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/channels/app/channel.go

@@ -13,6 +13,7 @@ import (
 	"strings"
 
 	"github.com/mattermost/mattermost/server/v8/channels/utils"
+	"github.com/mattermost/mattermost/server/v8/platform/services/sharedchannel"
 	"github.com/mattermost/mattermost/server/v8/platform/services/telemetry"
 
 	"github.com/mattermost/mattermost/server/public/model"
@@ -503,8 +504,8 @@ func (a *App) createDirectChannelWithUser(c request.CTX, user, otherUser *model.
 	return channel, nil
 }
 
-func (a *App) CreateGroupChannel(c request.CTX, userIDs []string, creatorId string) (*model.Channel, *model.AppError) {
+func (a *App) CreateGroupChannel(c request.CTX, userIDs []string, creatorId string, channelOptions ...model.ChannelOption) (*model.Channel, *model.AppError) {
-	channel, err := a.createGroupChannel(c, userIDs)
+	channel, err := a.createGroupChannel(c, userIDs, creatorId, channelOptions...)
 	if err != nil {
 		if err.Id == store.ChannelExistsError {
 			return channel, nil
@@ -524,7 +525,11 @@ func (a *App) CreateGroupChannel(c request.CTX, userIDs []string, creatorId stri
 	return channel, nil
 }
 
-func (a *App) createGroupChannel(c request.CTX, userIDs []string) (*model.Channel, *model.AppError) {
+// creatorId is used to determine if the group channel should have a
+// shared channel record attached. It can be empty if the caller
+// doesn't know who the creator is (e.g. the import process) and the
+// resulting group channel will not be shared
+func (a *App) createGroupChannel(c request.CTX, userIDs []string, creatorID string, channelOptions ...model.ChannelOption) (*model.Channel, *model.AppError) {
 	if len(userIDs) > model.ChannelGroupMaxUsers || len(userIDs) < model.ChannelGroupMinUsers {
 		return nil, model.NewAppError("CreateGroupChannel", "api.channel.create_group.bad_size.app_error", nil, "", http.StatusBadRequest)
 	}
@@ -538,7 +543,21 @@ func (a *App) createGroupChannel(c request.CTX, userIDs []string) (*model.Channe
 		return nil, model.NewAppError("CreateGroupChannel", "api.channel.create_group.bad_user.app_error", nil, "user_ids="+model.ArrayToJSON(userIDs), http.StatusBadRequest)
 	}
 
-	if !a.Config().FeatureFlags.EnableSharedChannelsDMs {
+	// extracts the creator and the remotes involved in the GM to
+	// decide how to handle the shared part of the creation
+	var creator *model.User
+	remoteIDs := map[string]bool{}
+	for _, user := range users {
+		if user.Id == creatorID {
+			creator = user
+		}
+		if user.IsRemote() {
+			remoteIDs[*user.RemoteId] = true
+		}
+	}
+	channelIsShared := len(remoteIDs) > 0
+
+	if channelIsShared && !a.Config().FeatureFlags.EnableSharedChannelsDMs {
 		for _, user := range users {
 			if user.IsRemote() {
 				return nil, model.NewAppError("createGroupChannel", "api.channel.create_group.remote_restricted.app_error", nil, "", http.StatusForbidden)
@@ -550,9 +569,10 @@ func (a *App) createGroupChannel(c request.CTX, userIDs []string) (*model.Channe
 		Name:        model.GetGroupNameFromUserIds(userIDs),
 		DisplayName: model.GetGroupDisplayNameFromUsers(users, true),
 		Type:        model.ChannelTypeGroup,
+		Shared:      model.NewPointer(channelIsShared),
 	}
 
-	channel, nErr := a.Srv().Store().Channel().Save(c, group, *a.Config().TeamSettings.MaxChannelsPerTeam)
+	channel, nErr := a.Srv().Store().Channel().Save(c, group, *a.Config().TeamSettings.MaxChannelsPerTeam, channelOptions...)
 	if nErr != nil {
 		var invErr *store.ErrInvalidInput
 		var cErr *store.ErrConflict
@@ -608,6 +628,48 @@ func (a *App) createGroupChannel(c request.CTX, userIDs []string) (*model.Channe
 		}
 	}
 
