MM-52600: [Shared Channels] Shared channels do not sync channel membership (#30976)

server/platform/services/sharedchannel/channelinvite.go

@@ -89,6 +89,7 @@ func (scs *Service) SendChannelInvite(channel *model.Channel, userId string, rc
 			RemoteId:          rc.RemoteId,
 			IsInviteAccepted:  true,
 			IsInviteConfirmed: false,
+			LastMembersSyncAt: 0,
 		}
 		if _, err = scs.server.GetStore().SharedChannel().SaveRemote(scr); err != nil {
 			scs.sendEphemeralPost(channel.Id, userId, fmt.Sprintf("Error saving channel invite for %s: %v", rc.DisplayName, err))
@@ -134,6 +135,7 @@ func (scs *Service) SendChannelInvite(channel *model.Channel, userId string, rc
 		}
 
 		curTime := model.GetMillis()
+		var sharedChannelRemote *model.SharedChannelRemote
 		if existingScr != nil {
 			if existingScr.DeleteAt == 0 && existingScr.IsInviteConfirmed {
 				// the shared channel remote exists and is not
@@ -153,6 +155,7 @@ func (scs *Service) SendChannelInvite(channel *model.Channel, userId string, rc
 				scs.sendEphemeralPost(channel.Id, userId, fmt.Sprintf("Error confirming channel invite for %s: %v", rc.DisplayName, sErr))
 				return
 			}
+			sharedChannelRemote = existingScr
 		} else {
 			// the shared channel remote doesn't exists, so we create it
 			scr := &model.SharedChannelRemote{
@@ -163,15 +166,26 @@ func (scs *Service) SendChannelInvite(channel *model.Channel, userId string, rc
 				IsInviteConfirmed: true,
 				LastPostCreateAt:  curTime,
 				LastPostUpdateAt:  curTime,
+				LastMembersSyncAt: 0,
 			}
 			if _, err = scs.server.GetStore().SharedChannel().SaveRemote(scr); err != nil {
 				scs.sendEphemeralPost(channel.Id, userId, fmt.Sprintf("Error confirming channel invite for %s: %v", rc.DisplayName, err))
 				return
 			}
+			sharedChannelRemote = scr
 		}
 
 		scs.NotifyChannelChanged(sc.ChannelId)
 		scs.sendEphemeralPost(channel.Id, userId, fmt.Sprintf("`%s` has been added to channel.", rc.DisplayName))
+
+		// Sync all channel members to the remote now that the remote entry exists
+		if syncErr := scs.SyncAllChannelMembers(sc.ChannelId, rc.RemoteId, sharedChannelRemote); syncErr != nil {
+			scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Failed to sync channel members after invite confirmation",
+				mlog.String("channel_id", sc.ChannelId),
+				mlog.String("remote_id", rc.RemoteId),
+				mlog.Err(syncErr),
+			)
+		}
 	}
 
 	if rc.IsPlugin() {
@@ -311,6 +325,15 @@ func (scs *Service) onReceiveChannelInvite(msg model.RemoteClusterMsg, rc *model
 		if _, err := scs.server.GetStore().SharedChannel().UpdateRemote(existingScr); err != nil {
 			return fmt.Errorf("cannot restore deleted shared channel remote (channel_id=%s): %w", invite.ChannelId, err)
 		}
+
+		// Sync local channel members to the remote after restoring the shared channel
+		if syncErr := scs.SyncAllChannelMembers(channel.Id, rc.RemoteId, existingScr); syncErr != nil {
+			scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Failed to sync local channel members after restoring shared channel",
+				mlog.String("channel_id", channel.Id),
+				mlog.String("remote_id", rc.RemoteId),
+				mlog.Err(syncErr),
+			)
+		}
 	} else {
 		creatorID := channel.CreatorId
 		if creatorID == "" {
@@ -325,6 +348,7 @@ func (scs *Service) onReceiveChannelInvite(msg model.RemoteClusterMsg, rc *model
 			RemoteId:          rc.RemoteId,
 			LastPostCreateAt:  model.GetMillis(),
 			LastPostUpdateAt:  model.GetMillis(),
+			LastMembersSyncAt: 0,
 		}
 
 		if _, err := scs.server.GetStore().SharedChannel().SaveRemote(scr); err != nil {
@@ -336,6 +360,15 @@ func (scs *Service) onReceiveChannelInvite(msg model.RemoteClusterMsg, rc *model
 			scs.server.GetStore().SharedChannel().Delete(sharedChannel.ChannelId)
 			return fmt.Errorf("cannot create shared channel remote (channel_id=%s): %w", invite.ChannelId, err)
 		}
+
+		// Sync local channel members to the remote after accepting the invitation
+		if syncErr := scs.SyncAllChannelMembers(channel.Id, rc.RemoteId, scr); syncErr != nil {
+			scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Failed to sync local channel members after accepting invitation",
+				mlog.String("channel_id", channel.Id),
+				mlog.String("remote_id", rc.RemoteId),
+				mlog.Err(syncErr),
+			)
+		}
 	}
 	return nil
 }

server/platform/services/sharedchannel/channelinvite_test.go

@@ -29,6 +29,18 @@ var (
 	mockTypeContext    = mock.MatchedBy(func(ctx context.Context) bool { return true })
 )
 
