Adds support for GMs in shared channels (#31403)

* Adds support for GMs in shared channels

* Fix linter

* Remove creatorID from slack call

---------

Co-authored-by: Miguel de la Cruz <miguel@ctrlz.es>

server/platform/services/sharedchannel/channelinvite.go

@@ -27,6 +27,7 @@ type channelInviteMsg struct {
 	Header               string            `json:"header"`
 	Purpose              string            `json:"purpose"`
 	Type                 model.ChannelType `json:"type"`
+	CreatorID            string            `json:"creator_id"`
 	DirectParticipantIDs []string          `json:"direct_participant_ids"`
 	DirectParticipants   []*model.User     `json:"direct_participants"`
 }
@@ -41,16 +42,24 @@ func (cim channelInviteMsg) DirectParticipantsMap() map[string]*model.User {
 
 type InviteOption func(msg *channelInviteMsg)
 
-func WithDirectParticipant(participant *model.User) InviteOption {
+func WithDirectParticipant(participant *model.User, remoteID string) InviteOption {
 	return func(msg *channelInviteMsg) {
 		msg.DirectParticipantIDs = append(msg.DirectParticipantIDs, participant.Id)
-		// if the participant is local, send it as part of the invite payload
-		if !participant.IsRemote() {
+		// if the participant doesn't belong to the remote we're
+		// sending the invite to, send it as part of the invite
+		// payload
+		if participant.GetRemoteID() != remoteID {
 			msg.DirectParticipants = append(msg.DirectParticipants, sanitizeUserForSync(participant))
 		}
 	}
 }
 
+func WithCreator(creatorID string) InviteOption {
+	return func(msg *channelInviteMsg) {
+		msg.CreatorID = creatorID
+	}
+}
+
 // SendChannelInvite asynchronously sends a channel invite to a remote cluster. The remote cluster is
 // expected to create a new channel with the same channel id, and respond with status OK.
 // If an error occurs on the remote cluster then an ephemeral message is posted to in the channel for userId.
@@ -241,9 +250,9 @@ func (scs *Service) onReceiveChannelInvite(msg model.RemoteClusterMsg, rc *model
 		// sanity check to ensure the channel returned has the expected id. Otherwise sync will not work as expected and will fail
 		// silently.
 		if invite.ChannelId != channel.Id {
-			// as of this writing, this scenario should only be possible if the invite included a DM channel invitation with a
-			// combination of two user ids (one remote, one local) that already have a DM on this server. Very unlikely unless
-			// the remote is compromised AND has knowledge of the local user id.
+			// as of this writing, this scenario should only be possible if the invite included a DM or GM channel
+			// invitation with a combination of user ids that already have a DM or GM on this server. Very unlikely
+			// unless the remote is compromised AND has knowledge of the local user ids.
 			// Another possibility would be an actual user ID collision between two servers, where the likelihood is
 			// infinitesimally small
 			scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Channel invite failed - channel created/fetched with wrong id",
@@ -254,7 +263,7 @@ func (scs *Service) onReceiveChannelInvite(msg model.RemoteClusterMsg, rc *model
 				mlog.String("team_id", invite.TeamId),
 				mlog.Array("dm_partics", invite.DirectParticipantIDs),
 			)
-			return fmt.Errorf("cannot create shared channel (DM channel_id=%s): %w", invite.ChannelId, model.ErrChannelAlreadyExists)
+			return fmt.Errorf("cannot create shared channel (channel_id=%s channel_type=%s): %w", invite.ChannelId, invite.Type, model.ErrChannelAlreadyExists)
 		}
 
