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)

services/sharedchannel/permalink.go

@@ -0,0 +1,81 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package sharedchannel
+
+import (
+	"context"
+	"net/url"
+	"regexp"
+	"strings"
+
+	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v5/shared/i18n"
+	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+)
+
+var (
+	// Team name regex taken from model.IsValidTeamName
+	permaLinkRegex       = regexp.MustCompile(`https?://[0-9.\-A-Za-z]+/[a-z0-9]+([a-z\-0-9]+|(__)?)[a-z0-9]+/pl/([a-zA-Z0-9]+)`)
+	permaLinkSharedRegex = regexp.MustCompile(`https?://[0-9.\-A-Za-z]+/[a-z0-9]+([a-z\-0-9]+|(__)?)[a-z0-9]+/plshared/([a-zA-Z0-9]+)`)
+)
+
+const (
+	permalinkMarker = "plshared"
+)
+
+// processPermalinkToRemote processes all permalinks going towards a remote site.
+func (scs *Service) processPermalinkToRemote(p *model.Post) string {
+	var sent bool
+	return permaLinkRegex.ReplaceAllStringFunc(p.Message, func(msg string) string {
+		// Extract the postID (This is simple enough not to warrant full-blown URL parsing.)
+		lastSlash := strings.LastIndexByte(msg, '/')
+		postID := msg[lastSlash+1:]
+		postList, err := scs.server.GetStore().Post().Get(context.Background(), postID, true, false, false, "")
+		if err != nil {
+			scs.server.GetLogger().Log(mlog.LvlSharedChannelServiceWarn, "Unable to get post during replacing permalinks", mlog.Err(err))
+			return msg
+		}
+		if len(postList.Order) == 0 {
+			scs.server.GetLogger().Log(mlog.LvlSharedChannelServiceWarn, "No post found for permalink", mlog.String("postID", postID))
+			return msg
+		}
+
+		// If postID is for a different channel
+		if postList.Posts[postList.Order[0]].ChannelId != p.ChannelId {
+			// Send ephemeral message to OP (only once per message).
+			if !sent {
+				scs.sendEphemeralPost(p.ChannelId, p.UserId, i18n.T("sharedchannel.permalink.not_found"))
+				sent = true
+			}
+			// But don't modify msg
+			return msg
+		}
+
+		// Otherwise, modify pl to plshared as a marker to be replaced by remote sites
+		return strings.Replace(msg, "/pl/", "/"+permalinkMarker+"/", 1)
+	})
+}
+
+// processPermalinkFromRemote processes all permalinks coming from a remote site.
+func (scs *Service) processPermalinkFromRemote(p *model.Post, team *model.Team) string {
+	return permaLinkSharedRegex.ReplaceAllStringFunc(p.Message, func(remoteLink string) string {
+		// Extract host name
+		parsed, err := url.Parse(remoteLink)
+		if err != nil {
+			scs.server.GetLogger().Log(mlog.LvlSharedChannelServiceWarn, "Unable to parse the remote link during replacing permalinks", mlog.Err(err))
+			return remoteLink
+		}
+
+		// Replace with local SiteURL
+		parsed.Scheme = scs.siteURL.Scheme
+		parsed.Host = scs.siteURL.Host
+
+		// Replace team name with local team
+		teamEnd := strings.Index(parsed.Path, "/"+permalinkMarker)
+		parsed.Path = "/" + team.Name + parsed.Path[teamEnd:]
+
+		// Replace plshared with pl
+		return strings.Replace(parsed.String(), "/"+permalinkMarker+"/", "/pl/", 1)
+	})
+}