+// setupMockServerWithConfig sets up the standard mocks that all tests need
+func setupMockServerWithConfig(mockServer *MockServerIface) {
+	// Mock Config for feature flag check - disable membership sync to avoid complex mocking
+	mockConfig := model.Config{}
+	mockConfig.SetDefaults()
+	mockConfig.FeatureFlags.EnableSharedChannelsMemberSync = false
+	mockServer.On("Config").Return(&mockConfig)
+
+	// Mock GetRemoteClusterService for feature flag check
+	mockServer.On("GetRemoteClusterService").Return(nil)
+}
+
 func TestOnReceiveChannelInvite(t *testing.T) {
 	t.Run("when msg payload is empty, it does nothing", func(t *testing.T) {
 		mockServer := &MockServerIface{}
@@ -92,6 +104,8 @@ func TestOnReceiveChannelInvite(t *testing.T) {
 		mockStore.On("SharedChannel").Return(&mockSharedChannelStore)
 
 		mockServer.On("GetStore").Return(mockStore)
+		setupMockServerWithConfig(mockServer)
+
 		createPostPermission := model.ChannelModeratedPermissionsMap[model.PermissionCreatePost.Id]
 		createReactionPermission := model.ChannelModeratedPermissionsMap[model.PermissionAddReaction.Id]
 		updateMap := model.ChannelModeratedRolesPatch{
@@ -216,6 +230,8 @@ func TestOnReceiveChannelInvite(t *testing.T) {
 		mockStore.On("SharedChannel").Return(&mockSharedChannelStore)
 
 		mockServer.On("GetStore").Return(mockStore)
+		setupMockServerWithConfig(mockServer)
+
 		defer mockApp.AssertExpectations(t)
 
 		err = scs.onReceiveChannelInvite(msg, remoteCluster, nil)
@@ -351,6 +367,7 @@ func TestOnReceiveChannelInvite(t *testing.T) {
 
 				mockServer = scs.server.(*MockServerIface)
 				mockServer.On("GetStore").Return(mockStore)
+				setupMockServerWithConfig(mockServer)
 
 				mockApp.On("GetOrCreateDirectChannel", mockTypeReqContext, mockTypeString, mockTypeString, mock.AnythingOfType("model.ChannelOption")).
 					Return(channel, nil).Maybe()

