MM-22845: Added support for permalink previews. (#17796)

* MM-22845: Added support for permalink previews.

* MM-22845: Adds license to new file.

* MM-22845: Adds endpoint to retrieve multiple posts by id.

* MM-22845: Fix for deleted post.

* MM-22845: Adds config setting for permalink previews.

* MM-22845: Adds API test for new endpoint.

* MM-22845: Fix typo.

* MM-22845: Tests that post create or updated via App get the previewed_post prop.

* MM-22845: Tests for matching permalinks.

* MM-22845: Adds PreparePostForClient test for permalink previews.

* MM-22845: Embeds entire post in permalink metadata.

* MM-22845: Filter WS message payload of created and edited post based on permissions.

* MM-22845: Runs app layer generator.

* MM-22845: Lint check fix.

* MM-22845: Adds feature flag.

* MM-22845: Clones WS message.

* MM-22845: Removes knowledge of permalink from LinkMetadata table. Removes knowledge of user id from post embedding methods in favour of a 'sanitize' method/step.

* MM-22845: Handle nil post metadata.

* MM-22845: Switch to cloning post.

* MM-22845: Removes unused code.

* MM-22845: Refactor.

* MM-22845: Reverts whitespace change.

* MM-22845: Removes unnecessary code.

* MM-22845: Removes unnecessary function.

* MM-22845: Warn but don't error if permalinked referenced post or channel is not found.

* MM-22845: Fix for clone method.

* MM-22845: Fix for clone method.

* MM-22845: Updates translations.

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

api4/channel.go

@@ -722,6 +722,11 @@ func getPinnedPosts(c *Context, w http.ResponseWriter, r *http.Request) {
 	}
 
 	clientPostList := c.App.PreparePostListForClient(posts)
+	clientPostList, err = c.App.SanitizePostListMetadataForUser(clientPostList, c.AppContext.Session().UserId)
+	if err != nil {
+		c.Err = err
+		return
+	}
 
 	w.Header().Set(model.HeaderEtagServer, clientPostList.Etag())
 	if err := json.NewEncoder(w).Encode(clientPostList); err != nil {

api4/post.go

@@ -128,6 +128,11 @@ func createEphemeralPost(c *Context, w http.ResponseWriter, r *http.Request) {
 	w.WriteHeader(http.StatusCreated)
 	rp = model.AddPostActionCookies(rp, c.App.PostActionCookieSecret())
 	rp = c.App.PreparePostForClient(rp, true, false)
+	rp, err := c.App.SanitizePostMetadataForUser(rp, c.AppContext.Session().UserId)
+	if err != nil {
+		c.Err = err
+		return
+	}
 	if err := json.NewEncoder(w).Encode(rp); err != nil {
 		mlog.Warn("Error while writing response", mlog.Err(err))
 	}
@@ -216,6 +221,11 @@ func getPostsForChannel(c *Context, w http.ResponseWriter, r *http.Request) {
 
 	c.App.AddCursorIdsForPostList(list, afterPost, beforePost, since, page, perPage, collapsedThreads)
 	clientPostList := c.App.PreparePostListForClient(list)
+	clientPostList, err = c.App.SanitizePostListMetadataForUser(clientPostList, c.AppContext.Session().UserId)
+	if err != nil {
+		c.Err = err
+		return
+	}
 
 	if err := json.NewEncoder(w).Encode(clientPostList); err != nil {
 		mlog.Warn("Error while writing response", mlog.Err(err))
@@ -274,6 +284,11 @@ func getPostsForChannelAroundLastUnread(c *Context, w http.ResponseWriter, r *ht
 	postList.PrevPostId = c.App.GetPrevPostIdFromPostList(postList, collapsedThreads)
 
 	clientPostList := c.App.PreparePostListForClient(postList)
+	clientPostList, err = c.App.SanitizePostListMetadataForUser(clientPostList, c.AppContext.Session().UserId)
+	if err != nil {
+		c.Err = err
+		return
+	}
 
 	if etag != "" {
 		w.Header().Set(model.HeaderEtagServer, etag)
@@ -337,7 +352,13 @@ func getFlaggedPostsForUser(c *Context, w http.ResponseWriter, r *http.Request)
 	}
 
 	pl.SortByCreateAt()
-	if err := json.NewEncoder(w).Encode(c.App.PreparePostListForClient(pl)); err != nil {
+	clientPostList := c.App.PreparePostListForClient(pl)
+	clientPostList, err = c.App.SanitizePostListMetadataForUser(clientPostList, c.AppContext.Session().UserId)
+	if err != nil {
+		c.Err = err
+		return
+	}
+	if err := json.NewEncoder(w).Encode(clientPostList); err != nil {
 		mlog.Warn("Error while writing response", mlog.Err(err))
 	}
 }
@@ -355,6 +376,11 @@ func getPost(c *Context, w http.ResponseWriter, r *http.Request) {
 	}
 
 	post = c.App.PreparePostForClient(post, false, false)
+	post, err = c.App.SanitizePostMetadataForUser(post, c.AppContext.Session().UserId)
+	if err != nil {
+		c.Err = err
+		return
+	}
 
 	if c.HandleEtag(post.Etag(), "Get Post", w, r) {
 		return
@@ -434,6 +460,11 @@ func getPostThread(c *Context, w http.ResponseWriter, r *http.Request) {
 	}
 
 	clientPostList := c.App.PreparePostListForClient(list)
+	clientPostList, err = c.App.SanitizePostListMetadataForUser(clientPostList, c.AppContext.Session().UserId)
+	if err != nil {
+		c.Err = err
+		return
+	}
 
 	w.Header().Set(model.HeaderEtagServer, clientPostList.Etag())
 
@@ -507,6 +538,11 @@ func searchPosts(c *Context, w http.ResponseWriter, r *http.Request) {
 	}
 
 	clientPostList := c.App.PreparePostListForClient(results.PostList)
+	clientPostList, err = c.App.SanitizePostListMetadataForUser(clientPostList, c.AppContext.Session().UserId)
+	if err != nil {
+		c.Err = err
+		return
+	}
 
 	results = model.MakePostSearchResults(clientPostList, results.Matches)
 

app/admin.go

@@ -176,6 +176,7 @@ func (s *Server) InvalidateAllCachesSkipSend() {
 	s.Store.Post().ClearCaches()
 	s.Store.FileInfo().ClearCaches()
 	s.Store.Webhook().ClearCaches()
+	linkCache.Purge()
 	s.LoadLicense()
 }
 

app/app_iface.go

@@ -815,6 +815,7 @@ type AppIface interface {
 	HasPermissionTo(askingUserId string, permission *model.Permission) bool
 	HasPermissionToChannel(askingUserId string, channelID string, permission *model.Permission) bool
 	HasPermissionToChannelByPost(askingUserId string, postID string, permission *model.Permission) bool
+	HasPermissionToReadChannel(userID string, channel *model.Channel) bool
 	HasPermissionToTeam(askingUserId string, teamID string, permission *model.Permission) bool
 	HasPermissionToUser(askingUserId string, userID string) bool
 	HasSharedChannel(channelID string) (bool, error)
@@ -941,6 +942,8 @@ type AppIface interface {
 	RevokeUserAccessToken(token *model.UserAccessToken) *model.AppError
 	RolesGrantPermission(roleNames []string, permissionId string) bool
 	Saml() einterfaces.SamlInterface
+	SanitizePostListMetadataForUser(postList *model.PostList, userID string) (*model.PostList, *model.AppError)
+	SanitizePostMetadataForUser(post *model.Post, userID string) (*model.Post, *model.AppError)
 	SanitizeProfile(user *model.User, asAdmin bool)
 	SanitizeTeam(session model.Session, team *model.Team) *model.Team
 	SanitizeTeams(session model.Session, teams []*model.Team) []*model.Team

app/authorization.go

@@ -278,3 +278,7 @@ func (a *App) SessionHasPermissionToManageBot(session model.Session, botUserId s
 
 	return nil
 }
+
+func (a *App) HasPermissionToReadChannel(userID string, channel *model.Channel) bool {
+	return a.HasPermissionToChannel(userID, channel.Id, model.PermissionReadChannel) || (channel.Type == model.ChannelTypeOpen && a.HasPermissionToTeam(userID, channel.TeamId, model.PermissionReadPublicChannel))
+}

app/notification.go

@@ -469,7 +469,13 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 		message.Add("mentions", model.ArrayToJson(mentionedUsersList))
 	}
 
+	published, err := a.publishWebsocketEventForPermalinkPost(post, message)
+	if err != nil {
+		return nil, err
+	}
+	if !published {
 		a.Publish(message)
+	}
 
