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)

api4/shared_channel.go

@@ -0,0 +1,76 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package api4
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/mattermost/mattermost-server/v5/model"
+)
+
+func (api *API) InitSharedChannels() {
+	api.BaseRoutes.SharedChannels.Handle("/{team_id:[A-Za-z0-9]+}", api.ApiSessionRequired(getSharedChannels)).Methods("GET")
+	api.BaseRoutes.SharedChannels.Handle("/remote_info/{remote_id:[A-Za-z0-9]+}", api.ApiSessionRequired(getRemoteClusterInfo)).Methods("GET")
+}
+
+func getSharedChannels(c *Context, w http.ResponseWriter, r *http.Request) {
+	c.RequireTeamId()
+	if c.Err != nil {
+		return
+	}
+
+	// make sure remote cluster service is enabled.
+	if _, appErr := c.App.GetRemoteClusterService(); appErr != nil {
+		c.Err = appErr
+		return
+	}
+
+	opts := model.SharedChannelFilterOpts{
+		TeamId: c.Params.TeamId,
+	}
+
+	channels, appErr := c.App.GetSharedChannels(c.Params.Page, c.Params.PerPage, opts)
+	if appErr != nil {
+		c.Err = appErr
+		return
+	}
+
+	b, err := json.Marshal(channels)
+	if err != nil {
+		c.SetJSONEncodingError()
+		return
+	}
+	w.Write(b)
+}
+
+func getRemoteClusterInfo(c *Context, w http.ResponseWriter, r *http.Request) {
+	c.RequireRemoteId()
+	if c.Err != nil {
+		return
+	}
+
+	// make sure remote cluster service is enabled.
+	if _, appErr := c.App.GetRemoteClusterService(); appErr != nil {
+		c.Err = appErr
+		return
+	}
+
+	// GetRemoteClusterForUser will only return a remote if the user is a member of at
+	// least one channel shared by the remote. All other cases return error.
+	rc, appErr := c.App.GetRemoteClusterForUser(c.Params.RemoteId, c.App.Session().UserId)
+	if appErr != nil {
+		c.Err = appErr
+		return
+	}
+
+	remoteInfo := rc.ToRemoteClusterInfo()
+
+	b, err := json.Marshal(remoteInfo)
+	if err != nil {
+		c.SetJSONEncodingError()
+		return
+	}
+	w.Write(b)
+}