server/platform/services/sharedchannel/membership.go

@@ -0,0 +1,409 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package sharedchannel
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"time"
+
+	"github.com/mattermost/mattermost/server/public/model"
+	"github.com/mattermost/mattermost/server/public/shared/mlog"
+	"github.com/mattermost/mattermost/server/v8/platform/services/remotecluster"
+)
+
+// isChannelMemberSyncEnabled checks if the feature flag is enabled and remote cluster service is available
+func (scs *Service) isChannelMemberSyncEnabled() bool {
+	featureFlagEnabled := scs.server.Config().FeatureFlags.EnableSharedChannelsMemberSync
+	remoteClusterService := scs.server.GetRemoteClusterService()
+	return featureFlagEnabled && remoteClusterService != nil
+}
+
+// queueMembershipSyncTask creates and queues a task to synchronize channel membership changes
+func (scs *Service) queueMembershipSyncTask(channelID, userID, remoteID string, syncMsg *model.SyncMsg, retryMsg *model.SyncMsg) {
+	task := newSyncTask(channelID, userID, remoteID, syncMsg, retryMsg)
+	task.schedule = time.Now().Add(NotifyMinimumDelay)
+
+	scs.addTask(task)
+}
+
+// HandleMembershipChange is called when users are added or removed from a shared channel.
+// It creates a task to notify all remote clusters about the membership change.
+func (scs *Service) HandleMembershipChange(channelID, userID string, isAdd bool, remoteID string) {
+	if !scs.isChannelMemberSyncEnabled() {
+		return
+	}
+
+	// Create timestamp for consistent usage
+	changeTime := model.GetMillis()
+
+	// Create membership change info
+	syncMsg := model.NewSyncMsg(channelID)
+	syncMsg.MembershipChanges = []*model.MembershipChangeMsg{
+		{
+			ChannelId:  channelID,
+			UserId:     userID,
+			IsAdd:      isAdd,
+			RemoteId:   remoteID, // which remote initiated this change
+			ChangeTime: changeTime,
+		},
+	}
+
+	// Queue the membership change task
+	scs.queueMembershipSyncTask(channelID, userID, "", syncMsg, nil)
+}
+
+// HandleMembershipBatchChange is called to process a batch of membership changes for a shared channel.
+// It creates a task to notify all remote clusters about the batch membership changes.
+func (scs *Service) HandleMembershipBatchChange(channelID string, userIDs []string, isAdd bool, remoteID string) {
+	if !scs.isChannelMemberSyncEnabled() {
+		return
+	}
+
+	if len(userIDs) == 0 {
+		return
+	}
+
+	// Create timestamp for consistent usage
+	changeTime := model.GetMillis()
+
+	// Create sync message with membership changes
+	syncMsg := model.NewSyncMsg(channelID)
+	syncMsg.MembershipChanges = make([]*model.MembershipChangeMsg, 0, len(userIDs))
+
+	// Add each user to the batch
+	for _, userID := range userIDs {
+		syncMsg.MembershipChanges = append(syncMsg.MembershipChanges, &model.MembershipChangeMsg{
+			ChannelId:  channelID,
+			UserId:     userID,
+			IsAdd:      isAdd,
+			RemoteId:   remoteID,
+			ChangeTime: changeTime,
+		})
+	}
+
+	// Queue the batch membership sync task
+	scs.queueMembershipSyncTask(channelID, "", "", syncMsg, nil)
+}
+
+// SyncAllChannelMembers synchronizes all channel members to a specific remote.
+// This is typically called when a channel is first shared with a remote cluster.
+// If remote is provided, it will be used instead of fetching from the database.
+func (scs *Service) SyncAllChannelMembers(channelID string, remoteID string, remote *model.SharedChannelRemote) error {
+	if !scs.isChannelMemberSyncEnabled() {
+		return nil
+	}
+
+	// Verify the channel exists and is shared
+	if _, err := scs.server.GetStore().SharedChannel().Get(channelID); err != nil {
+		scs.server.Log().Log(mlog.LvlSharedChannelServiceDebug, "Failed to get shared channel",
+			mlog.String("channel_id", channelID),
+			mlog.Err(err),
+		)
+		return fmt.Errorf("failed to get shared channel %s: %w", channelID, err)
+	}
+
+	// Get the remote to ensure it exists (if not provided)
+	if remote == nil {
+		var err error
+		remote, err = scs.server.GetStore().SharedChannel().GetRemoteByIds(channelID, remoteID)
+		if err != nil {
+			scs.server.Log().Log(mlog.LvlSharedChannelServiceDebug, "Failed to get remote",
+				mlog.String("channel_id", channelID),
+				mlog.String("remote_id", remoteID),
+				mlog.Err(err),
+			)
+			return fmt.Errorf("failed to get remote for channel %s: %w", channelID, err)
+		}
+	}
+
+	// Use offset-based pagination to handle channels with many members
+	// This ensures we don't skip members when multiple members have the same LastUpdateAt timestamp
+	maxPerPage := scs.GetMemberSyncBatchSize()
+	var allMembers model.ChannelMembers
+	lastSyncAt := remote.LastMembersSyncAt
+	offset := 0
+
+	// Process members incrementally with offset-based pagination
+	for {
+		opts := model.ChannelMembersGetOptions{
+			ChannelID:    channelID,
+			UpdatedAfter: lastSyncAt,
+			Limit:        maxPerPage,
+			Offset:       offset,
+		}
+
+		members, err1 := scs.server.GetStore().Channel().GetMembers(opts)
+		if err1 != nil {
+			return fmt.Errorf("failed to get members for channel %s: %w", channelID, err1)
+		}
+
+		if len(members) == 0 {
+			break // No more members to process
+		}
+
+		// Add to our collection
+		allMembers = append(allMembers, members...)
+
+		// Log progress when processing large channels
+		if len(allMembers)%1000 == 0 {
+			scs.server.Log().Log(mlog.LvlSharedChannelServiceDebug, "Processing channel members in batches",
+				mlog.String("channel_id", channelID),
+				mlog.String("remote_id", remoteID),
+				mlog.Int("processed_so_far", len(allMembers)),
+			)
+		}
+
+		if len(members) < maxPerPage {
+			break // Last page
+		}
+
+		// Move to next page
+		offset += maxPerPage
+	}
+
+	if len(allMembers) == 0 {
+		scs.server.Log().Log(mlog.LvlSharedChannelServiceDebug, "No members to sync for channel",
+			mlog.String("channel_id", channelID),
+			mlog.String("remote_id", remoteID),
+		)
+		return nil
+	}
+
+	scs.server.Log().Log(mlog.LvlSharedChannelServiceDebug, "Syncing all channel members",
+		mlog.String("channel_id", channelID),
+		mlog.String("remote_id", remoteID),
+		mlog.Int("member_count", len(allMembers)),
+	)
+
+	// Get batch size from config
+	batchSize := scs.GetMemberSyncBatchSize()
+
+	// For small channels, queue individual membership changes
+	if len(allMembers) <= batchSize {
+		return scs.syncMembersIndividually(channelID, remoteID, allMembers, remote)
+	}
+
+	// For larger channels, use batch processing
+	return scs.syncMembersInBatches(channelID, remoteID, allMembers, remote)
+}
+
+// syncMembersIndividually processes each member individually
+// This is more efficient for small channels
+func (scs *Service) syncMembersIndividually(channelID, remoteID string, members model.ChannelMembers, remote *model.SharedChannelRemote) error {
+	// Queue individual membership changes for each member
+	for _, member := range members {
+		// Queue membership change for this user (isAdd=true)
+		scs.HandleMembershipChange(channelID, member.UserId, true, "")
+	}
+
+	return nil
+}
+
+// syncMembersInBatches processes members in batches for greater efficiency
+// This is better for channels with many members