 	// If this is a reply in a thread, notify participants
 	if a.Config().FeatureFlags.CollapsedThreads && *a.Config().ServiceSettings.CollapsedThreads != model.CollapsedThreadsDisabled && post.RootId != "" {
@@ -502,6 +508,18 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 				if userThread != nil {
 					a.sanitizeProfiles(userThread.Participants, false)
 					userThread.Post.SanitizeProps()
+
+					previewPost := post.GetPreviewPost()
+					if previewPost != nil {
+						previewedChannel, err := a.GetChannel(previewPost.Post.ChannelId)
+						if err != nil {
+							return nil, err
+						}
+						if previewedChannel != nil && !a.HasPermissionToReadChannel(uid, previewedChannel) {
+							userThread.Post.Metadata.Embeds[0].Data = nil
+						}
+					}
+
 					message.Add("thread", userThread.ToJson())
 					a.Publish(message)
 				}

app/opentracing/opentracing_layer.go

@@ -10431,6 +10431,23 @@ func (a *OpenTracingAppLayer) HasPermissionToChannelByPost(askingUserId string,
 	return resultVar0
 }
 
+func (a *OpenTracingAppLayer) HasPermissionToReadChannel(userID string, channel *model.Channel) bool {
+	origCtx := a.ctx
+	span, newCtx := tracing.StartSpanWithParentByContext(a.ctx, "app.HasPermissionToReadChannel")
+
+	a.ctx = newCtx
+	a.app.Srv().Store.SetContext(newCtx)
+	defer func() {
+		a.app.Srv().Store.SetContext(origCtx)
+		a.ctx = origCtx
+	}()
+
+	defer span.Finish()
+	resultVar0 := a.app.HasPermissionToReadChannel(userID, channel)
+
+	return resultVar0
+}
+
 func (a *OpenTracingAppLayer) HasPermissionToTeam(askingUserId string, teamID string, permission *model.Permission) bool {
 	origCtx := a.ctx
 	span, newCtx := tracing.StartSpanWithParentByContext(a.ctx, "app.HasPermissionToTeam")
@@ -13432,6 +13449,50 @@ func (a *OpenTracingAppLayer) RolesGrantPermission(roleNames []string, permissio
 	return resultVar0
 }
 
+func (a *OpenTracingAppLayer) SanitizePostListMetadataForUser(postList *model.PostList, userID string) (*model.PostList, *model.AppError) {
+	origCtx := a.ctx
+	span, newCtx := tracing.StartSpanWithParentByContext(a.ctx, "app.SanitizePostListMetadataForUser")
+
+	a.ctx = newCtx
+	a.app.Srv().Store.SetContext(newCtx)
+	defer func() {
+		a.app.Srv().Store.SetContext(origCtx)
+		a.ctx = origCtx
+	}()
+
+	defer span.Finish()
+	resultVar0, resultVar1 := a.app.SanitizePostListMetadataForUser(postList, userID)
+
+	if resultVar1 != nil {
+		span.LogFields(spanlog.Error(resultVar1))
+		ext.Error.Set(span, true)
+	}
+
+	return resultVar0, resultVar1
+}
+
+func (a *OpenTracingAppLayer) SanitizePostMetadataForUser(post *model.Post, userID string) (*model.Post, *model.AppError) {
+	origCtx := a.ctx
+	span, newCtx := tracing.StartSpanWithParentByContext(a.ctx, "app.SanitizePostMetadataForUser")
+
+	a.ctx = newCtx
+	a.app.Srv().Store.SetContext(newCtx)
+	defer func() {
+		a.app.Srv().Store.SetContext(origCtx)
+		a.ctx = origCtx
+	}()
+
+	defer span.Finish()
+	resultVar0, resultVar1 := a.app.SanitizePostMetadataForUser(post, userID)
+
+	if resultVar1 != nil {
+		span.LogFields(spanlog.Error(resultVar1))
+		ext.Error.Set(span, true)
+	}
+
+	return resultVar0, resultVar1
+}
+
 func (a *OpenTracingAppLayer) SanitizeProfile(user *model.User, asAdmin bool) {
 	origCtx := a.ctx
 	span, newCtx := tracing.StartSpanWithParentByContext(a.ctx, "app.SanitizeProfile")

app/post.go

@@ -359,6 +359,11 @@ func (a *App) CreatePost(c *request.Context, post *model.Post, channel *model.Ch
 	// to be done when we send the post over the websocket in handlePostEvents
 	rpost = a.PreparePostForClient(rpost, true, false)
 
+	rpost, nErr = a.addPostPreviewProp(rpost)
+	if nErr != nil {
+		return nil, model.NewAppError("CreatePost", "app.post.save.app_error", nil, nErr.Error(), http.StatusInternalServerError)
+	}
+
 	// Make sure poster is following the thread
 	if *a.Config().ServiceSettings.ThreadAutoFollow && rpost.RootId != "" {
 		_, err := a.Srv().Store.Thread().MaintainMembership(user.Id, rpost.RootId, store.ThreadMembershipOpts{
@@ -379,9 +384,25 @@ func (a *App) CreatePost(c *request.Context, post *model.Post, channel *model.Ch
 		a.SendEphemeralPost(post.UserId, ephemeralPost)
 	}
 
+	rpost, err = a.SanitizePostMetadataForUser(rpost, c.Session().UserId)
+	if err != nil {
+		return nil, err
+	}
+
 	return rpost, nil
 }
 
+func (a *App) addPostPreviewProp(post *model.Post) (*model.Post, error) {
+	previewPost := post.GetPreviewPost()
+	if previewPost != nil {
+		updatedPost := post.Clone()
+		updatedPost.AddProp(model.PostPropsPreviewedPost, previewPost.PostID)
+		updatedPost, err := a.Srv().Store.Post().Update(updatedPost, post)
+		return updatedPost, err
+	}
+	return post, nil
+}
+
 func (a *App) attachFilesToPost(post *model.Post) *model.AppError {
 	var attachedIds []string
 	for _, fileID := range post.FileIds {
@@ -671,15 +692,79 @@ func (a *App) UpdatePost(c *request.Context, post *model.Post, safeUpdate bool)
 	// individually.
 	rpost.IsFollowing = nil
 
+	rpost, nErr = a.addPostPreviewProp(rpost)
+	if nErr != nil {
+		return nil, model.NewAppError("UpdatePost", "app.post.update.app_error", nil, nErr.Error(), http.StatusInternalServerError)
+	}
+
 	message := model.NewWebSocketEvent(model.WebsocketEventPostEdited, "", rpost.ChannelId, "", nil)
 	message.Add("post", rpost.ToJson())
+
+	published, err := a.publishWebsocketEventForPermalinkPost(rpost, message)
+	if err != nil {
+		return nil, err
+	}
+	if !published {
 		a.Publish(message)
+	}
 
 	a.invalidateCacheForChannelPosts(rpost.ChannelId)
 
 	return rpost, nil
 }
 
+func (a *App) publishWebsocketEventForPermalinkPost(post *model.Post, message *model.WebSocketEvent) (published bool, err *model.AppError) {
+	var previewedPostID string
+	if val, ok := post.GetProp(model.PostPropsPreviewedPost).(string); ok {
+		previewedPostID = val
+	} else {
+		return false, nil
+	}
+
+	if !model.IsValidId(previewedPostID) {
+		mlog.Warn("invalid post prop value", mlog.String("prop_key", model.PostPropsPreviewedPost), mlog.String("prop_value", previewedPostID))
+		return false, nil
+	}
+
+	previewedPost, err := a.GetSinglePost(previewedPostID)
+	if err != nil {
+		if err.StatusCode == http.StatusNotFound {
+			mlog.Warn("permalinked post not found", mlog.String("referenced_post_id", previewedPostID))
+			return false, nil
+		}
+		return false, err
+	}
+
+	previewedChannel, err := a.GetChannel(previewedPost.ChannelId)
+	if err != nil {
+		if err.StatusCode == http.StatusNotFound {
+			mlog.Warn("channel containing permalinked post not found", mlog.String("referenced_channel_id", previewedPost.ChannelId))
+			return false, nil
+		}
+		return false, err
+	}
+
+	channelMembers, err := a.GetChannelMembersPage(post.ChannelId, 0, 10000000)
+	if err != nil {
+		return false, err
+	}
+
+	for _, cm := range *channelMembers {
+		postForUser := post.Clone()
+		if !a.HasPermissionToReadChannel(cm.UserId, previewedChannel) {
+			postForUser.Metadata.Embeds[0].Data = nil
+		}
+		messageCopy := message.Copy()
+		broadcastCopy := messageCopy.GetBroadcast()
+		broadcastCopy.UserId = cm.UserId
+		messageCopy.SetBroadcast(broadcastCopy)
+		messageCopy.Add("post", postForUser.ToJson())
+		a.Publish(messageCopy)
+	}
+
+	return true, nil
+}
+
 func (a *App) PatchPost(c *request.Context, postID string, patch *model.PostPatch) (*model.Post, *model.AppError) {
 	post, err := a.GetSinglePost(postID)
 	if err != nil {
@@ -1063,15 +1148,13 @@ func (a *App) DeletePost(postID, deleteByID string) (*model.Post, *model.AppErro
 		}
 	}
 
