
 	}
 }

+func (d *DiscordClient) selfEventSender() bridgev2.EventSender {
+	return d.makeEventSenderWithID(d.Session.State.User.ID)
+}
+
 func (d *DiscordClient) makeEventSender(user *discordgo.User) bridgev2.EventSender {
 	return d.makeEventSenderWithID(user.ID)
 }

-func (d *DiscordClient) syncChannel(_ context.Context, ch *discordgo.Channel, selfIsInChannel bool) {
-	isGroup := len(ch.RecipientIDs) > 1
-
-	var roomType database.RoomType
-	if isGroup {
-		roomType = database.RoomTypeGroupDM
-	} else {
-		roomType = database.RoomTypeDM
-	}
-
-	selfEventSender := d.makeEventSender(d.Session.State.User)
-
-	var members bridgev2.ChatMemberList
-	members.IsFull = true
-	members.MemberMap = make(bridgev2.ChatMemberMap, len(ch.Recipients))
-	if len(ch.Recipients) > 0 {
-		// Private channels' array of participants doesn't include ourselves,
-		// so this boolean can be used to inject ourselves as a member.
-		if selfIsInChannel {
-			members.MemberMap[selfEventSender.Sender] = bridgev2.ChatMember{EventSender: selfEventSender}
-		}
-
-		for _, recipient := range ch.Recipients {
-			sender := d.makeEventSender(recipient)
-			members.MemberMap[sender.Sender] = bridgev2.ChatMember{EventSender: sender}
-		}
-
-		members.TotalMemberCount = len(ch.Recipients)
-	}
-
+func (d *DiscordClient) syncChannel(_ context.Context, ch *discordgo.Channel) {
 	d.connector.Bridge.QueueRemoteEvent(d.UserLogin, &DiscordChatResync{
+		Client:    d,
 		channel:   ch,
 		portalKey: discordid.MakePortalKey(ch, d.UserLogin.ID, true),
-		info: &bridgev2.ChatInfo{
-			Name:        &ch.Name,
-			Members:     &members,
-			Avatar:      makeChannelAvatar(ch),
-			Type:        &roomType,
-			CanBackfill: true,
-		},
 	})
 }
--- a/pkg/connector/config.go
+++ b/pkg/connector/config.go
@@ -17,9 +17,24 @@
 package connector

 import (
-	"go.mau.fi/util/configupgrade"
+	_ "embed"
+
+	up "go.mau.fi/util/configupgrade"
 )

-func (d *DiscordConnector) GetConfig() (example string, data any, upgrader configupgrade.Upgrader) {
-	return "", nil, configupgrade.NoopUpgrader
+//go:embed example-config.yaml
+var ExampleConfig string
+
+type Config struct {
+	Guilds struct {
+		BridgingGuildIDs []string `yaml:"bridging_guild_ids"`
+	} `yaml:"guilds"`
+}
+
+func upgradeConfig(helper up.Helper) {
+	helper.Copy(up.List, "guilds", "bridging_guild_ids")
+}
+
+func (d *DiscordConnector) GetConfig() (example string, data any, upgrader up.Upgrader) {
+	return ExampleConfig, &d.Config, up.SimpleUpgrader(upgradeConfig)
 }
--- a/pkg/connector/connector.go
+++ b/pkg/connector/connector.go
@@ -26,6 +26,7 @@ import (

 type DiscordConnector struct {
 	Bridge  *bridgev2.Bridge
+	Config  *Config
 	MsgConv *msgconv.MessageConverter
 }

--- a/pkg/connector/dbmeta.go
+++ b/pkg/connector/dbmeta.go
@@ -24,6 +24,9 @@ import (

 func (d *DiscordConnector) GetDBMetaTypes() database.MetaTypes {
 	return database.MetaTypes{
+		Portal: func() any {
+			return &discordid.PortalMetadata{}
+		},
 		UserLogin: func() any {
 			return &discordid.UserLoginMetadata{}
 		},
--- a/pkg/connector/events.go
+++ b/pkg/connector/events.go
@@ -21,15 +21,18 @@ import (