+func (scs *Service) syncMembersInBatches(channelID, remoteID string, members model.ChannelMembers, remote *model.SharedChannelRemote) error {
+	// Get batch size from config
+	batchSize := scs.GetMemberSyncBatchSize()
+
+	for i := 0; i < len(members); i += batchSize {
+		end := i + batchSize
+		if end > len(members) {
+			end = len(members)
+		}
+
+		// Create a batch of members
+		batchMembers := members[i:end]
+
+		// Extract user IDs from the batch
+		userIDs := make([]string, len(batchMembers))
+		for j, member := range batchMembers {
+			userIDs[j] = member.UserId
+		}
+
+		// Use the batch handling function to queue the changes
+		scs.HandleMembershipBatchChange(channelID, userIDs, true, "")
+	}
+
+	return nil
+}
+
+// processMembershipChange processes a channel membership change task.
+// It determines which remotes should receive the update and creates tasks for each.
+func (scs *Service) processMembershipChange(syncMsg *model.SyncMsg) {
+	if len(syncMsg.MembershipChanges) == 0 {
+		scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Invalid membership change task - no membership changes",
+			mlog.String("channel_id", syncMsg.ChannelId),
+		)
+		return
+	}
+
+	// Get the shared channel (to verify it exists)
+	_, err := scs.server.GetStore().SharedChannel().Get(syncMsg.ChannelId)
+	if err != nil {
+		scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Failed to get shared channel for membership change",
+			mlog.String("channel_id", syncMsg.ChannelId),
+			mlog.Int("change_count", len(syncMsg.MembershipChanges)),
+			mlog.Err(err),
+		)
+		return
+	}
+
+	// Get all remotes for this channel
+	remotes, err := scs.server.GetStore().SharedChannel().GetRemotes(0, 999999, model.SharedChannelRemoteFilterOpts{
+		ChannelId: syncMsg.ChannelId,
+	})
+	if err != nil {
+		scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Failed to get shared channel remotes for membership change",
+			mlog.String("channel_id", syncMsg.ChannelId),
+			mlog.Err(err),
+		)
+		return
+	}
+
+	// Always use batch processing for consistency (works for single or multiple changes)
+	scs.syncMembershipBatchToRemotes(syncMsg, remotes)
+}
+
+// syncMembershipBatchToRemotes synchronizes membership changes (single or batch) with remote clusters.
+func (scs *Service) syncMembershipBatchToRemotes(syncMsg *model.SyncMsg, remotes []*model.SharedChannelRemote) {
+	if len(syncMsg.MembershipChanges) == 0 {
+		return
+	}
+
+	// Get the initiating remote ID from the first change (all should be the same)
+	initiatingRemoteId := ""
+	if len(syncMsg.MembershipChanges) > 0 {
+		initiatingRemoteId = syncMsg.MembershipChanges[0].RemoteId
+	}
+
+	// Send to all remotes except the one that initiated this change
+	for _, remote := range remotes {
+		// Skip the remote that initiated this change to prevent loops
+		if remote.RemoteId == initiatingRemoteId {
+			continue
+		}
+
+		// Get the remote cluster
+		rc, err := scs.server.GetStore().RemoteCluster().Get(remote.RemoteId, false)
+		if err != nil {
+			scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Failed to get remote cluster for batch membership sync",
+				mlog.String("remote_id", remote.RemoteId),
+				mlog.String("channel_id", syncMsg.ChannelId),
+				mlog.Err(err),
+			)
+			continue
+		}
+
+		// Create a copy of the sync message to potentially add user profiles
+		enrichedSyncMsg := &model.SyncMsg{
+			Id:                syncMsg.Id,
+			ChannelId:         syncMsg.ChannelId,
+			MembershipChanges: syncMsg.MembershipChanges,
+			Users:             make(map[string]*model.User),
+		}
+
+		// Add user profiles for all users being added
+		for _, change := range syncMsg.MembershipChanges {
+			if change.IsAdd {
+				user, pErr := scs.server.GetStore().User().Get(context.Background(), change.UserId)
+				if pErr != nil {
+					scs.server.Log().Log(mlog.LvlSharedChannelServiceWarn, "Failed to get user for batch membership sync",
+						mlog.String("user_id", change.UserId),
+						mlog.String("channel_id", syncMsg.ChannelId),
+						mlog.String("remote_id", remote.RemoteId),
+						mlog.Err(pErr),
+					)
+					continue
+				}
+
+				// Check if user profile needs to be synced
+				doSync, _, sErr := scs.shouldUserSync(user, syncMsg.ChannelId, rc)
+				if sErr == nil && doSync {
+					enrichedSyncMsg.Users[user.Id] = user
+				}
+			}
+		}
+
+		// Send message using the existing remote cluster framework
+		payload, err := json.Marshal(enrichedSyncMsg)
+		if err != nil {
+			scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Failed to marshal batch membership message",
+				mlog.String("remote_id", remote.RemoteId),
+				mlog.String("channel_id", syncMsg.ChannelId),
+				mlog.Err(err),
+			)
+			continue
+		}
+
+		msg := model.RemoteClusterMsg{
+			Id:       model.NewId(),
+			Topic:    TopicChannelMembership,
+			CreateAt: model.GetMillis(),
+			Payload:  payload,
+		}
+
+		ctx, cancel := context.WithTimeout(context.Background(), remotecluster.SendTimeout)
+		defer cancel()
+
+		// Define a callback function
+		callback := func(msg model.RemoteClusterMsg, rc *model.RemoteCluster, resp *remotecluster.Response, err error) {
+			if err != nil {
+				scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Error sending batch membership changes to remote",
+					mlog.String("remote", remote.RemoteId),
+					mlog.String("channel_id", syncMsg.ChannelId),
+					mlog.Int("change_count", len(syncMsg.MembershipChanges)),
+					mlog.Err(err),
+				)
+				return
+			}
+
+			if resp != nil && resp.Err != "" {
+				scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Remote error when processing batch membership changes",
+					mlog.String("remote", remote.RemoteId),
+					mlog.String("channel_id", syncMsg.ChannelId),
+					mlog.String("remote_error", resp.Err),
+				)
+				return
+			}
+
+			// Update sync timestamps
+			for _, change := range syncMsg.MembershipChanges {
+				if err := scs.server.GetStore().SharedChannel().UpdateUserLastMembershipSyncAt(change.UserId, change.ChannelId, remote.RemoteId, change.ChangeTime); err != nil {
+					scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Failed to update user membership sync timestamp in batch",
+						mlog.String("user_id", change.UserId),
+						mlog.Err(err),
+					)
+				}
+			}
+
+			// Update the cursor with the latest change time
+			var maxChangeTime int64
+			for _, change := range syncMsg.MembershipChanges {
+				if change.ChangeTime > maxChangeTime {
+					maxChangeTime = change.ChangeTime
+				}
+			}
+
+			if err := scs.updateMembershipSyncCursor(syncMsg.ChannelId, remote.RemoteId, maxChangeTime); err != nil {
+				scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Failed to update membership sync cursor for batch",
+					mlog.String("remote_id", remote.RemoteId),
+					mlog.String("channel_id", syncMsg.ChannelId),
+					mlog.Err(err),
+				)
+			}
+		}
+
+		err = scs.server.GetRemoteClusterService().SendMsg(ctx, msg, rc, callback)
+
+		if err != nil {
+			scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Failed to send batch membership changes to remote",
+				mlog.String("remote_id", remote.RemoteId),
+				mlog.String("channel_id", syncMsg.ChannelId),
+				mlog.Err(err),
+			)
+		}
+	}
+}