-	postData := a.PreparePostForClient(post, false, false).ToJson()
-
 	userMessage := model.NewWebSocketEvent(model.WebsocketEventPostDeleted, "", post.ChannelId, "", nil)
-	userMessage.Add("post", postData)
+	userMessage.Add("post", post)
 	userMessage.GetBroadcast().ContainsSanitizedData = true
 	a.Publish(userMessage)
 
 	adminMessage := model.NewWebSocketEvent(model.WebsocketEventPostDeleted, "", post.ChannelId, "", nil)
-	adminMessage.Add("post", postData)
+	adminMessage.Add("post", post)
 	adminMessage.Add("delete_by", deleteByID)
 	adminMessage.GetBroadcast().ContainsSensitiveData = true
 	a.Publish(adminMessage)

app/post_metadata.go

@@ -5,11 +5,13 @@ package app
 
 import (
 	"bytes"
+	"fmt"
 	"image"
 	"io"
 	"io/ioutil"
 	"net/http"
 	"net/url"
+	"regexp"
 	"strconv"
 	"strings"
 	"time"
@@ -26,6 +28,7 @@ import (
 type linkMetadataCache struct {
 	OpenGraph *opengraph.OpenGraph
 	PostImage *model.PostImage
+	Permalink *model.Permalink
 }
 
 const LinkCacheSize = 10000
@@ -138,6 +141,41 @@ func (a *App) PreparePostForClient(originalPost *model.Post, isNewPost bool, isE
 	return post
 }
 
+func (a *App) SanitizePostMetadataForUser(post *model.Post, userID string) (*model.Post, *model.AppError) {
+	if post.Metadata == nil || len(post.Metadata.Embeds) == 0 {
+		return post, nil
+	}
+
+	previewPost := post.GetPreviewPost()
+	if previewPost == nil {
+		return post, nil
+	}
+
+	previewedChannel, err := a.GetChannel(previewPost.Post.ChannelId)
+	if err != nil {
+		return nil, err
+	}
+
+	if previewedChannel != nil && !a.HasPermissionToReadChannel(userID, previewedChannel) {
+		post = post.Clone()
+		post.Metadata.Embeds[0].Data = nil
+	}
+
+	return post, nil
+}
+
+func (a *App) SanitizePostListMetadataForUser(postList *model.PostList, userID string) (*model.PostList, *model.AppError) {
+	clonedPostList := postList.Clone()
+	for postID, post := range clonedPostList.Posts {
+		sanitizedPost, err := a.SanitizePostMetadataForUser(post, userID)
+		if err != nil {
+			return nil, err
+		}
+		clonedPostList.Posts[postID] = sanitizedPost
+	}
+	return clonedPostList, nil
+}
+
 func (a *App) getFileMetadataForPost(post *model.Post, fromMaster bool) ([]*model.FileInfo, *model.AppError) {
 	if len(post.FileIds) == 0 {
 		return nil, nil
@@ -171,15 +209,24 @@ func (a *App) getEmbedForPost(post *model.Post, firstLink string, isNewPost bool
 		}, nil
 	}
 
-	if firstLink == "" || !*a.Config().ServiceSettings.EnableLinkPreviews {
+	if firstLink == "" {
 		return nil, nil
 	}
 
-	og, image, err := a.getLinkMetadata(firstLink, post.CreateAt, isNewPost)
+	// Permalink previews are not toggled via the ServiceSettings.EnableLinkPreviews config setting.
+	if !*a.Config().ServiceSettings.EnableLinkPreviews && !looksLikeAPermalink(firstLink, *a.Config().ServiceSettings.SiteURL) {
+		return nil, nil
+	}
+
+	og, image, permalink, err := a.getLinkMetadata(firstLink, post.CreateAt, isNewPost)
 	if err != nil {
 		return nil, err
 	}
 
+	if !*a.Config().ServiceSettings.EnablePermalinkPreviews || !a.Config().FeatureFlags.PermalinkPreviews {
+		permalink = nil
+	}
+
 	if og != nil {
 		return &model.PostEmbed{
 			Type: model.PostEmbedOpengraph,
@@ -196,6 +243,10 @@ func (a *App) getEmbedForPost(post *model.Post, firstLink string, isNewPost bool
 		}, nil
 	}
 
+	if permalink != nil {
+		return &model.PostEmbed{Type: model.PostEmbedPermalink, Data: permalink.PreviewPost}, nil
+	}
+
 	return &model.PostEmbed{
 		Type: model.PostEmbedLink,
 		URL:  firstLink,
@@ -238,7 +289,7 @@ func (a *App) getImagesForPost(post *model.Post, imageURLs []string, isNewPost b
 	}
 
 	for _, imageURL := range imageURLs {
-		if _, image, err := a.getLinkMetadata(imageURL, post.CreateAt, isNewPost); err != nil {
+		if _, image, _, err := a.getLinkMetadata(imageURL, post.CreateAt, isNewPost); err != nil {
 			mlog.Debug("Failed to get dimensions of an image in a post",
 				mlog.String("post_id", post.Id), mlog.String("image_url", imageURL), mlog.Err(err))
 		} else if image != nil {
@@ -393,31 +444,70 @@ func (a *App) getImagesInMessageAttachments(post *model.Post) []string {
 	return images
 }
 
-func (a *App) getLinkMetadata(requestURL string, timestamp int64, isNewPost bool) (*opengraph.OpenGraph, *model.PostImage, error) {
+func looksLikeAPermalink(url, siteURL string) bool {
+	expression := fmt.Sprintf(`^(%s).*(/pl/)[a-z0-9]{26}$`, siteURL)
+	matched, err := regexp.MatchString(expression, strings.TrimSpace(url))
+	if err != nil {
+		mlog.Warn("error matching regex", mlog.Err(err))
+	}
+	return matched
+}
+
+func (a *App) getLinkMetadata(requestURL string, timestamp int64, isNewPost bool) (*opengraph.OpenGraph, *model.PostImage, *model.Permalink, error) {
 	requestURL = resolveMetadataURL(requestURL, a.GetSiteURL())
 
 	timestamp = model.FloorToNearestHour(timestamp)
 
 	// Check cache
-	og, image, ok := getLinkMetadataFromCache(requestURL, timestamp)
+	og, image, permalink, ok := getLinkMetadataFromCache(requestURL, timestamp)
+	if !*a.Config().ServiceSettings.EnablePermalinkPreviews || !a.Config().FeatureFlags.PermalinkPreviews {
+		permalink = nil
+	}
+
 	if ok {
-		return og, image, nil
+		return og, image, permalink, nil
 	}
 
 	// Check the database if this isn't a new post. If it is a new post and the data is cached, it should be in memory.
 	if !isNewPost {
 		og, image, ok = a.getLinkMetadataFromDatabase(requestURL, timestamp)
 		if ok {
-			cacheLinkMetadata(requestURL, timestamp, og, image)
+			cacheLinkMetadata(requestURL, timestamp, og, image, nil)
-
+			return og, image, nil, nil
-			return og, image, nil
 		}
 	}
 
+	var err error
+
+	if looksLikeAPermalink(requestURL, a.GetSiteURL()) && *a.Config().ServiceSettings.EnablePermalinkPreviews && a.Config().FeatureFlags.PermalinkPreviews {
+		referencedPostID := requestURL[len(requestURL)-26:]
+
+		referencedPost, appErr := a.GetSinglePost(referencedPostID)
+		// Ignore 'not found' errors; post could have been deleted via retention policy so we don't want to permanently log a warning.
+		//
+		// TODO: Look into saving a value in the LinkMetadat.Data field to prevent perpetually re-querying for the deleted post.
+		if appErr != nil && appErr.StatusCode != http.StatusNotFound {
+			return nil, nil, nil, appErr
+		}
+
+		referencedChannel, appErr := a.GetChannel(referencedPost.ChannelId)
+		if appErr != nil {
+			return nil, nil, nil, appErr
+		}
+
+		referencedTeam, appErr := a.GetTeam(referencedChannel.TeamId)
+		if appErr != nil {
+			return nil, nil, nil, appErr
+		}
+
+		permalink = &model.Permalink{PreviewPost: model.NewPreviewPost(referencedPost, referencedTeam, referencedChannel)}
+	} else {
+
+		var request *http.Request
 		// Make request for a web page or an image
-	request, err := http.NewRequest("GET", requestURL, nil)
+		request, err = http.NewRequest("GET", requestURL, nil)
 		if err != nil {
-		return nil, nil, err
+			return nil, nil, nil, err
 		}
 
 		var body io.ReadCloser
@@ -455,12 +545,13 @@ func (a *App) getLinkMetadata(requestURL string, timestamp int64, isNewPost bool
 		}
 		og = model.TruncateOpenGraph(og) // remove unwanted length of texts
 