 	"github.com/bwmarrin/discordgo"
 	"github.com/rs/zerolog"
+	"go.mau.fi/util/ptr"
 	"maunium.net/go/mautrix/bridgev2"
 	"maunium.net/go/mautrix/bridgev2/database"
 	"maunium.net/go/mautrix/bridgev2/networkid"
+
+	"go.mau.fi/mautrix-discord/pkg/discordid"
 )

 type DiscordChatResync struct {
+	Client    *DiscordClient
 	channel   *discordgo.Channel
 	portalKey networkid.PortalKey
-	info      *bridgev2.ChatInfo
 }

 var (
@@ -55,11 +58,112 @@ func (d *DiscordChatResync) GetType() bridgev2.RemoteEventType {
 	return bridgev2.RemoteEventChatResync
 }

-func (d *DiscordChatResync) GetChatInfo(ctx context.Context, portal *bridgev2.Portal) (*bridgev2.ChatInfo, error) {
-	if d.info == nil {
-		return nil, nil
+func (d *DiscordChatResync) avatar(ctx context.Context) *bridgev2.Avatar {
+	ch := d.channel
+
+	// TODO make this configurable (ala workspace_avatar_in_rooms)
+	if !d.isPrivate() {
+		guild, err := d.Client.Session.State.Guild(ch.GuildID)
+
+		if err != nil || guild == nil {
+			zerolog.Ctx(ctx).Err(err).Msg("Couldn't look up guild in cache in order to create room avatar")
+			return nil
 		}
-	return d.info, nil
+
+		return d.Client.makeAvatarForGuild(guild)
+	}
+
+	return &bridgev2.Avatar{
+		ID: networkid.AvatarID(ch.Icon),
+		Get: func(ctx context.Context) ([]byte, error) {
+			url := discordgo.EndpointGroupIcon(ch.ID, ch.Icon)
+			return simpleDownload(ctx, url, "group dm icon")
+		},
+		Remove: ch.Icon == "",
+	}
+}
+
+func (d *DiscordChatResync) privateChannelMemberList() bridgev2.ChatMemberList {
+	ch := d.channel
+
+	var members bridgev2.ChatMemberList
+	members.IsFull = true
+	members.MemberMap = make(bridgev2.ChatMemberMap, len(ch.Recipients))
+	if len(ch.Recipients) > 0 {
+		selfEventSender := d.Client.selfEventSender()
+
+		// Private channels' array of participants doesn't include ourselves,
+		// so inject ourselves as a member.
+		members.MemberMap[selfEventSender.Sender] = bridgev2.ChatMember{EventSender: selfEventSender}
+
+		for _, recipient := range ch.Recipients {
+			sender := d.Client.makeEventSender(recipient)
+			members.MemberMap[sender.Sender] = bridgev2.ChatMember{EventSender: sender}
+		}
+
+		members.TotalMemberCount = len(ch.Recipients)
+	}
+
+	return members
+}
+
+func (d *DiscordChatResync) memberList() bridgev2.ChatMemberList {
+	if d.isPrivate() {
+		return d.privateChannelMemberList()
+	}
+
+	// TODO we're _always_ sending partial member lists for guilds; we can probably
+	// do better
+	selfEventSender := d.Client.selfEventSender()
+
+	return bridgev2.ChatMemberList{
+		IsFull: false,
+		MemberMap: map[networkid.UserID]bridgev2.ChatMember{
+			selfEventSender.Sender: {EventSender: selfEventSender},
+		},
+	}
+}
+
+func (d *DiscordChatResync) isPrivate() bool {
+	ch := d.channel
+	return ch.Type == discordgo.ChannelTypeDM || ch.Type == discordgo.ChannelTypeGroupDM
+}
+
+func (d *DiscordChatResync) GetChatInfo(ctx context.Context, portal *bridgev2.Portal) (*bridgev2.ChatInfo, error) {
+	ch := d.channel
+
+	var roomType database.RoomType
+
+	switch ch.Type {
+	case discordgo.ChannelTypeDM:
+		roomType = database.RoomTypeDM
+	case discordgo.ChannelTypeGroupDM:
+		roomType = database.RoomTypeGroupDM
+	}
+
+	info := &bridgev2.ChatInfo{
+		Name:        &ch.Name,
+		Members:     ptr.Ptr(d.memberList()),
+		Avatar:      d.avatar(ctx),
+		Type:        &roomType,
+		CanBackfill: true,
+		ExtraUpdates: func(ctx context.Context, portal *bridgev2.Portal) (changed bool) {
+			meta := portal.Metadata.(*discordid.PortalMetadata)
+			if meta.GuildID != ch.GuildID {
+				meta.GuildID = ch.GuildID
+				changed = true
+			}
+
+			return
+		},
+	}
+
+	if !d.isPrivate() {
+		// Channel belongs to a guild; associate it with the respective space.
+		info.ParentID = ptr.Ptr(d.Client.guildPortalKeyFromID(ch.GuildID).ID)
+	}
+
+	return info, nil
 }