server/platform/services/sharedchannel/membership_recv.go

@@ -0,0 +1,213 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package sharedchannel
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/mattermost/mattermost/server/public/model"
+	"github.com/mattermost/mattermost/server/public/shared/mlog"
+	"github.com/mattermost/mattermost/server/public/shared/request"
+	"github.com/mattermost/mattermost/server/v8/platform/services/remotecluster"
+)
+
+// checkMembershipConflict checks if there are newer changes that would conflict with this one
+// Returns true if this change should be skipped due to a conflict
+func (scs *Service) checkMembershipConflict(userID, channelID string, changeTime int64) (bool, error) {
+	conflicts, err := scs.server.GetStore().SharedChannel().GetUserChanges(userID, channelID, changeTime)
+	if err != nil {
+		scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Failed to check for membership change conflicts",
+			mlog.String("user_id", userID),
+			mlog.String("channel_id", channelID),
+			mlog.Err(err),
+		)
+		return false, err
+	}
+
+	// If there are conflicting operations, the latest one wins
+	for _, conflict := range conflicts {
+		if conflict.LastMembershipSyncAt > changeTime {
+			scs.server.Log().Log(mlog.LvlSharedChannelServiceDebug, "Ignoring older membership change due to conflict",
+				mlog.String("user_id", userID),
+				mlog.String("channel_id", channelID),
+				mlog.Int("change_time", int(changeTime)),
+				mlog.Int("conflicting_time", int(conflict.LastMembershipSyncAt)),
+			)
+			return true, nil
+		}
+	}
+
+	return false, nil
+}
+
+// onReceiveMembershipChanges processes channel membership changes from a remote cluster
+func (scs *Service) onReceiveMembershipChanges(syncMsg *model.SyncMsg, rc *model.RemoteCluster, response *remotecluster.Response) error {
+	// Check if feature flag is enabled
+	if !scs.server.Config().FeatureFlags.EnableSharedChannelsMemberSync {
+		return nil
+	}
+
+	if len(syncMsg.MembershipChanges) == 0 {
+		return fmt.Errorf("onReceiveMembershipChanges: no membership changes")
+	}
+
+	// Get the channel to make sure it exists and is shared
+	channel, err := scs.server.GetStore().Channel().Get(syncMsg.ChannelId, true)
+	if err != nil {
+		return fmt.Errorf("cannot get channel for membership changes: %w", err)
+	}
+
+	// Verify this is a valid shared channel
+	_, err = scs.server.GetStore().SharedChannel().Get(syncMsg.ChannelId)
+	if err != nil {
+		return fmt.Errorf("cannot get shared channel for membership changes: %w", err)
+	}
+
+	// Calculate the maximum ChangeTime from all changes in the batch
+	var maxChangeTime int64
+	for _, change := range syncMsg.MembershipChanges {
+		if change.ChangeTime > maxChangeTime {
+			maxChangeTime = change.ChangeTime
+		}
+	}
+
+	// Process each change
+	var successCount, skipCount, failCount int
+
+	for _, change := range syncMsg.MembershipChanges {
+		// Check for conflicts
+		shouldSkip, _ := scs.checkMembershipConflict(change.UserId, change.ChannelId, change.ChangeTime)
+		if shouldSkip {
+			skipCount++
+			continue
+		}
+
+		// Process the membership change based on whether it's an add or remove
+		var processErr error
+		if change.IsAdd {
+			scs.server.Log().Log(mlog.LvlSharedChannelServiceDebug, "Adding user to channel from remote cluster",
+				mlog.String("user_id", change.UserId),
+				mlog.String("channel_id", change.ChannelId),
+				mlog.String("remote_id", rc.RemoteId),
+			)
+			processErr = scs.processMemberAdd(change, channel, rc, maxChangeTime)
+		} else {
+			scs.server.Log().Log(mlog.LvlSharedChannelServiceDebug, "Removing user from channel from remote cluster",
+				mlog.String("user_id", change.UserId),
+				mlog.String("channel_id", change.ChannelId),
+				mlog.String("remote_id", rc.RemoteId),
+			)
+			processErr = scs.processMemberRemove(change, rc, maxChangeTime)
+		}
+
+		if processErr != nil {
+			scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Failed to process membership change",
+				mlog.String("user_id", change.UserId),
+				mlog.String("channel_id", change.ChannelId),
+				mlog.String("remote_id", rc.RemoteId),
+				mlog.Bool("is_add", change.IsAdd),
+				mlog.Err(processErr),
+			)
+			failCount++
+			continue
+		}
+
+		successCount++
+	}
+
+	return nil
+}
+
+// processMemberAdd handles adding a user to a channel as part of batch processing
+func (scs *Service) processMemberAdd(change *model.MembershipChangeMsg, channel *model.Channel, rc *model.RemoteCluster, maxChangeTime int64) error {
+	// Get the user if they exist
+	user, err := scs.server.GetStore().User().Get(request.EmptyContext(scs.server.Log()).Context(), change.UserId)
+	if err != nil {
+		return fmt.Errorf("cannot get user for channel add: %w", err)
+	}
+
+	// Check user permissions for private channels
+	if channel.Type == model.ChannelTypePrivate {
+		// Add user to team if needed for private channel
+		rctx := request.EmptyContext(scs.server.Log())
+		appErr := scs.app.AddUserToTeamByTeamId(rctx, channel.TeamId, user)
+		if appErr != nil {
+			return fmt.Errorf("cannot add user to team for private channel: %w", appErr)
+		}
+	}
+
+	// Use the app layer to add the user to the channel
+	// This ensures proper processing of all side effects
+	rctx := request.EmptyContext(scs.server.Log())
+	_, appErr := scs.app.AddUserToChannel(rctx, user, channel, false)
+	if appErr != nil {
+		// Skip "already added" errors
+		if appErr.Error() != "api.channel.add_user.to_channel.failed.app_error" &&
+			!strings.Contains(appErr.Error(), "channel_member_exists") {
+			return fmt.Errorf("cannot add user to channel: %w", appErr)
+		}
+		// User is already in the channel, which is fine
+	}
+
+	// Update the sync status
+	if syncErr := scs.server.GetStore().SharedChannel().UpdateUserLastMembershipSyncAt(change.UserId, change.ChannelId, rc.RemoteId, maxChangeTime); syncErr != nil {
+		scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Failed to update user LastMembershipSyncAt after batch member add",
+			mlog.String("user_id", change.UserId),
+			mlog.String("channel_id", change.ChannelId),
+			mlog.String("remote_id", rc.RemoteId),
+			mlog.Err(syncErr),
+		)
+		// Continue despite the error - this is not critical
+	}
+
+	return nil
+}
+
+// processMemberRemove handles removing a user from a channel as part of batch processing
+func (scs *Service) processMemberRemove(change *model.MembershipChangeMsg, rc *model.RemoteCluster, maxChangeTime int64) error {
+	// Get channel so we can use app layer methods properly
+	channel, err := scs.server.GetStore().Channel().Get(change.ChannelId, true)
+	if err != nil {
+		scs.server.Log().Log(mlog.LvlSharedChannelServiceWarn, "Cannot find channel for member removal",
+			mlog.String("channel_id", change.ChannelId),
+			mlog.String("user_id", change.UserId),
+			mlog.Err(err),
+		)
+		// Continue anyway to update sync status - the channel might be deleted
+	}
+
+	// Use the app layer's remove user method if channel still exists
+	if channel != nil {
+		rctx := request.EmptyContext(scs.server.Log())
+		// We use empty string for removerUserId to indicate system-initiated removal
+		// This also ensures we bypass permission checks intended for user-initiated removals
+		appErr := scs.app.RemoveUserFromChannel(rctx, change.UserId, "", channel)
+		if appErr != nil {
+			// Ignore "not found" errors - the user might already be removed
+			if !strings.Contains(appErr.Error(), "store.sql_channel.remove_member.missing.app_error") {
+				scs.server.Log().Log(mlog.LvlSharedChannelServiceWarn, "Error removing user from channel",
+					mlog.String("channel_id", change.ChannelId),
+					mlog.String("user_id", change.UserId),
+					mlog.Err(appErr),
+				)
+				// Continue anyway to update sync status - don't return error here
+				// to ensure sync status still gets updated
+			}
+		}
+	}
+
+	// Update the sync status
+	if syncErr := scs.server.GetStore().SharedChannel().UpdateUserLastMembershipSyncAt(change.UserId, change.ChannelId, rc.RemoteId, maxChangeTime); syncErr != nil {
+		scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Failed to update user LastMembershipSyncAt after batch member remove",
+			mlog.String("user_id", change.UserId),
+			mlog.String("channel_id", change.ChannelId),
+			mlog.String("remote_id", rc.RemoteId),
+			mlog.Err(syncErr),
+		)
+		// Continue despite the error - this is not critical
+	}
+
+	return nil
+}