+	// When the newly created channel is shared, the creator is local
+	// and one of the participants is remote create a local shared
+	// channel record
+	if channel.IsShared() && creator != nil && !creator.IsRemote() {
+		sc := &model.SharedChannel{
+			ChannelId:        channel.Id,
+			TeamId:           channel.TeamId,
+			Home:             true,
+			ReadOnly:         false,
+			ShareName:        channel.Name,
+			ShareDisplayName: channel.DisplayName,
+			SharePurpose:     channel.Purpose,
+			ShareHeader:      channel.Header,
+			CreatorId:        creatorID,
+			Type:             channel.Type,
+		}
+
+		if _, err := a.ShareChannel(c, sc); err != nil {
+			c.Logger().Error("Failed to share newly created group channel", mlog.String("channel_id", channel.Id), mlog.Err(err))
+		} else {
+			// if we could successfully share the channel, we invite
+			// the remotes involved to it
+			if sc, _ := a.getSharedChannelsService(); sc != nil {
+				for remoteID := range remoteIDs {
+					rc, err := a.Srv().Store().RemoteCluster().Get(remoteID, false)
+					if err != nil {
+						c.Logger().Error("Failed to send invite to group message channel, can't retrieve remote cluster", mlog.String("channel_id", channel.Id), mlog.String("remote_id", remoteID), mlog.Err(err))
+						continue
+					}
+
+					opts := []sharedchannel.InviteOption{sharedchannel.WithCreator(creatorID)}
+					for _, user := range users {
+						opts = append(opts, sharedchannel.WithDirectParticipant(user, remoteID))
+					}
+					if err := sc.SendChannelInvite(channel, creatorID, rc, opts...); err != nil {
+						c.Logger().Error("Failed to send invite to group message channel, error sending the invite", mlog.String("channel_id", channel.Id), mlog.String("remote_id", remoteID), mlog.Err(err))
+					}
+				}
+			}
+		}
+	}
+
 	a.Srv().Go(func() {
 		pluginContext := pluginContext(c)
 		a.ch.RunMultiHook(func(hooks plugin.Hooks, _ *model.Manifest) bool {

server/channels/app/import_functions.go

@@ -2105,7 +2105,7 @@ func (a *App) importDirectChannel(rctx request.CTX, data *imports.DirectChannelI
 		}
 		channel = ch
 	} else {
-		ch, err2 := a.createGroupChannel(rctx, userIDs)
+		ch, err2 := a.createGroupChannel(rctx, userIDs, "")
 		if err2 != nil && err2.Id != store.ChannelExistsError {
 			return model.NewAppError("BulkImport", "app.import.import_direct_channel.create_group_channel.error", nil, "", http.StatusBadRequest).Wrap(err2)
 		}
@@ -2350,7 +2350,7 @@ func (a *App) importMultipleDirectPostLines(rctx request.CTX, lines []imports.Li
 			}
 			channel = ch
 		} else if len(userIDs) > 2 {
-			ch, err = a.createGroupChannel(rctx, userIDs)
+			ch, err = a.createGroupChannel(rctx, userIDs, "")
 			if err != nil && err.Id != store.ChannelExistsError {
 				return line.LineNumber, model.NewAppError("BulkImport", "app.import.import_direct_post.create_group_channel.error", nil, "", http.StatusBadRequest).Wrap(err)
 			}

server/channels/app/import_functions_test.go

@@ -4050,7 +4050,7 @@ func TestImportImportDirectChannel(t *testing.T) {
 					th.BasicUser2.Id,
 					user3.Id,
 				}
-				channel, appErr := th.App.createGroupChannel(th.Context, userIDs)
+				channel, appErr := th.App.createGroupChannel(th.Context, userIDs, th.BasicUser.Id)
 				require.Equal(t, appErr.Id, store.ChannelExistsError)
 				require.Equal(t, channel.Header, *data.Header)
 			})
@@ -4677,7 +4677,7 @@ func TestImportImportDirectPost(t *testing.T) {
 		th.BasicUser2.Id,
 		user3.Id,
 	}
-	channel, appErr = th.App.createGroupChannel(th.Context, userIDs)
+	channel, appErr = th.App.createGroupChannel(th.Context, userIDs, th.BasicUser.Id)
 	require.Equal(t, appErr.Id, store.ChannelExistsError)
 	groupChannel = channel
 