-	// Write back to cache and database, even if there was an error and the results are nil
-	cacheLinkMetadata(requestURL, timestamp, og, image)
-
 		a.saveLinkMetadataToDatabase(requestURL, timestamp, og, image)
+	}
 
-	return og, image, err
+	// Write back to cache and database, even if there was an error and the results are nil
+	cacheLinkMetadata(requestURL, timestamp, og, image, permalink)
+
+	return og, image, permalink, err
 }
 
 // resolveMetadataURL resolves a given URL relative to the server's site URL.
@@ -478,14 +569,14 @@ func resolveMetadataURL(requestURL string, siteURL string) string {
 	return resolved.String()
 }
 
-func getLinkMetadataFromCache(requestURL string, timestamp int64) (*opengraph.OpenGraph, *model.PostImage, bool) {
+func getLinkMetadataFromCache(requestURL string, timestamp int64) (*opengraph.OpenGraph, *model.PostImage, *model.Permalink, bool) {
 	var cached linkMetadataCache
 	err := linkCache.Get(strconv.FormatInt(model.GenerateLinkMetadataHash(requestURL, timestamp), 16), &cached)
 	if err != nil {
-		return nil, nil, false
+		return nil, nil, nil, false
 	}
 
-	return cached.OpenGraph, cached.PostImage, true
+	return cached.OpenGraph, cached.PostImage, cached.Permalink, true
 }
 
 func (a *App) getLinkMetadataFromDatabase(requestURL string, timestamp int64) (*opengraph.OpenGraph, *model.PostImage, bool) {
@@ -528,10 +619,11 @@ func (a *App) saveLinkMetadataToDatabase(requestURL string, timestamp int64, og
 	}
 }
 
-func cacheLinkMetadata(requestURL string, timestamp int64, og *opengraph.OpenGraph, image *model.PostImage) {
+func cacheLinkMetadata(requestURL string, timestamp int64, og *opengraph.OpenGraph, image *model.PostImage, permalink *model.Permalink) {
 	metadata := linkMetadataCache{
 		OpenGraph: og,
 		PostImage: image,
+		Permalink: permalink,
 	}
 
 	linkCache.SetWithExpiry(strconv.FormatInt(model.GenerateLinkMetadataHash(requestURL, timestamp), 16), metadata, LinkCacheDuration)

app/post_metadata_test.go

@@ -550,6 +550,38 @@ func TestPreparePostForClient(t *testing.T) {
 		assert.Nil(t, clientPost.Metadata.Reactions, "should not have populated Reactions")
 		assert.Nil(t, clientPost.Metadata.Files, "should not have populated Files")
 	})
+
+	t.Run("permalink preview", func(t *testing.T) {
+		th := setup(t)
+		defer th.TearDown()
+
+		th.App.UpdateConfig(func(cfg *model.Config) {
+			*cfg.ServiceSettings.SiteURL = "http://mymattermost.com"
+		})
+
+		th.Context.Session().UserId = th.BasicUser.Id
+
+		referencedPost, err := th.App.CreatePost(th.Context, &model.Post{
+			UserId:    th.BasicUser.Id,
+			ChannelId: th.BasicChannel.Id,
+			Message:   "hello world",
+		}, th.BasicChannel, false, true)
+		require.Nil(t, err)
+
+		link := fmt.Sprintf("%s/%s/pl/%s", *th.App.Config().ServiceSettings.SiteURL, th.BasicTeam.Name, referencedPost.Id)
+
+		previewPost, err := th.App.CreatePost(th.Context, &model.Post{
+			UserId:    th.BasicUser.Id,
+			ChannelId: th.BasicChannel.Id,
+			Message:   link,
+		}, th.BasicChannel, false, true)
+		require.Nil(t, err)
+
+		clientPost := th.App.PreparePostForClient(previewPost, false, false)
+		firstEmbed := clientPost.Metadata.Embeds[0]
+		preview := firstEmbed.Data.(*model.PreviewPost)
+		require.Equal(t, referencedPost.Id, preview.PostID)
+	})
 }
 
 func TestPreparePostForClientWithImageProxy(t *testing.T) {
@@ -1688,16 +1720,16 @@ func TestGetLinkMetadata(t *testing.T) {
 		timestamp := int64(1547510400000)
 		title := "from cache"
 
-		cacheLinkMetadata(requestURL, timestamp, &opengraph.OpenGraph{Title: title}, nil)
+		cacheLinkMetadata(requestURL, timestamp, &opengraph.OpenGraph{Title: title}, nil, nil)
 
 		t.Run("should use cache if cached entry exists", func(t *testing.T) {
-			_, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
+			_, _, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
 			require.True(t, ok, "data should already exist in in-memory cache")
 
 			_, _, ok = th.App.getLinkMetadataFromDatabase(requestURL, timestamp)
 			require.False(t, ok, "data should not exist in database")
 
-			og, img, err := th.App.getLinkMetadata(requestURL, timestamp, false)
+			og, img, _, err := th.App.getLinkMetadata(requestURL, timestamp, false)
 
 			require.NotNil(t, og)
 			assert.Nil(t, img)
@@ -1706,13 +1738,13 @@ func TestGetLinkMetadata(t *testing.T) {
 		})
 
 		t.Run("should use cache if cached entry exists near time", func(t *testing.T) {
-			_, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
+			_, _, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
 			require.True(t, ok, "data should already exist in in-memory cache")
 
 			_, _, ok = th.App.getLinkMetadataFromDatabase(requestURL, timestamp)
 			require.False(t, ok, "data should not exist in database")
 
-			og, img, err := th.App.getLinkMetadata(requestURL, timestamp+60*1000, false)
+			og, img, _, err := th.App.getLinkMetadata(requestURL, timestamp+60*1000, false)
 
 			require.NotNil(t, og)
 			assert.Nil(t, img)
@@ -1723,13 +1755,13 @@ func TestGetLinkMetadata(t *testing.T) {
 		t.Run("should not use cache if URL is different", func(t *testing.T) {
 			differentURL := server.URL + "/other"
 
-			_, _, ok := getLinkMetadataFromCache(differentURL, timestamp)
+			_, _, _, ok := getLinkMetadataFromCache(differentURL, timestamp)
 			require.False(t, ok, "data should not exist in in-memory cache")
 
 			_, _, ok = th.App.getLinkMetadataFromDatabase(differentURL, timestamp)
 			require.False(t, ok, "data should not exist in database")
 
-			og, img, err := th.App.getLinkMetadata(differentURL, timestamp, false)
+			og, img, _, err := th.App.getLinkMetadata(differentURL, timestamp, false)
 
 			assert.Nil(t, og)
 			assert.Nil(t, img)
@@ -1739,13 +1771,13 @@ func TestGetLinkMetadata(t *testing.T) {
 		t.Run("should not use cache if timestamp is different", func(t *testing.T) {
 			differentTimestamp := timestamp + 60*60*1000
 
-			_, _, ok := getLinkMetadataFromCache(requestURL, differentTimestamp)
+			_, _, _, ok := getLinkMetadataFromCache(requestURL, differentTimestamp)
 			require.False(t, ok, "data should not exist in in-memory cache")
 
 			_, _, ok = th.App.getLinkMetadataFromDatabase(requestURL, differentTimestamp)
 			require.False(t, ok, "data should not exist in database")
 
-			og, img, err := th.App.getLinkMetadata(requestURL, differentTimestamp, false)
+			og, img, _, err := th.App.getLinkMetadata(requestURL, differentTimestamp, false)
 
 			assert.Nil(t, og)
 			assert.Nil(t, img)
@@ -1766,13 +1798,13 @@ func TestGetLinkMetadata(t *testing.T) {
 		t.Run("should use database if saved entry exists", func(t *testing.T) {
 			linkCache.Purge()
 
-			_, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
+			_, _, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
 			require.False(t, ok, "data should not exist in in-memory cache")
 