server/platform/services/sharedchannel/mock_AppIface_test.go

@@ -513,6 +513,26 @@ func (_m *MockAppIface) Publish(message *model.WebSocketEvent) {
 	_m.Called(message)
 }
 
+// RemoveUserFromChannel provides a mock function with given fields: c, userID, removerUserId, channel
+func (_m *MockAppIface) RemoveUserFromChannel(c request.CTX, userID string, removerUserId string, channel *model.Channel) *model.AppError {
+	ret := _m.Called(c, userID, removerUserId, channel)
+
+	if len(ret) == 0 {
+		panic("no return value specified for RemoveUserFromChannel")
+	}
+
+	var r0 *model.AppError
+	if rf, ok := ret.Get(0).(func(request.CTX, string, string, *model.Channel) *model.AppError); ok {
+		r0 = rf(c, userID, removerUserId, channel)
+	} else {
+		if ret.Get(0) != nil {
+			r0 = ret.Get(0).(*model.AppError)
+		}
+	}
+
+	return r0
+}
+
 // SaveAndBroadcastStatus provides a mock function with given fields: status
 func (_m *MockAppIface) SaveAndBroadcastStatus(status *model.Status) {
 	_m.Called(status)

server/platform/services/sharedchannel/service.go

@@ -24,6 +24,7 @@ const (
 	TopicSync                    = "sharedchannel_sync"
 	TopicChannelInvite           = "sharedchannel_invite"
 	TopicUploadCreate            = "sharedchannel_upload"
+	TopicChannelMembership       = "sharedchannel_membership"
 	TopicGlobalUserSync          = "sharedchannel_global_user_sync"
 	MaxRetries                   = 3
 	MaxUsersPerSync              = 25
@@ -31,6 +32,7 @@ const (
 	NotifyMinimumDelay           = time.Second * 2
 	MaxUpsertRetries             = 25
 	ProfileImageSyncTimeout      = time.Second * 5
+	// Default value for MaxMembersPerBatch is defined in config.go as ConnectedWorkspacesSettingsDefaultMemberSyncBatchSize
 )
 