server/channels/app/platform/shared_channel_notifier.go

@@ -145,7 +145,7 @@ func handleInvitation(ps *PlatformService, syncService SharedChannelServiceIFace
 		return errors.Wrap(err, fmt.Sprintf("couldn't find remote cluster %s, for creating shared channel invitation for a DM", *participant.RemoteId))
 	}
 
-	return syncService.SendChannelInvite(channel, creator.Id, rc, sharedchannel.WithDirectParticipant(creator), sharedchannel.WithDirectParticipant(participant))
+	return syncService.SendChannelInvite(channel, creator.Id, rc, sharedchannel.WithDirectParticipant(creator, rc.RemoteId), sharedchannel.WithDirectParticipant(participant, rc.RemoteId))
 }
 
 func getUserFromEvent(ps *PlatformService, event *model.WebSocketEvent, key string) (*model.User, error) {

server/channels/app/post_metadata_test.go

@@ -681,7 +681,7 @@ func TestPreparePostForClient(t *testing.T) {
 		directChannel, err := th.App.createDirectChannel(th.Context, th.BasicUser.Id, th.BasicUser2.Id)
 		require.Nil(t, err)
 
-		groupChannel, err := th.App.createGroupChannel(th.Context, []string{th.BasicUser.Id, th.BasicUser2.Id, th.CreateUser().Id})
+		groupChannel, err := th.App.createGroupChannel(th.Context, []string{th.BasicUser.Id, th.BasicUser2.Id, th.CreateUser().Id}, th.BasicUser.Id)
 		require.Nil(t, err)
 
 		testCases := []struct {

server/channels/app/post_test.go

@@ -1162,7 +1162,7 @@ func TestCreatePost(t *testing.T) {
 		user1 := th.CreateUser()
 		user2 := th.CreateUser()
 		user3 := th.CreateUser()
-		gm, appErr := th.App.createGroupChannel(th.Context, []string{user1.Id, user2.Id, user3.Id})
+		gm, appErr := th.App.createGroupChannel(th.Context, []string{user1.Id, user2.Id, user3.Id}, user1.Id)
 		require.Nil(t, appErr)
 		require.NotNil(t, gm)
 
@@ -2423,7 +2423,7 @@ func TestCountMentionsFromPost(t *testing.T) {
 		user2 := th.BasicUser2
 		user3 := th.SystemAdminUser
 
-		channel, err := th.App.createGroupChannel(th.Context, []string{user1.Id, user2.Id, user3.Id})
+		channel, err := th.App.createGroupChannel(th.Context, []string{user1.Id, user2.Id, user3.Id}, user1.Id)
 		require.Nil(t, err)
 
 		post1, err := th.App.CreatePost(th.Context, &model.Post{

server/channels/store/retrylayer/retrylayer.go

@@ -2858,11 +2858,11 @@ func (s *RetryLayerChannelStore) Restore(channelID string, timestamp int64) erro
 
 }
 
-func (s *RetryLayerChannelStore) Save(rctx request.CTX, channel *model.Channel, maxChannelsPerTeam int64) (*model.Channel, error) {
+func (s *RetryLayerChannelStore) Save(rctx request.CTX, channel *model.Channel, maxChannelsPerTeam int64, channelOptions ...model.ChannelOption) (*model.Channel, error) {
 
 	tries := 0
 	for {
-		result, err := s.ChannelStore.Save(rctx, channel, maxChannelsPerTeam)
+		result, err := s.ChannelStore.Save(rctx, channel, maxChannelsPerTeam, channelOptions...)
 		if err == nil {
 			return result, nil
 		}

server/channels/store/searchlayer/channel_layer.go

@@ -66,8 +66,8 @@ func (c *SearchChannelStore) indexChannel(rctx request.CTX, channel *model.Chann
 	}
 }
 