 			_, _, ok = th.App.getLinkMetadataFromDatabase(requestURL, timestamp)
 			require.True(t, ok, "data should already exist in database")
 
-			og, img, err := th.App.getLinkMetadata(requestURL, timestamp, false)
+			og, img, _, err := th.App.getLinkMetadata(requestURL, timestamp, false)
 
 			require.NotNil(t, og)
 			assert.Nil(t, img)
@@ -1783,13 +1815,13 @@ func TestGetLinkMetadata(t *testing.T) {
 		t.Run("should use database if saved entry exists near time", func(t *testing.T) {
 			linkCache.Purge()
 
-			_, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
+			_, _, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
 			require.False(t, ok, "data should not exist in in-memory cache")
 
 			_, _, ok = th.App.getLinkMetadataFromDatabase(requestURL, timestamp)
 			require.True(t, ok, "data should already exist in database")
 
-			og, img, err := th.App.getLinkMetadata(requestURL, timestamp+60*1000, false)
+			og, img, _, err := th.App.getLinkMetadata(requestURL, timestamp+60*1000, false)
 
 			require.NotNil(t, og)
 			assert.Nil(t, img)
@@ -1802,13 +1834,13 @@ func TestGetLinkMetadata(t *testing.T) {
 
 			differentURL := requestURL + "/other"
 
-			_, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
+			_, _, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
 			require.False(t, ok, "data should not exist in in-memory cache")
 
 			_, _, ok = th.App.getLinkMetadataFromDatabase(differentURL, timestamp)
 			require.False(t, ok, "data should not exist in database")
 
-			og, img, err := th.App.getLinkMetadata(differentURL, timestamp, false)
+			og, img, _, err := th.App.getLinkMetadata(differentURL, timestamp, false)
 
 			assert.Nil(t, og)
 			assert.Nil(t, img)
@@ -1820,13 +1852,13 @@ func TestGetLinkMetadata(t *testing.T) {
 
 			differentTimestamp := timestamp + 60*60*1000
 
-			_, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
+			_, _, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
 			require.False(t, ok, "data should not exist in in-memory cache")
 
 			_, _, ok = th.App.getLinkMetadataFromDatabase(requestURL, differentTimestamp)
 			require.False(t, ok, "data should not exist in database")
 
-			og, img, err := th.App.getLinkMetadata(requestURL, differentTimestamp, false)
+			og, img, _, err := th.App.getLinkMetadata(requestURL, differentTimestamp, false)
 
 			assert.Nil(t, og)
 			assert.Nil(t, img)
@@ -1841,13 +1873,13 @@ func TestGetLinkMetadata(t *testing.T) {
 		requestURL := server.URL + "/opengraph?title=Remote&name=" + t.Name()
 		timestamp := int64(1547510400000)
 
-		_, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
+		_, _, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
 		require.False(t, ok, "data should not exist in in-memory cache")
 
 		_, _, ok = th.App.getLinkMetadataFromDatabase(requestURL, timestamp)
 		require.False(t, ok, "data should not exist in database")
 
-		og, img, err := th.App.getLinkMetadata(requestURL, timestamp, false)
+		og, img, _, err := th.App.getLinkMetadata(requestURL, timestamp, false)
 
 		assert.NotNil(t, og)
 		assert.Nil(t, img)
@@ -1861,19 +1893,19 @@ func TestGetLinkMetadata(t *testing.T) {
 		requestURL := server.URL + "/opengraph?title=Remote&name=" + t.Name()
 		timestamp := int64(1547510400000)
 
-		_, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
+		_, _, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
 		require.False(t, ok, "data should not exist in in-memory cache")
 
 		_, _, ok = th.App.getLinkMetadataFromDatabase(requestURL, timestamp)
 		require.False(t, ok, "data should not exist in database")
 
-		og, img, err := th.App.getLinkMetadata(requestURL, timestamp, false)
+		og, img, _, err := th.App.getLinkMetadata(requestURL, timestamp, false)
 
 		assert.NotNil(t, og)
 		assert.Nil(t, img)
 		assert.NoError(t, err)
 
-		fromCache, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
+		fromCache, _, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
 		assert.True(t, ok)
 		assert.Exactly(t, og, fromCache)
 
@@ -1889,19 +1921,19 @@ func TestGetLinkMetadata(t *testing.T) {
 		requestURL := server.URL + "/image?height=300&width=400&name=" + t.Name()
 		timestamp := int64(1547510400000)
 
-		_, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
+		_, _, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
 		require.False(t, ok, "data should not exist in in-memory cache")
 
 		_, _, ok = th.App.getLinkMetadataFromDatabase(requestURL, timestamp)
 		require.False(t, ok, "data should not exist in database")
 
-		og, img, err := th.App.getLinkMetadata(requestURL, timestamp, false)
+		og, img, _, err := th.App.getLinkMetadata(requestURL, timestamp, false)
 
 		assert.Nil(t, og)
 		assert.NotNil(t, img)
 		assert.NoError(t, err)
 
-		_, fromCache, ok := getLinkMetadataFromCache(requestURL, timestamp)
+		_, fromCache, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
 		assert.True(t, ok)
 		assert.Exactly(t, img, fromCache)
 
@@ -1917,19 +1949,19 @@ func TestGetLinkMetadata(t *testing.T) {
 		requestURL := server.URL + "/error"
 		timestamp := int64(1547510400000)
 
-		_, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
+		_, _, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
 		require.False(t, ok, "data should not exist in in-memory cache")
 
 		_, _, ok = th.App.getLinkMetadataFromDatabase(requestURL, timestamp)
 		require.False(t, ok, "data should not exist in database")
 
-		og, img, err := th.App.getLinkMetadata(requestURL, timestamp, false)
+		og, img, _, err := th.App.getLinkMetadata(requestURL, timestamp, false)
 
 		assert.Nil(t, og)
 		assert.Nil(t, img)
 		assert.NoError(t, err)
 
-		ogFromCache, imgFromCache, ok := getLinkMetadataFromCache(requestURL, timestamp)
+		ogFromCache, imgFromCache, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
 		assert.True(t, ok)
 		assert.Nil(t, ogFromCache)
 		assert.Nil(t, imgFromCache)
@@ -1947,19 +1979,19 @@ func TestGetLinkMetadata(t *testing.T) {
 		requestURL := "http://notarealdomainthatactuallyexists.ca/?name=" + t.Name()
 		timestamp := int64(1547510400000)
 
-		_, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
+		_, _, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
 		require.False(t, ok, "data should not exist in in-memory cache")
 
 		_, _, ok = th.App.getLinkMetadataFromDatabase(requestURL, timestamp)
 		require.False(t, ok, "data should not exist in database")
 
-		og, img, err := th.App.getLinkMetadata(requestURL, timestamp, false)
+		og, img, _, err := th.App.getLinkMetadata(requestURL, timestamp, false)
 
 		assert.Nil(t, og)
 		assert.Nil(t, img)
 		assert.IsType(t, &url.Error{}, err)
 
-		ogFromCache, imgFromCache, ok := getLinkMetadataFromCache(requestURL, timestamp)
+		ogFromCache, imgFromCache, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
 		assert.True(t, ok)
 		assert.Nil(t, ogFromCache)
 		assert.Nil(t, imgFromCache)
@@ -1981,20 +2013,20 @@ func TestGetLinkMetadata(t *testing.T) {
 		requestURL := server.URL + "/timeout?name=" + t.Name()
 		timestamp := int64(1547510400000)
 
-		_, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
+		_, _, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
 		require.False(t, ok, "data should not exist in in-memory cache")
 
 		_, _, ok = th.App.getLinkMetadataFromDatabase(requestURL, timestamp)
 		require.False(t, ok, "data should not exist in database")
 
-		og, img, err := th.App.getLinkMetadata(requestURL, timestamp, false)
+		og, img, _, err := th.App.getLinkMetadata(requestURL, timestamp, false)
 
 		assert.Nil(t, og)
 		assert.Nil(t, img)
 		assert.Error(t, err)
 		assert.True(t, os.IsTimeout(err))
 
-		ogFromCache, imgFromCache, ok := getLinkMetadataFromCache(requestURL, timestamp)
+		ogFromCache, imgFromCache, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
 		assert.True(t, ok)
 		assert.Nil(t, ogFromCache)
 		assert.Nil(t, imgFromCache)
@@ -2012,20 +2044,20 @@ func TestGetLinkMetadata(t *testing.T) {
 		requestURL := server.URL + "/image?height=300&width=400&name=" + t.Name()
 		timestamp := int64(1547510400000)
 