 // Mocks can be re-generated with `make sharedchannel-mocks`.
@@ -58,6 +60,7 @@ type AppIface interface {
 	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
+	RemoveUserFromChannel(c request.CTX, userID string, removerUserId string, channel *model.Channel) *model.AppError
 	PermanentDeleteChannel(c request.CTX, channel *model.Channel) *model.AppError
 	CreatePost(c request.CTX, post *model.Post, channel *model.Channel, flags model.CreatePostFlags) (savedPost *model.Post, err *model.AppError)
 	UpdatePost(c request.CTX, post *model.Post, updatePostOptions *model.UpdatePostOptions) (*model.Post, *model.AppError)
@@ -90,9 +93,10 @@ type Service struct {
 	changeSignal chan struct{}
 
 	// everything below guarded by `mux`
-	mux                       sync.RWMutex
-	active                    bool
-	leaderListenerId          string
+	mux              sync.RWMutex
+	active           bool
+	leaderListenerId string
+
 	connectionStateListenerId string
 	done                      chan struct{}
 	tasks                     map[string]syncTask
@@ -136,6 +140,8 @@ func (scs *Service) Start() error {
 	scs.connectionStateListenerId = rcs.AddConnectionStateListener(scs.onConnectionStateChange)
 	scs.mux.Unlock()
 
+	rcs.AddTopicListener(TopicChannelMembership, scs.onReceiveSyncMessage)
+
 	scs.onClusterLeaderChange()
 
 	return nil
@@ -220,6 +226,14 @@ func (scs *Service) pause() {
 	scs.server.Log().Debug("Shared Channel Service inactive")
 }
 
+// GetMemberSyncBatchSize returns the configured batch size for member synchronization
+func (scs *Service) GetMemberSyncBatchSize() int {
+	if scs.server.Config().ConnectedWorkspacesSettings.MemberSyncBatchSize != nil {
+		return *scs.server.Config().ConnectedWorkspacesSettings.MemberSyncBatchSize
+	}
+	return model.ConnectedWorkspacesSettingsDefaultMemberSyncBatchSize
+}
+
 // Makes the remote channel to be read-only(announcement mode, only admins can create posts and reactions).
 func (scs *Service) makeChannelReadOnly(channel *model.Channel) *model.AppError {
 	createPostPermission := model.ChannelModeratedPermissionsMap[model.PermissionCreatePost.Id]
@@ -314,17 +328,19 @@ func (scs *Service) scheduleGlobalUserSync(rc *model.RemoteCluster) {
 	}()
 }
 
-// OnReceiveSyncMessageForTesting exposes onReceiveSyncMessage for testing
-func (scs *Service) OnReceiveSyncMessageForTesting(msg model.RemoteClusterMsg, rc *model.RemoteCluster, response *remotecluster.Response) error {
-	return scs.onReceiveSyncMessage(msg, rc, response)
-}
-
-// GetUserSyncBatchSizeForTesting returns the configured batch size for user syncing (exported for testing)
-func (scs *Service) GetUserSyncBatchSizeForTesting() int {
-	return scs.getGlobalUserSyncBatchSize()
+// HasPendingTasksForTesting returns true if there are pending sync tasks in the queue
+func (scs *Service) HasPendingTasksForTesting() bool {
+	scs.mux.RLock()
+	defer scs.mux.RUnlock()
+	return len(scs.tasks) > 0
 }
 
 // HandleSyncAllUsersForTesting exposes syncAllUsers for testing
 func (scs *Service) HandleSyncAllUsersForTesting(rc *model.RemoteCluster) error {
 	return scs.syncAllUsers(rc)
 }
+
+// OnReceiveSyncMessageForTesting exposes onReceiveSyncMessage for testing
+func (scs *Service) OnReceiveSyncMessageForTesting(msg model.RemoteClusterMsg, rc *model.RemoteCluster, response *remotecluster.Response) error {
+	return scs.onReceiveSyncMessage(msg, rc, response)
+}

server/platform/services/sharedchannel/service_api.go

@@ -277,3 +277,41 @@ func (scs *Service) CheckCanInviteToSharedChannel(channelId string) error {
 	}
 	return nil
 }
+
+// updateMembershipSyncCursor updates the LastMembersSyncAt value for the shared channel remote
+// This provides centralized and consistent cursor management
+func (scs *Service) updateMembershipSyncCursor(channelID string, remoteID string, newTimestamp int64) error {
+	// Get the remote record
+	scr, err := scs.server.GetStore().SharedChannel().GetRemoteByIds(channelID, remoteID)
+	if err != nil {
+		scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Failed to get shared channel remote for cursor update",
+			mlog.String("channel_id", channelID),
+			mlog.String("remote_id", remoteID),
+			mlog.Int("timestamp", int(newTimestamp)),
+			mlog.Err(err),
+		)
+		return fmt.Errorf("failed to get shared channel remote for cursor update: %w", err)
+	}
+
+	if scr == nil {
+		scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Shared channel remote not found for cursor update",
+			mlog.String("channel_id", channelID),
+			mlog.String("remote_id", remoteID),
+		)
+		return fmt.Errorf("shared channel remote not found for channel %s and remote %s", channelID, remoteID)
+	}
+
+	// Update the cursor - the store will handle ensuring it only moves forward
+	err = scs.server.GetStore().SharedChannel().UpdateRemoteMembershipCursor(scr.Id, newTimestamp)
+	if err != nil {
+		scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Failed to update membership cursor",
+			mlog.String("channel_id", channelID),
+			mlog.String("remote_id", remoteID),
+			mlog.String("remote_record_id", scr.Id),
+			mlog.Int("timestamp", int(newTimestamp)),
+			mlog.Err(err),
+		)
+	}
+
+	return err
+}

server/platform/services/sharedchannel/sync_recv.go

@@ -24,10 +24,9 @@ var (
 )
 
 func (scs *Service) onReceiveSyncMessage(msg model.RemoteClusterMsg, rc *model.RemoteCluster, response *remotecluster.Response) error {
-	if msg.Topic != TopicSync && msg.Topic != TopicGlobalUserSync {
-		return fmt.Errorf("wrong topic, expected `%s` or `%s`, got `%s`", TopicSync, TopicGlobalUserSync, msg.Topic)
+	if msg.Topic != TopicSync && msg.Topic != TopicChannelMembership && msg.Topic != TopicGlobalUserSync {
+		return fmt.Errorf("wrong topic, expected sync-related topic, got `%s`", msg.Topic)
 	}
-
 	if len(msg.Payload) == 0 {
 		return errors.New("empty sync message")
 	}
@@ -89,6 +88,15 @@ func (scs *Service) processSyncMessage(c request.CTX, syncMsg *model.SyncMsg, rc
 		ReactionErrors: make([]string, 0),
 	}
 