-func (c *SearchChannelStore) Save(rctx request.CTX, channel *model.Channel, maxChannels int64) (*model.Channel, error) {
+func (c *SearchChannelStore) Save(rctx request.CTX, channel *model.Channel, maxChannels int64, channelOptions ...model.ChannelOption) (*model.Channel, error) {
-	newChannel, err := c.ChannelStore.Save(rctx, channel, maxChannels)
+	newChannel, err := c.ChannelStore.Save(rctx, channel, maxChannels, channelOptions...)
 	if err == nil {
 		c.indexChannel(rctx, newChannel)
 	}

server/channels/store/sqlstore/channel_store.go

@@ -616,7 +616,11 @@ func (s SqlChannelStore) upsertPublicChannelT(transaction *sqlxTxWrapper, channe
 }
 
 // Save writes the (non-direct) channel to the database.
-func (s SqlChannelStore) Save(rctx request.CTX, channel *model.Channel, maxChannelsPerTeam int64) (_ *model.Channel, err error) {
+func (s SqlChannelStore) Save(rctx request.CTX, channel *model.Channel, maxChannelsPerTeam int64, channelOptions ...model.ChannelOption) (_ *model.Channel, err error) {
+	for _, option := range channelOptions {
+		option(channel)
+	}
+
 	if channel.DeleteAt != 0 {
 		return nil, store.NewErrInvalidInput("Channel", "DeleteAt", channel.DeleteAt)
 	}

server/channels/store/store.go

@@ -188,7 +188,7 @@ type TeamStore interface {
 }
 
 type ChannelStore interface {
-	Save(rctx request.CTX, channel *model.Channel, maxChannelsPerTeam int64) (*model.Channel, error)
+	Save(rctx request.CTX, channel *model.Channel, maxChannelsPerTeam int64, channelOptions ...model.ChannelOption) (*model.Channel, error)
 	CreateDirectChannel(ctx request.CTX, userID *model.User, otherUserID *model.User, channelOptions ...model.ChannelOption) (*model.Channel, error)
 	SaveDirectChannel(ctx request.CTX, channel *model.Channel, member1 *model.ChannelMember, member2 *model.ChannelMember) (*model.Channel, error)
 	Update(ctx request.CTX, channel *model.Channel) (*model.Channel, error)

server/channels/store/storetest/mocks/ChannelStore.go

@@ -2537,9 +2537,16 @@ func (_m *ChannelStore) Restore(channelID string, timestamp int64) error {
 	return r0
 }
 
-// Save provides a mock function with given fields: rctx, channel, maxChannelsPerTeam
+// Save provides a mock function with given fields: rctx, channel, maxChannelsPerTeam, channelOptions
-func (_m *ChannelStore) Save(rctx request.CTX, channel *model.Channel, maxChannelsPerTeam int64) (*model.Channel, error) {
+func (_m *ChannelStore) Save(rctx request.CTX, channel *model.Channel, maxChannelsPerTeam int64, channelOptions ...model.ChannelOption) (*model.Channel, error) {
-	ret := _m.Called(rctx, channel, maxChannelsPerTeam)
+	_va := make([]interface{}, len(channelOptions))
+	for _i := range channelOptions {
+		_va[_i] = channelOptions[_i]
+	}
+	var _ca []interface{}
+	_ca = append(_ca, rctx, channel, maxChannelsPerTeam)
+	_ca = append(_ca, _va...)
+	ret := _m.Called(_ca...)
 