-		_, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
+		_, _, _, ok := getLinkMetadataFromCache(requestURL, timestamp)
 		require.False(t, ok, "data should not exist in in-memory cache")
 
 		_, _, ok = th.App.getLinkMetadataFromDatabase(requestURL, timestamp)
 		require.False(t, ok, "data should not exist in database")
 
-		_, img, err := th.App.getLinkMetadata(requestURL, timestamp, false)
+		_, img, _, err := th.App.getLinkMetadata(requestURL, timestamp, false)
 		require.NoError(t, err)
 
-		_, _, ok = getLinkMetadataFromCache(requestURL, timestamp)
+		_, _, _, ok = getLinkMetadataFromCache(requestURL, timestamp)
 		require.True(t, ok, "data should now exist in in-memory cache")
 
 		linkCache.Purge()
-		_, _, ok = getLinkMetadataFromCache(requestURL, timestamp)
+		_, _, _, ok = getLinkMetadataFromCache(requestURL, timestamp)
 		require.False(t, ok, "data should no longer exist in in-memory cache")
 
 		_, fromDatabase, ok := th.App.getLinkMetadataFromDatabase(requestURL, timestamp)
@@ -2040,7 +2072,7 @@ func TestGetLinkMetadata(t *testing.T) {
 		requestURL := server.URL + "/json?name=" + t.Name()
 		timestamp := int64(1547510400000)
 
-		og, img, err := th.App.getLinkMetadata(requestURL, timestamp, false)
+		og, img, _, err := th.App.getLinkMetadata(requestURL, timestamp, false)
 		assert.Nil(t, og)
 		assert.Nil(t, img)
 		assert.NoError(t, err)
@@ -2053,9 +2085,9 @@ func TestGetLinkMetadata(t *testing.T) {
 		requestURL := server.URL + "/error?name=" + t.Name()
 		timestamp := int64(1547510400000)
 
-		cacheLinkMetadata(requestURL, timestamp, &opengraph.OpenGraph{Title: "cached"}, nil)
+		cacheLinkMetadata(requestURL, timestamp, &opengraph.OpenGraph{Title: "cached"}, nil, nil)
 
-		og, img, err := th.App.getLinkMetadata(requestURL, timestamp, true)
+		og, img, _, err := th.App.getLinkMetadata(requestURL, timestamp, true)
 		assert.NotNil(t, og)
 		assert.Nil(t, img)
 		assert.NoError(t, err)
@@ -2070,7 +2102,7 @@ func TestGetLinkMetadata(t *testing.T) {
 
 		th.App.saveLinkMetadataToDatabase(requestURL, timestamp, &opengraph.OpenGraph{Title: "cached"}, nil)
 
-		og, img, err := th.App.getLinkMetadata(requestURL, timestamp, true)
+		og, img, _, err := th.App.getLinkMetadata(requestURL, timestamp, true)
 		assert.Nil(t, og)
 		assert.Nil(t, img)
 		assert.NoError(t, err)
@@ -2093,7 +2125,7 @@ func TestGetLinkMetadata(t *testing.T) {
 		requestURL := "/image?height=200&width=300&name=" + t.Name()
 		timestamp := int64(1547510400000)
 
-		og, img, err := th.App.getLinkMetadata(requestURL, timestamp, false)
+		og, img, _, err := th.App.getLinkMetadata(requestURL, timestamp, false)
 		assert.Nil(t, og)
 		assert.NotNil(t, img)
 		assert.NoError(t, err)
@@ -2121,7 +2153,7 @@ func TestGetLinkMetadata(t *testing.T) {
 		requestURL := server.URL + "/image?height=200&width=300&name=" + t.Name()
 		timestamp := int64(1547510400000)
 
-		og, img, err := th.App.getLinkMetadata(requestURL, timestamp, false)
+		og, img, _, err := th.App.getLinkMetadata(requestURL, timestamp, false)
 		assert.Nil(t, og)
 		assert.Nil(t, img)
 		assert.Error(t, err)
@@ -2131,7 +2163,7 @@ func TestGetLinkMetadata(t *testing.T) {
 		requestURL = th.App.GetSiteURL() + "/api/v4/image?url=" + url.QueryEscape(requestURL)
 
 		// Note that this request still fails while testing because the request made by the image proxy is blocked
-		og, img, err = th.App.getLinkMetadata(requestURL, timestamp, false)
+		og, img, _, err = th.App.getLinkMetadata(requestURL, timestamp, false)
 		assert.Nil(t, og)
 		assert.Nil(t, img)
 		assert.Error(t, err)
@@ -2145,7 +2177,7 @@ func TestGetLinkMetadata(t *testing.T) {
 		requestURL := server.URL + "/mixed?name=" + t.Name()
 		timestamp := int64(1547510400000)
 
-		og, img, err := th.App.getLinkMetadata(requestURL, timestamp, true)
+		og, img, _, err := th.App.getLinkMetadata(requestURL, timestamp, true)
 		assert.Nil(t, og)
 		assert.NotNil(t, img)
 		assert.NoError(t, err)
@@ -2323,3 +2355,33 @@ func TestParseImages(t *testing.T) {
 		})
 	}
 }
+
+func TestLooksLikeAPermalink(t *testing.T) {
+	const siteURLWithSubpath = "http://localhost:8065/foo"
+	const siteURLWithTrailingSlash = "http://test.com/"
+	const siteURL = "http://test.com"
+	tests := map[string]struct {
+		input   string
+		siteURL string
+		expect  bool
+	}{
+		"happy path":                       {input: fmt.Sprintf("%s/private-core/pl/dppezk51jp8afbhwxf1jpag66r", siteURLWithSubpath), siteURL: siteURLWithSubpath, expect: true},
+		"looks nothing like a permalink":   {input: "foobar", siteURL: siteURLWithSubpath, expect: false},
+		"link has no subpath":              {input: fmt.Sprintf("%s/private-core/pl/dppezk51jp8afbhwxf1jpag66r", "http://localhost:8065"), siteURL: siteURLWithSubpath, expect: false},
+		"without port":                     {input: fmt.Sprintf("%s/private-core/pl/dppezk51jp8afbhwxf1jpag66r", "http://localhost/foo"), siteURL: siteURLWithSubpath, expect: false},
+		"wrong port":                       {input: fmt.Sprintf("%s/private-core/pl/dppezk51jp8afbhwxf1jpag66r", "http://localhost:8066"), siteURL: siteURLWithSubpath, expect: false},
+		"invalid post ID length":           {input: fmt.Sprintf("%s/private-core/pl/dppezk51jp8afbhwxf1jpag66", siteURLWithSubpath), siteURL: siteURLWithSubpath, expect: false},
+		"invalid post ID character":        {input: fmt.Sprintf("%s/private-core/pl/dppezk51jp8$fbhwxf1jpag66r", siteURLWithSubpath), siteURL: siteURLWithSubpath, expect: false},
+		"leading whitespace":               {input: fmt.Sprintf(" %s/private-core/pl/dppezk51jp8afbhwxf1jpag66r", siteURLWithSubpath), siteURL: siteURLWithSubpath, expect: true},
+		"trailing whitespace":              {input: fmt.Sprintf("%s/private-core/pl/dppezk51jp8afbhwxf1jpag66r ", siteURLWithSubpath), siteURL: siteURLWithSubpath, expect: true},
+		"siteURL without a subpath":        {input: fmt.Sprintf("%sprivate-core/pl/dppezk51jp8afbhwxf1jpag66r", siteURLWithTrailingSlash), siteURL: siteURLWithTrailingSlash, expect: true},
+		"siteURL without a trailing slash": {input: fmt.Sprintf("%s/private-core/pl/dppezk51jp8afbhwxf1jpag66r", siteURL), siteURL: siteURL, expect: true},
+	}
+
+	for name, tc := range tests {
+		t.Run(name, func(t *testing.T) {
+			actual := looksLikeAPermalink(tc.input, tc.siteURL)
+			assert.Equal(t, tc.expect, actual)
+		})
+	}
+}

app/post_test.go

@@ -651,7 +651,7 @@ func TestDeletePostWithFileAttachments(t *testing.T) {
 	assert.Nil(t, err)
 
 	// Delete the post.
-	post, err = th.App.DeletePost(post.Id, userID)
+	_, err = th.App.DeletePost(post.Id, userID)
 	assert.Nil(t, err)
 