+	// Check if feature flag is enabled for membership changes
+	membershipSyncEnabled := scs.server.Config().FeatureFlags.EnableSharedChannelsMemberSync
+	hasMembershipChanges := len(syncMsg.MembershipChanges) > 0
+
+	// If this message only contains membership changes and feature is disabled, skip it
+	if hasMembershipChanges && !membershipSyncEnabled && len(syncMsg.Users) == 0 && len(syncMsg.Posts) == 0 && len(syncMsg.Reactions) == 0 {
+		return nil
+	}
+
 	scs.server.Log().Log(mlog.LvlSharedChannelServiceDebug, "Sync msg received",
 		mlog.String("remote", rc.Name),
 		mlog.String("channel_id", syncMsg.ChannelId),
@@ -96,6 +104,7 @@ func (scs *Service) processSyncMessage(c request.CTX, syncMsg *model.SyncMsg, rc
 		mlog.Int("post_count", len(syncMsg.Posts)),
 		mlog.Int("reaction_count", len(syncMsg.Reactions)),
 		mlog.Int("status_count", len(syncMsg.Statuses)),
+		mlog.Int("membership_change_count", len(syncMsg.MembershipChanges)),
 	)
 
 	// Check if this is a global user sync message (no channel ID and only users)
@@ -229,6 +238,19 @@ func (scs *Service) processSyncMessage(c request.CTX, syncMsg *model.SyncMsg, rc
 		scs.app.SaveAndBroadcastStatus(status)
 	}
 
+	// Process membership changes after users have been synced
+	if hasMembershipChanges && membershipSyncEnabled {
+		if err := scs.onReceiveMembershipChanges(syncMsg, rc, response); err != nil {
+			scs.server.Log().Log(mlog.LvlSharedChannelServiceError, "Error processing membership changes",
+				mlog.String("remote", rc.Name),
+				mlog.String("channel_id", syncMsg.ChannelId),
+				mlog.Int("change_count", len(syncMsg.MembershipChanges)),
+				mlog.Err(err),
+			)
+			// Don't fail the entire sync if membership changes fail
+		}
+	}
+
 	response.SetPayload(syncResp)
 
 	return nil

server/platform/services/sharedchannel/sync_send.go

@@ -37,8 +37,17 @@ func newSyncTask(channelID, userID string, remoteID string, existingMsg, retryMs
 		retryID = retryMsg.Id
 	}
 
+	// Generate a unique task ID
+	taskID := channelID + userID + remoteID + retryID // combination of ids to avoid duplicates
+
+	// For batch tasks, add a batch identifier to make the ID unique
+	if existingMsg != nil && len(existingMsg.MembershipChanges) > 1 {
+		batchID := model.NewId()[:8] // Use a short unique ID for the batch
+		taskID = channelID + "batch" + batchID + remoteID + retryID
+	}
+
 	return syncTask{
-		id:          channelID + userID + remoteID + retryID, // combination of ids to avoid duplicates
+		id:          taskID,
 		channelID:   channelID,
 		userID:      userID,
 		remoteID:    remoteID, // empty means update all remote clusters
@@ -235,6 +244,7 @@ func (scs *Service) ForceSyncForRemote(rc *model.RemoteCluster) {
 // addTask adds or re-adds a task to the queue.
 func (scs *Service) addTask(task syncTask) {
 	task.AddedAt = time.Now()
+
 	scs.mux.Lock()
 	if originalTask, ok := scs.tasks[task.id]; ok {
 		// if the task was already scheduled, we only update the
@@ -365,6 +375,16 @@ func (scs *Service) removeOldestTask() (syncTask, bool, time.Duration) {
 
 // processTask updates one or more remote clusters with any new channel content.
 func (scs *Service) processTask(task syncTask) error {
+	// Check if this is a membership change task
+	if task.existingMsg != nil && len(task.existingMsg.MembershipChanges) > 0 {
+		// Check if feature flag is enabled
+		if !scs.server.Config().FeatureFlags.EnableSharedChannelsMemberSync {
+			return nil
+		}
+		scs.processMembershipChange(task.existingMsg)
+		return nil
+	}
+
 	// map is used to ensure remotes don't get sync'd twice, such as when
 	// they have the autoinvited flag and have explicitly subscribed to a channel.
 	remotesMap := make(map[string]*model.RemoteCluster)

server/platform/services/sharedchannel/sync_send_remote.go

@@ -119,6 +119,7 @@ func (scs *Service) syncForRemote(task syncTask, rc *model.RemoteCluster) error
 			RemoteId:          rc.RemoteId,
 			LastPostCreateAt:  model.GetMillis(),
 			LastPostUpdateAt:  model.GetMillis(),
+			LastMembersSyncAt: 0,
 		}
 		if scr, err = scs.server.GetStore().SharedChannel().SaveRemote(scr); err != nil {
 			return fmt.Errorf("cannot auto-create shared channel remote (channel_id=%s, remote_id=%s): %w", task.channelID, rc.RemoteId, err)