 	if len(ret) == 0 {
 		panic("no return value specified for Save")
@@ -2547,19 +2554,19 @@ func (_m *ChannelStore) Save(rctx request.CTX, channel *model.Channel, maxChanne
 
 	var r0 *model.Channel
 	var r1 error
-	if rf, ok := ret.Get(0).(func(request.CTX, *model.Channel, int64) (*model.Channel, error)); ok {
+	if rf, ok := ret.Get(0).(func(request.CTX, *model.Channel, int64, ...model.ChannelOption) (*model.Channel, error)); ok {
-		return rf(rctx, channel, maxChannelsPerTeam)
+		return rf(rctx, channel, maxChannelsPerTeam, channelOptions...)
 	}
-	if rf, ok := ret.Get(0).(func(request.CTX, *model.Channel, int64) *model.Channel); ok {
+	if rf, ok := ret.Get(0).(func(request.CTX, *model.Channel, int64, ...model.ChannelOption) *model.Channel); ok {
-		r0 = rf(rctx, channel, maxChannelsPerTeam)
+		r0 = rf(rctx, channel, maxChannelsPerTeam, channelOptions...)
 	} else {
 		if ret.Get(0) != nil {
 			r0 = ret.Get(0).(*model.Channel)
 		}
 	}
 
-	if rf, ok := ret.Get(1).(func(request.CTX, *model.Channel, int64) error); ok {
+	if rf, ok := ret.Get(1).(func(request.CTX, *model.Channel, int64, ...model.ChannelOption) error); ok {
-		r1 = rf(rctx, channel, maxChannelsPerTeam)
+		r1 = rf(rctx, channel, maxChannelsPerTeam, channelOptions...)
 	} else {
 		r1 = ret.Error(1)
 	}

server/channels/store/timerlayer/timerlayer.go

@@ -2394,10 +2394,10 @@ func (s *TimerLayerChannelStore) Restore(channelID string, timestamp int64) erro
 	return err
 }
 
-func (s *TimerLayerChannelStore) Save(rctx request.CTX, channel *model.Channel, maxChannelsPerTeam int64) (*model.Channel, error) {
+func (s *TimerLayerChannelStore) Save(rctx request.CTX, channel *model.Channel, maxChannelsPerTeam int64, channelOptions ...model.ChannelOption) (*model.Channel, error) {
 	start := time.Now()
 
-	result, err := s.ChannelStore.Save(rctx, channel, maxChannelsPerTeam)
+	result, err := s.ChannelStore.Save(rctx, channel, maxChannelsPerTeam, channelOptions...)
 
 	elapsed := float64(time.Since(start)) / float64(time.Second)
 	if s.Root.Metrics != nil {

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 the participant doesn't belong to the remote we're
-		if !participant.IsRemote() {
+		// 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
+			// as of this writing, this scenario should only be possible if the invite included a DM or GM channel
-			// combination of two user ids (one remote, one local) that already have a DM on this server. Very unlikely unless
+			// invitation with a combination of user ids that already have a DM or GM on this server. Very unlikely
-			// the remote is compromised AND has knowledge of the local user id.
+			// 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
+}

server/platform/services/sharedchannel/mock_AppIface_test.go

@@ -102,6 +102,45 @@ func (_m *MockAppIface) CreateChannelWithUser(c request.CTX, channel *model.Chan
 	return r0, r1
 }
 
+// CreateGroupChannel provides a mock function with given fields: c, userIDs, creatorId, channelOptions
+func (_m *MockAppIface) CreateGroupChannel(c request.CTX, userIDs []string, creatorId string, channelOptions ...model.ChannelOption) (*model.Channel, *model.AppError) {
+	_va := make([]interface{}, len(channelOptions))
+	for _i := range channelOptions {
+		_va[_i] = channelOptions[_i]
+	}
+	var _ca []interface{}
+	_ca = append(_ca, c, userIDs, creatorId)
+	_ca = append(_ca, _va...)
+	ret := _m.Called(_ca...)
+
+	if len(ret) == 0 {
+		panic("no return value specified for CreateGroupChannel")
+	}
+
+	var r0 *model.Channel
+	var r1 *model.AppError
+	if rf, ok := ret.Get(0).(func(request.CTX, []string, string, ...model.ChannelOption) (*model.Channel, *model.AppError)); ok {
+		return rf(c, userIDs, creatorId, channelOptions...)
+	}
+	if rf, ok := ret.Get(0).(func(request.CTX, []string, string, ...model.ChannelOption) *model.Channel); ok {
+		r0 = rf(c, userIDs, creatorId, channelOptions...)
+	} else {
+		if ret.Get(0) != nil {
+			r0 = ret.Get(0).(*model.Channel)
+		}
+	}
+
+	if rf, ok := ret.Get(1).(func(request.CTX, []string, string, ...model.ChannelOption) *model.AppError); ok {
+		r1 = rf(c, userIDs, creatorId, channelOptions...)
+	} else {
+		if ret.Get(1) != nil {
+			r1 = ret.Get(1).(*model.AppError)
+		}
+	}
+
+	return r0, r1
+}
+
 // CreatePost provides a mock function with given fields: c, post, channel, flags
 func (_m *MockAppIface) CreatePost(c request.CTX, post *model.Post, channel *model.Channel, flags model.CreatePostFlags) (*model.Post, *model.AppError) {
 	ret := _m.Called(c, post, channel, flags)