 	// Wait for the cleanup routine to finish.
@@ -755,6 +755,41 @@ func TestCreatePost(t *testing.T) {
 			th.AddPermissionToRole(model.PermissionUseChannelMentions.Id, model.ChannelAdminRoleId)
 		})
 	})
+
+	t.Run("Sets PostPropsPreviewedPost when a permalink is the first link", func(t *testing.T) {
+		th := Setup(t).InitBasic()
+		defer th.TearDown()
+		th.AddUserToChannel(th.BasicUser, th.BasicChannel)
+
+		referencedPost := &model.Post{
+			ChannelId: th.BasicChannel.Id,
+			Message:   "hello world",
+			UserId:    th.BasicUser.Id,
+		}
+
+		th.App.UpdateConfig(func(cfg *model.Config) {
+			*cfg.ServiceSettings.SiteURL = "http://mymattermost.com"
+		})
+
+		th.Context.Session().UserId = th.BasicUser.Id
+
+		referencedPost, err := th.App.CreatePost(th.Context, referencedPost, th.BasicChannel, false, false)
+		require.Nil(t, err)
+
+		permalink := fmt.Sprintf("%s/%s/pl/%s", *th.App.Config().ServiceSettings.SiteURL, th.BasicTeam.Name, referencedPost.Id)
+
+		channelForPreview := th.CreateChannel(th.BasicTeam)
+		previewPost := &model.Post{
+			ChannelId: channelForPreview.Id,
+			Message:   permalink,
+			UserId:    th.BasicUser.Id,
+		}
+
+		previewPost, err = th.App.CreatePost(th.Context, previewPost, channelForPreview, false, false)
+		require.Nil(t, err)
+
+		assert.Equal(t, previewPost.GetProps(), model.StringInterface{"previewed_post": referencedPost.Id})
+	})
 }
 
 func TestPatchPost(t *testing.T) {
@@ -1085,6 +1120,45 @@ func TestUpdatePost(t *testing.T) {
 		require.Nil(t, err)
 		assert.Equal(t, "![image]("+proxiedImageURL+")", rpost.Message)
 	})
+
+	t.Run("Sets PostPropsPreviewedPost when a post is updated to have a permalink as the first link", func(t *testing.T) {
+		th := Setup(t).InitBasic()
+		defer th.TearDown()
+		th.AddUserToChannel(th.BasicUser, th.BasicChannel)
+
+		referencedPost := &model.Post{
+			ChannelId: th.BasicChannel.Id,
+			Message:   "hello world",
+			UserId:    th.BasicUser.Id,
+		}
+
+		th.App.UpdateConfig(func(cfg *model.Config) {
+			*cfg.ServiceSettings.SiteURL = "http://mymattermost.com"
+		})
+
+		th.Context.Session().UserId = th.BasicUser.Id
+
+		referencedPost, err := th.App.CreatePost(th.Context, referencedPost, th.BasicChannel, false, false)
+		require.Nil(t, err)
+
+		permalink := fmt.Sprintf("%s/%s/pl/%s", *th.App.Config().ServiceSettings.SiteURL, th.BasicTeam.Name, referencedPost.Id)
+
+		channelForTestPost := th.CreateChannel(th.BasicTeam)
+		testPost := &model.Post{
+			ChannelId: channelForTestPost.Id,
+			Message:   "hello world",
+			UserId:    th.BasicUser.Id,
+		}
+
+		testPost, err = th.App.CreatePost(th.Context, testPost, channelForTestPost, false, false)
+		require.Nil(t, err)
+		assert.Equal(t, testPost.GetProps(), model.StringInterface{})
+
+		testPost.Message = permalink
+		testPost, err = th.App.UpdatePost(th.Context, testPost, false)
+		require.Nil(t, err)
+		assert.Equal(t, testPost.GetProps(), model.StringInterface{"previewed_post": referencedPost.Id})
+	})
 }
 
 func TestSearchPostsInTeamForUser(t *testing.T) {

app/ratelimit.go

@@ -86,7 +86,7 @@ func (rl *RateLimiter) RateLimitWriter(key string, w http.ResponseWriter) bool {
 
 	if limited {
 		mlog.Debug("Denied due to throttling settings code=429", mlog.String("key", key))
-		http.Error(w, "limit exceeded", 429)
+		http.Error(w, "limit exceeded", http.StatusTooManyRequests)
 	}
 
 	return limited

app/role_test.go

@@ -221,7 +221,7 @@ func testPermissionInheritance(t *testing.T, testCallback func(t *testing.T, th
 
 	// assign the scheme to the team
 	team.SchemeId = &teamScheme.Id
-	team, err = th.App.UpdateTeamScheme(team)
+	_, err = th.App.UpdateTeamScheme(team)
 	require.Nil(t, err)
 
 	// test 24 combinations where the higher-scoped scheme is a TEAM scheme

app/shared_channel.go

@@ -22,7 +22,7 @@ func (a *App) checkChannelNotShared(channelId string) error {
 	if _, err := a.GetSharedChannel(channelId); err == nil {
 		var errNotFound *store.ErrNotFound
 		if errors.As(err, &errNotFound) {
-			return errors.New("channel is already shared.")
+			return errors.New("channel is already shared")
 		}
 		return fmt.Errorf("cannot find channel: %w", err)
 	}
@@ -33,7 +33,7 @@ func (a *App) checkChannelIsShared(channelId string) error {
 	if _, err := a.GetSharedChannel(channelId); err != nil {
 		var errNotFound *store.ErrNotFound
 		if errors.As(err, &errNotFound) {
-			return errors.New("channel is not shared.")
+			return errors.New("channel is not shared")
 		}
 		return fmt.Errorf("cannot find channel: %w", err)
 	}
@@ -45,13 +45,13 @@ func (a *App) CheckCanInviteToSharedChannel(channelId string) error {
 	if err != nil {
 		var errNotFound *store.ErrNotFound
 		if errors.As(err, &errNotFound) {
-			return errors.New("channel is not shared.")
+			return errors.New("channel is not shared")
 		}
 		return fmt.Errorf("cannot find channel: %w", err)
 	}
 
 	if !sc.Home {
-		return errors.New("channel is homed on a remote cluster.")
+		return errors.New("channel is homed on a remote cluster")
 	}
 	return nil
 }

app/syncables_test.go

@@ -169,7 +169,7 @@ func TestCreateDefaultMemberships(t *testing.T) {
 
 	// update AutoAdd to true
 	scienceTeamGroupSyncable.AutoAdd = true
-	scienceTeamGroupSyncable, err = th.App.UpdateGroupSyncable(scienceTeamGroupSyncable)
+	_, err = th.App.UpdateGroupSyncable(scienceTeamGroupSyncable)
 	if err != nil {
 		t.Errorf("error updating group syncable: %s", err.Error())
 	}

app/user_test.go

@@ -1162,7 +1162,7 @@ func TestPromoteGuestToUser(t *testing.T) {
 		assert.Nil(t, err)
 		assert.False(t, teamMember.SchemeGuest)
 		assert.True(t, teamMember.SchemeUser)
-		channelMember, err = th.App.GetChannelMember(context.Background(), th.BasicChannel.Id, guest.Id)
+		_, err = th.App.GetChannelMember(context.Background(), th.BasicChannel.Id, guest.Id)
 		assert.Nil(t, err)
 		assert.False(t, teamMember.SchemeGuest)
 		assert.True(t, teamMember.SchemeUser)
@@ -1194,7 +1194,7 @@ func TestPromoteGuestToUser(t *testing.T) {
 		assert.Nil(t, err)
 		assert.False(t, teamMember.SchemeGuest)
 		assert.True(t, teamMember.SchemeUser)
-		channelMember, err = th.App.GetChannelMember(context.Background(), th.BasicChannel.Id, guest.Id)
+		_, err = th.App.GetChannelMember(context.Background(), th.BasicChannel.Id, guest.Id)
 		assert.Nil(t, err)
 		assert.False(t, teamMember.SchemeGuest)
 		assert.True(t, teamMember.SchemeUser)
@@ -1325,7 +1325,7 @@ func TestDemoteUserToGuest(t *testing.T) {
 		assert.Nil(t, err)
 		assert.False(t, teamMember.SchemeUser)
 		assert.True(t, teamMember.SchemeGuest)
-		channelMember, err = th.App.GetChannelMember(context.Background(), th.BasicChannel.Id, user.Id)
+		_, err = th.App.GetChannelMember(context.Background(), th.BasicChannel.Id, user.Id)
 		assert.Nil(t, err)
 		assert.False(t, teamMember.SchemeUser)
 		assert.True(t, teamMember.SchemeGuest)
@@ -1357,7 +1357,7 @@ func TestDemoteUserToGuest(t *testing.T) {
 		assert.Nil(t, err)
 		assert.False(t, teamMember.SchemeUser)
 		assert.True(t, teamMember.SchemeGuest)
-		channelMember, err = th.App.GetChannelMember(context.Background(), th.BasicChannel.Id, user.Id)
+		_, err = th.App.GetChannelMember(context.Background(), th.BasicChannel.Id, user.Id)
 		assert.Nil(t, err)
 		assert.False(t, teamMember.SchemeUser)
 		assert.True(t, teamMember.SchemeGuest)