 		// mark the newly created channel read-only if requested in the invite
@@ -303,10 +312,14 @@ func (scs *Service) onReceiveChannelInvite(msg model.RemoteClusterMsg, rc *model
 			return fmt.Errorf("cannot restore deleted shared channel remote (channel_id=%s): %w", invite.ChannelId, err)
 		}
 	} else {
+		creatorID := channel.CreatorId
+		if creatorID == "" {
+			creatorID = invite.CreatorID
+		}
 		scr := &model.SharedChannelRemote{
 			Id:                model.NewId(),
 			ChannelId:         channel.Id,
-			CreatorId:         channel.CreatorId,
+			CreatorId:         creatorID,
 			IsInviteAccepted:  true,
 			IsInviteConfirmed: true,
 			RemoteId:          rc.RemoteId,
@@ -335,6 +348,10 @@ func (scs *Service) handleChannelCreation(invite channelInviteMsg, rc *model.Rem
 		return scs.createDirectChannel(invite, rc)
 	}
 
+	if invite.Type == model.ChannelTypeGroup {
+		return scs.createGroupChannel(invite, rc)
+	}
+
 	teamId := rc.DefaultTeamId
 	// if the remote doesn't have a teamId associated and until the
 	// acceptance of an invite includes selecting a team, we use the
@@ -472,3 +489,62 @@ func (scs *Service) createDirectChannel(invite channelInviteMsg, rc *model.Remot
 
 	return channel, true, nil
 }
+
+// createGroupChannel creates a DM channel, or fetches an existing channel, and returns the channel plus a boolean
+// indicating if the channel is new.
+func (scs *Service) createGroupChannel(invite channelInviteMsg, rc *model.RemoteCluster) (*model.Channel, bool, error) {
+	if len(invite.DirectParticipantIDs) > model.ChannelGroupMaxUsers || len(invite.DirectParticipantIDs) < model.ChannelGroupMinUsers {
+		return nil, false, fmt.Errorf("cannot create group channel `%s` bad participant count `%d`", invite.ChannelId, len(invite.DirectParticipantIDs))
+	}
+
+	participantsMap := invite.DirectParticipantsMap()
+
+	remoteIDMap := map[string]bool{}
+	hasLocalUsers := false
+	for _, participantID := range invite.DirectParticipantIDs {
+		user, err := scs.getOrCreateUser(participantID, participantsMap, rc)
+		if err != nil {
+			return nil, false, fmt.Errorf("cannot create group channel `%s` from invite: %w", invite.ChannelId, err)
+		}
+
+		// we keep track of the origin of the users to check if the
+		// invite is valid
+		if user.IsRemote() {
+			remoteIDMap[user.GetRemoteID()] = true
+		} else {
+			hasLocalUsers = true
+		}
+	}
+
+	// if the invite doesn't contain remote users, GM should not be created via remote invite
+	if len(remoteIDMap) == 0 {
+		return nil, false, fmt.Errorf("cannot create group channel `%s` there are no remote users", invite.ChannelId)
+	}
+
+	// if the channel doesn't contain local users, the GM channel doesn't belong to this server
+	if !hasLocalUsers {
+		return nil, false, fmt.Errorf("cannot create group channel `%s` there are no local users", invite.ChannelId)
+	}
+
+	// check if this DM already exists.
+	channelName := model.GetGroupNameFromUserIds(invite.DirectParticipantIDs)
+	channelExists, err := scs.server.GetStore().Channel().GetByName("", channelName, true)
+	if err != nil && !isNotFoundError(err) {
+		return nil, false, fmt.Errorf("cannot check GM channel exists (%s): %w", channelName, err)
+	}
+	if channelExists != nil {
+		if channelExists.Id == invite.ChannelId {
+			return channelExists, false, nil
+		}
+
+		return nil, false, fmt.Errorf("cannot create group channel `%s`: channel exists with wrong id", channelName)
+	}
+
+	// create the channel
+	channel, appErr := scs.app.CreateGroupChannel(request.EmptyContext(scs.server.Log()), invite.DirectParticipantIDs, invite.CreatorID, model.WithID(invite.ChannelId))
+	if appErr != nil {
+		return nil, false, fmt.Errorf("cannot create group channel `%s`: %w", invite.ChannelId, appErr)
+	}
+
+	return channel, true, nil
+}