 func (d *DiscordChatResync) ShouldCreatePortal() bool {
--- a/pkg/connector/example-config.yaml
+++ b/pkg/connector/example-config.yaml
@@ -0,0 +1,6 @@
+# Configuration options related to Discord guilds (also known as "servers").
+guilds:
+    # UNSTABLE: The IDs of the guilds to bridge. This is a stopgap measure
+    # during bridge development. If no guild IDs are specified, then no guilds
+    # are bridged at all.
+    bridging_guild_ids: []
--- a/pkg/connector/handlematrix.go
+++ b/pkg/connector/handlematrix.go
@@ -25,6 +25,8 @@ import (
 	"maunium.net/go/mautrix/bridgev2"
 	"maunium.net/go/mautrix/bridgev2/database"
 	"maunium.net/go/mautrix/bridgev2/networkid"
+
+	"go.mau.fi/mautrix-discord/pkg/discordid"
 )

 var (
@@ -41,6 +43,7 @@ func (d *DiscordClient) HandleMatrixMessage(ctx context.Context, msg *bridgev2.M
 	}

 	portal := msg.Portal
+	guildID := portal.Metadata.(*discordid.PortalMetadata).GuildID
 	channelID := string(portal.ID)

 	sendReq, err := d.connector.MsgConv.ToDiscord(ctx, d.Session, msg)
@@ -50,8 +53,7 @@ func (d *DiscordClient) HandleMatrixMessage(ctx context.Context, msg *bridgev2.M

 	var options []discordgo.RequestOption
 	// TODO: When supporting threads (and not a bot user), send a thread referer.
-	// TODO: Pass the guild ID when send messages in guild channels.
-	options = append(options, discordgo.WithChannelReferer("", channelID))
+	options = append(options, discordgo.WithChannelReferer(guildID, channelID))

 	sentMsg, err := d.Session.ChannelMessageSendComplex(string(msg.Portal.ID), sendReq, options...)
 	if err != nil {
@@ -84,12 +86,11 @@ func (d *DiscordClient) PreHandleMatrixReaction(ctx context.Context, reaction *b
 }

 func (d *DiscordClient) HandleMatrixReaction(ctx context.Context, reaction *bridgev2.MatrixReaction) (*database.Reaction, error) {
-	key := reaction.Content.RelatesTo.Key
+	relatesToKey := reaction.Content.RelatesTo.Key
 	portal := reaction.Portal
-	// TODO: Support guilds.
-	guildID := ""
+	meta := portal.Metadata.(*discordid.PortalMetadata)

-	err := d.Session.MessageReactionAddUser(guildID, string(portal.ID), string(reaction.TargetMessage.ID), key)
+	err := d.Session.MessageReactionAddUser(meta.GuildID, string(portal.ID), string(reaction.TargetMessage.ID), relatesToKey)
 	return nil, err
 }

@@ -97,8 +98,7 @@ func (d *DiscordClient) HandleMatrixReactionRemove(ctx context.Context, removal
 	removing := removal.TargetReaction
 	emojiID := removing.EmojiID
 	channelID := string(removing.Room.ID)
-	// TODO: Support guilds.
-	guildID := ""
+	guildID := removal.Portal.Metadata.(*discordid.PortalMetadata).GuildID

 	err := d.Session.MessageReactionRemoveUser(guildID, channelID, string(removing.MessageID), string(emojiID), string(d.UserLogin.ID))
 	return err
@@ -149,9 +149,10 @@ func (d *DiscordClient) HandleMatrixReadReceipt(ctx context.Context, msg *bridge
 		}
 	}