server/platform/services/sharedchannel/service.go

@@ -53,6 +53,7 @@ type AppIface interface {
 	SendEphemeralPost(c request.CTX, userId string, post *model.Post) *model.Post
 	CreateChannelWithUser(c request.CTX, channel *model.Channel, userId string) (*model.Channel, *model.AppError)
 	GetOrCreateDirectChannel(c request.CTX, userId, otherUserId string, channelOptions ...model.ChannelOption) (*model.Channel, *model.AppError)
+	CreateGroupChannel(c request.CTX, userIDs []string, creatorId string, channelOptions ...model.ChannelOption) (*model.Channel, *model.AppError)
 	UserCanSeeOtherUser(c request.CTX, userID string, otherUserId string) (bool, *model.AppError)
 	AddUserToChannel(c request.CTX, user *model.User, channel *model.Channel, skipTeamMemberIntegrityCheck bool) (*model.ChannelMember, *model.AppError)
 	AddUserToTeamByTeamId(c request.CTX, teamId string, user *model.User) *model.AppError

server/platform/services/sharedchannel/sync_recv.go

@@ -115,7 +115,7 @@ func (scs *Service) processSyncMessage(c request.CTX, syncMsg *model.SyncMsg, rc
 			continue
 		}
 
-		if targetChannel.Type != model.ChannelTypeDirect && team == nil {
+		if (targetChannel.Type != model.ChannelTypeDirect && targetChannel.Type != model.ChannelTypeGroup) && team == nil {
 			var err2 error
 			team, err2 = scs.server.GetStore().Channel().GetTeamForChannel(syncMsg.ChannelId)
 			if err2 != nil {

server/platform/services/slackimport/slackimport.go

@@ -88,7 +88,7 @@ type Actions struct {
 	AddUserToChannel       func(request.CTX, *model.User, *model.Channel, bool) (*model.ChannelMember, *model.AppError)
 	JoinUserToTeam         func(*model.Team, *model.User, string) (*model.TeamMember, *model.AppError)
 	CreateDirectChannel    func(request.CTX, string, string, ...model.ChannelOption) (*model.Channel, *model.AppError)
-	CreateGroupChannel     func(request.CTX, []string) (*model.Channel, *model.AppError)
+	CreateGroupChannel     func(request.CTX, []string, string, ...model.ChannelOption) (*model.Channel, *model.AppError)
 	CreateChannel          func(*model.Channel, bool) (*model.Channel, *model.AppError)
 	DoUploadFile           func(time.Time, string, string, string, string, []byte) (*model.FileInfo, *model.AppError)
 	GenerateThumbnailImage func(request.CTX, image.Image, string, string)
@@ -764,7 +764,7 @@ func (si *SlackImporter) oldImportChannel(rctx request.CTX, channel *model.Chann
 		if creator == nil {
 			return nil
 		}
-		sc, err := si.actions.CreateGroupChannel(rctx, members)
+		sc, err := si.actions.CreateGroupChannel(rctx, members, "")
 		if err != nil {
 			return nil
 		}