app/web_conn.go

@@ -126,7 +126,7 @@ func (a *App) PopulateWebConnConfig(s *model.Session, cfg *WebConnConfig, seqVal
 		if seqVal == "" {
 			// Sequence_number must be sent with connection id.
 			// A client must be either non-compliant or fully compliant.
-			return nil, errors.New("Sequence number not present in websocket request")
+			return nil, errors.New("sequence number not present in websocket request")
 		}
 		var err error
 		cfg.sequence, err = strconv.Atoi(seqVal)

config/client.go

@@ -35,6 +35,7 @@ func GenerateClientConfig(c *model.Config, telemetryID string, license *model.Li
 	props["EnablePostIconOverride"] = strconv.FormatBool(*c.ServiceSettings.EnablePostIconOverride)
 	props["EnableUserAccessTokens"] = strconv.FormatBool(*c.ServiceSettings.EnableUserAccessTokens)
 	props["EnableLinkPreviews"] = strconv.FormatBool(*c.ServiceSettings.EnableLinkPreviews)
+	props["EnablePermalinkPreviews"] = strconv.FormatBool(*c.ServiceSettings.EnablePermalinkPreviews)
 	props["EnableTesting"] = strconv.FormatBool(*c.ServiceSettings.EnableTesting)
 	props["EnableDeveloper"] = strconv.FormatBool(*c.ServiceSettings.EnableDeveloper)
 	props["PostEditTimeLimit"] = fmt.Sprintf("%v", *c.ServiceSettings.PostEditTimeLimit)

model/config.go

@@ -303,6 +303,7 @@ type ServiceSettings struct {
 	GoogleDeveloperKey                                *string  `access:"site_posts,write_restrictable,cloud_restrictable"`
 	DEPRECATED_DO_NOT_USE_EnableOnlyAdminIntegrations *bool    `json:"EnableOnlyAdminIntegrations" mapstructure:"EnableOnlyAdminIntegrations"` // Deprecated: do not use
 	EnableLinkPreviews                                *bool    `access:"site_posts"`
+	EnablePermalinkPreviews                           *bool    `access:"site_posts"`
 	RestrictLinkPreviews                              *string  `access:"site_posts"`
 	EnableTesting                                     *bool    `access:"environment_developer,write_restrictable,cloud_restrictable"`
 	EnableDeveloper                                   *bool    `access:"environment_developer,write_restrictable,cloud_restrictable"`
@@ -413,6 +414,10 @@ func (s *ServiceSettings) SetDefaults(isUpdate bool) {
 		s.EnableLinkPreviews = NewBool(true)
 	}
 
+	if s.EnablePermalinkPreviews == nil {
+		s.EnablePermalinkPreviews = NewBool(true)
+	}
+
 	if s.RestrictLinkPreviews == nil {
 		s.RestrictLinkPreviews = NewString("")
 	}

model/feature_flags.go

@@ -36,6 +36,8 @@ type FeatureFlags struct {
 	// Enable timed dnd support for user status
 	TimedDND bool
 
+	PermalinkPreviews bool
+
 	// Enable the Global Header
 	GlobalHeader bool
 
@@ -54,6 +56,7 @@ func (f *FeatureFlags) SetDefaults() {
 	f.PluginApps = ""
 	f.PluginFocalboard = ""
 	f.TimedDND = false
+	f.PermalinkPreviews = true
 	f.GlobalHeader = false
 	f.InviteMembersButton = "none"
 }

model/message_export.go

@@ -3,6 +3,8 @@
 
 package model
 
+import "encoding/json"
+
 type MessageExport struct {
 	TeamId          *string
 	TeamName        *string
@@ -34,3 +36,15 @@ type MessageExportCursor struct {
 	LastPostUpdateAt int64
 	LastPostId       string
 }
+
+// PreviewID returns the value of the post's previewed_post prop, if present, or an empty string.
+func (m *MessageExport) PreviewID() string {
+	var previewID string
+	props := map[string]interface{}{}
+	if m.PostProps != nil && json.Unmarshal([]byte(*m.PostProps), &props) == nil {
+		if val, ok := props[PostPropsPreviewedPost]; ok {
+			previewID = val.(string)
+		}
+	}
+	return previewID
+}

model/permalink.go

@@ -0,0 +1,34 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package model
+
+import "encoding/json"
+
+type Permalink struct {
+	PreviewPost *PreviewPost `json:"preview_post"`
+}
+
+type PreviewPost struct {
+	PostID             string `json:"post_id"`
+	Post               *Post  `json:"post"`
+	TeamName           string `json:"team_name"`
+	ChannelDisplayName string `json:"channel_display_name"`
+}
+
+func NewPreviewPost(post *Post, team *Team, channel *Channel) *PreviewPost {
+	if post == nil {
+		return nil
+	}
+	return &PreviewPost{
+		PostID:             post.Id,
+		Post:               post,
+		TeamName:           team.Name,
+		ChannelDisplayName: channel.DisplayName,
+	}
+}
+
+func (o *Permalink) ToJson() string {
+	b, _ := json.Marshal(o)
+	return string(b)
+}

model/post.go

@@ -67,6 +67,8 @@ const (
 
 	PostPropsMentionHighlightDisabled = "mentionHighlightDisabled"
 	PostPropsGroupHighlightDisabled   = "disable_group_highlight"
+
+	PostPropsPreviewedPost = "previewed_post"
 )
 
 type Post struct {
@@ -749,3 +751,14 @@ func (o *Post) ToNilIfInvalid() *Post {
 	}
 	return o
 }
+
+func (o *Post) GetPreviewPost() *PreviewPost {
+	for _, embed := range o.Metadata.Embeds {
+		if embed.Type == PostEmbedPermalink {
+			if previewPost, ok := embed.Data.(*PreviewPost); ok {
+				return previewPost
+			}
+		}
+	}
+	return nil
+}

model/post_embed.go

@@ -8,6 +8,7 @@ const (
 	PostEmbedMessageAttachment PostEmbedType = "message_attachment"
 	PostEmbedOpengraph         PostEmbedType = "opengraph"
 	PostEmbedLink              PostEmbedType = "link"
+	PostEmbedPermalink         PostEmbedType = "permalink"
 )
 
 type PostEmbedType string

model/post_list.go

@@ -25,6 +25,23 @@ func NewPostList() *PostList {
 	}
 }
 
+func (o *PostList) Clone() *PostList {
+	orderCopy := make([]string, len(o.Order))
+	postsCopy := make(map[string]*Post)
+	for i, v := range o.Order {
+		orderCopy[i] = v
+	}
+	for k, v := range o.Posts {
+		postsCopy[k] = v.Clone()
+	}
+	return &PostList{
+		Order:      orderCopy,
+		Posts:      postsCopy,
+		NextPostId: o.NextPostId,
+		PrevPostId: o.PrevPostId,
+	}
+}
+
 func (o *PostList) ToSlice() []*Post {
 	var posts []*Post
 

services/telemetry/telemetry.go

@@ -442,6 +442,7 @@ func (ts *TelemetryService) trackConfig() {
 		"enable_legacy_sidebar":                                   *cfg.ServiceSettings.EnableLegacySidebar,
 		"thread_auto_follow":                                      *cfg.ServiceSettings.ThreadAutoFollow,
 		"enable_link_previews":                                    *cfg.ServiceSettings.EnableLinkPreviews,
+		"enable_permalink_previews":                               *cfg.ServiceSettings.EnablePermalinkPreviews,
 		"enable_file_search":                                      *cfg.ServiceSettings.EnableFileSearch,
 		"restrict_link_previews":                                  isDefault(*cfg.ServiceSettings.RestrictLinkPreviews, ""),
 	})

templates/globalrelay_compliance_export_message.html

@@ -6,5 +6,6 @@
     <span class="usertype">{{.Props.UserType}}</span>
     <span class="email">({{.Props.Email}}):</span>
     <span class="message">{{.Props.Message}}</span>
+    <span class="previews_post">{{.Props.PreviewsPost}}</span>
 </li>
 {{end}}

tests/test-config.json

@@ -23,6 +23,7 @@
         "EnablePostUsernameOverride": false,
         "EnablePostIconOverride": false,
         "EnableLinkPreviews": false,
+        "EnablePermalinkPreviews": false,
         "EnableTesting": false,
         "EnableDeveloper": false,
         "EnableSecurityFixAlert": true,