MM-27493 Shared channels (MVP) (#17301)

Remote Cluster Service
- provides ability for multiple Mattermost cluster instances to create a trusted connection with each other and exchange messages
- trusted connections are managed via slash commands (for now)
- facilitates features requiring inter-cluster communication, such as Shared Channels
Shared Channels Service
- provides ability to shared channels between one or more Mattermost cluster instances (using trusted connection)
- sharing/unsharing of channels is managed via slash commands (for now)

app/server.go

@@ -47,8 +47,10 @@ import (
 	"github.com/mattermost/mattermost-server/v5/services/cache"
 	"github.com/mattermost/mattermost-server/v5/services/httpservice"
 	"github.com/mattermost/mattermost-server/v5/services/imageproxy"
+	"github.com/mattermost/mattermost-server/v5/services/remotecluster"
 	"github.com/mattermost/mattermost-server/v5/services/searchengine"
 	"github.com/mattermost/mattermost-server/v5/services/searchengine/bleveengine"
+	"github.com/mattermost/mattermost-server/v5/services/sharedchannel"
 	"github.com/mattermost/mattermost-server/v5/services/telemetry"
 	"github.com/mattermost/mattermost-server/v5/services/timezones"
 	"github.com/mattermost/mattermost-server/v5/services/tracing"
@@ -157,6 +159,9 @@ type Server struct {
 
 	telemetryService *telemetry.TelemetryService
 
+	remoteClusterService remotecluster.RemoteClusterServiceIFace
+	sharedChannelService SharedChannelServiceIFace
+
 	phase2PermissionsMigrationComplete bool
 
 	HTTPService httpservice.HTTPService
@@ -808,6 +813,64 @@ func (s *Server) removeUnlicensedLogTargets(license *model.License) {
 	})
 }
 
+func (s *Server) startInterClusterServices(license *model.License, app *App) error {
+	if license == nil {
+		mlog.Debug("No license provided; Remote Cluster services disabled")
+		return nil
+	}
+
+	// Remote Cluster service
+
+	// License check
+	if !*license.Features.RemoteClusterService {
+		mlog.Debug("License does not have Remote Cluster services enabled")
+		return nil
+	}
+
+	// Config check
+	if !*s.Config().ExperimentalSettings.EnableRemoteClusterService {
+		mlog.Debug("Remote Cluster Service disabled via config")
+		return nil
+	}
+
+	var err error
+
+	s.remoteClusterService, err = remotecluster.NewRemoteClusterService(s)
+	if err != nil {
+		return err
+	}
+
+	if err = s.remoteClusterService.Start(); err != nil {
+		s.remoteClusterService = nil
+		return err
+	}
+
+	// Shared Channels service
+
+	// License check
+	if !*license.Features.SharedChannels {
+		mlog.Debug("License does not have shared channels enabled")
+		return nil
+	}
+
+	// Config check
+	if !*s.Config().ExperimentalSettings.EnableSharedChannels {
+		mlog.Debug("Shared Channels Service disabled via config")
+		return nil
+	}
+
+	s.sharedChannelService, err = sharedchannel.NewSharedChannelService(s, app)
+	if err != nil {
+		return err
+	}
+
+	if err = s.sharedChannelService.Start(); err != nil {
+		s.remoteClusterService = nil
+		return err
+	}
+	return nil
+}
+
 func (s *Server) enableLoggingMetrics() {
 	if s.Metrics == nil {
 		return
@@ -866,6 +929,12 @@ func (s *Server) Shutdown() {
 		mlog.Warn("Unable to cleanly shutdown telemetry client", mlog.Err(err))
 	}
 
+	if s.remoteClusterService != nil {
+		if err = s.remoteClusterService.Shutdown(); err != nil {
+			mlog.Error("Error shutting down intercluster services", mlog.Err(err))
+		}
+	}
+
 	s.StopHTTPServer()
 	s.stopLocalModeServer()
 	// Push notification hub needs to be shutdown after HTTP server
@@ -1231,6 +1300,10 @@ func (s *Server) Start() error {
 		}
 	}
 
+	if err := s.startInterClusterServices(s.License(), s.WebSocketRouter.app); err != nil {
+		mlog.Error("Error starting inter-cluster services", mlog.Err(err))
+	}
+
 	return nil
 }
 
@@ -1799,6 +1872,46 @@ func (s *Server) SetLog(l *mlog.Logger) {
 	s.Log = l
 }
 
+func (s *Server) GetLogger() mlog.LoggerIFace {
+	return s.Log
+}
+
+// GetStore returns the server's Store. Exposing via a method
+// allows interfaces to be created with subsets of server APIs.
+func (s *Server) GetStore() store.Store {
+	return s.Store
+}
+
+// GetRemoteClusterService returns the `RemoteClusterService` instantiated by the server.
+// May be nil if the service is not enabled via license.
+func (s *Server) GetRemoteClusterService() remotecluster.RemoteClusterServiceIFace {
+	return s.remoteClusterService
+}
+
+// GetSharedChannelSyncService returns the `SharedChannelSyncService` instantiated by the server.
+// May be nil if the service is not enabled via license.
+func (s *Server) GetSharedChannelSyncService() SharedChannelServiceIFace {
+	return s.sharedChannelService
+}
+
+// GetMetrics returns the server's Metrics interface. Exposing via a method
+// allows interfaces to be created with subsets of server APIs.
+func (s *Server) GetMetrics() einterfaces.MetricsInterface {
+	return s.Metrics
+}
+
+// SetRemoteClusterService sets the `RemoteClusterService` to be used by the server.
+// For testing only.
+func (s *Server) SetRemoteClusterService(remoteClusterService remotecluster.RemoteClusterServiceIFace) {
+	s.remoteClusterService = remoteClusterService
+}
+
+// SetSharedChannelSyncService sets the `SharedChannelSyncService` to be used by the server.
+// For testing only.
+func (s *Server) SetSharedChannelSyncService(sharedChannelService SharedChannelServiceIFace) {
+	s.sharedChannelService = sharedChannelService
+}
+
 func (a *App) GenerateSupportPacket() []model.FileData {
 	// If any errors we come across within this function, we will log it in a warning.txt file so that we know why certain files did not get produced if any
 	var warnings []string