-	// TODO: Support guilds and threads.
+	// TODO: Support threads.
+	guildID := msg.Portal.Metadata.(*discordid.PortalMetadata).GuildID
 	channelID := string(msg.Portal.ID)
-	resp, err := d.Session.ChannelMessageAckNoToken(channelID, targetMessageID, discordgo.WithChannelReferer("", channelID))
+	resp, err := d.Session.ChannelMessageAckNoToken(channelID, targetMessageID, discordgo.WithChannelReferer(guildID, channelID))
 	if err != nil {
 		log.Err(err).Msg("Failed to send read receipt to Discord")
 		return err
@@ -167,7 +168,11 @@ func (d *DiscordClient) HandleMatrixReadReceipt(ctx context.Context, msg *bridge
 }

 func (d *DiscordClient) viewingChannel(ctx context.Context, portal *bridgev2.Portal) error {
-	// TODO: When guilds are supported, explicitly bail out if this method is called with a guild channel.
+	if portal.Metadata.(*discordid.PortalMetadata).GuildID != "" {
+		// Only private channels need this logic.
+		return nil
+	}
+
 	d.markedOpenedLock.Lock()
 	defer d.markedOpenedLock.Unlock()

@@ -201,9 +206,10 @@ func (d *DiscordClient) HandleMatrixTyping(ctx context.Context, msg *bridgev2.Ma
 	// Don't mind if this fails.
 	_ = d.viewingChannel(ctx, msg.Portal)

+	guildID := msg.Portal.Metadata.(*discordid.PortalMetadata).GuildID
 	channelID := string(msg.Portal.ID)
-	// TODO: Support guilds and threads properly when sending the referer.
-	err := d.Session.ChannelTyping(channelID, discordgo.WithChannelReferer("", channelID))
+	// TODO: Support threads properly when sending the referer.
+	err := d.Session.ChannelTyping(channelID, discordgo.WithChannelReferer(guildID, channelID))

 	if err != nil {
 		log.Warn().Err(err).Msg("Failed to mark user as typing")
--- a/pkg/discordid/dbmeta.go
+++ b/pkg/discordid/dbmeta.go
@@ -18,6 +18,15 @@ package discordid

 import "github.com/bwmarrin/discordgo"

+type PortalMetadata struct {
+	// The ID of the Discord guild that the channel corresponding to this portal
+	// belongs to.
+	//
+	// For private channels (DMs and group DMs), this will be the zero value
+	// (an empty string).
+	GuildID string `json:"guild_id"`
+}
+
 type UserLoginMetadata struct {
 	Token            string                     `json:"token"`
 	HeartbeatSession discordgo.HeartbeatSession `json:"heartbeat_session"`
--- a/pkg/msgconv/from-matrix.go
+++ b/pkg/msgconv/from-matrix.go
@@ -31,6 +31,8 @@ import (
 	"maunium.net/go/mautrix/bridgev2"
 	"maunium.net/go/mautrix/event"
 	"maunium.net/go/mautrix/format"
+
+	"go.mau.fi/mautrix-discord/pkg/discordid"
 )

 const discordEpochMillis = 1420070400000
@@ -109,6 +111,7 @@ func (mc *MessageConverter) ToDiscord(
 	}

 	portal := msg.Portal
+	guildID := msg.Portal.Metadata.(*discordid.PortalMetadata).GuildID
 	channelID := string(portal.ID)
 	content := msg.Content

@@ -152,8 +155,8 @@ func (mc *MessageConverter) ToDiscord(
 				Name: att.Filename,
 				ID:   mc.NextDiscordUploadID(),
 			}},
-			// TODO: Populate with guild ID. Support threads.
-		}, discordgo.WithChannelReferer("", channelID))
+			// TODO: Support threads.
+		}, discordgo.WithChannelReferer(guildID, channelID))

 		if err != nil {
 			log.Err(err).Msg("Failed to create attachment in preparation for attachment reupload")