MM-38657 - Sentry crash: Fix nil dereference when post not found (#18440)

* Revert "Add debug log for nil referencedPost (#18437)"

This reverts commit e575315275.

* Ignores post not found errors at a higher level

And adds test

```release-note
NONE
```

app/post_metadata.go

@@ -5,7 +5,6 @@ package app
 
 import (
 	"bytes"
-	"errors"
 	"fmt"
 	"image"
 	"io"
@@ -130,7 +129,12 @@ func (a *App) PreparePostForClient(originalPost *model.Post, isNewPost bool, isE
 	firstLink, images := a.getFirstLinkAndImages(post.Message)
 
 	if embed, err := a.getEmbedForPost(post, firstLink, isNewPost); err != nil {
+		appErr, ok := err.(*model.AppError)
+		isNotFound := ok && appErr.StatusCode == http.StatusNotFound
+		// Ignore NotFound errors.
+		if !isNotFound {
 			mlog.Debug("Failed to get embedded content for a post", mlog.String("post_id", post.Id), mlog.Err(err))
+		}
 	} else if embed == nil {
 		post.Metadata.Embeds = []*model.PostEmbed{}
 	} else {
@@ -291,8 +295,13 @@ func (a *App) getImagesForPost(post *model.Post, imageURLs []string, isNewPost b
 
 	for _, imageURL := range imageURLs {
 		if _, image, _, err := a.getLinkMetadata(imageURL, post.CreateAt, isNewPost, post.GetPreviewedPostProp()); err != nil {
+			appErr, ok := err.(*model.AppError)
+			isNotFound := ok && appErr.StatusCode == http.StatusNotFound
+			// Ignore NotFound errors.
+			if !isNotFound {
 				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 {
 			images[imageURL] = image
 		}
@@ -479,46 +488,25 @@ func (a *App) getLinkMetadata(requestURL string, timestamp int64, isNewPost bool
 	}
 
 	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 {
+		if appErr != nil {
 			return nil, nil, nil, appErr
 		}
 
-		if referencedPost == nil {
-			msg := "Referenced post is nil"
-			mlog.Debug(msg, mlog.String("post_id", referencedPostID))
-			return nil, nil, nil, errors.New(msg)
-		}
-
 		referencedChannel, appErr := a.GetChannel(referencedPost.ChannelId)
 		if appErr != nil {
 			return nil, nil, nil, appErr
 		}
 
-		if referencedChannel == nil {
-			msg := "Referenced channel is nil"
-			mlog.Debug(msg, mlog.String("channel_id", referencedPost.ChannelId))
-			return nil, nil, nil, errors.New(msg)
-		}
-
 		referencedTeam, appErr := a.GetTeam(referencedChannel.TeamId)
 		if appErr != nil {
 			return nil, nil, nil, appErr
 		}
 
-		if referencedTeam == nil {
-			msg := "Referenced team is nil"
-			mlog.Debug(msg, mlog.String("team_id", referencedChannel.TeamId))
-			return nil, nil, nil, errors.New(msg)
-		}
-
 		permalink = &model.Permalink{PreviewPost: model.NewPreviewPost(referencedPost, referencedTeam, referencedChannel)}
 	} else {
 

app/post_metadata_test.go

@@ -2229,6 +2229,23 @@ func TestGetLinkMetadata(t *testing.T) {
 		assert.NotNil(t, img)
 		assert.NoError(t, err)
 	})
+
+	t.Run("should throw error if post doesn't exist", func(t *testing.T) {
+		th := setup(t)
+		defer th.TearDown()
+
+		th.App.UpdateConfig(func(cfg *model.Config) {
+			*cfg.ServiceSettings.EnablePermalinkPreviews = true
+			*cfg.ServiceSettings.SiteURL = server.URL
+			cfg.FeatureFlags.PermalinkPreviews = true
+		})
+
+		requestURL := server.URL + "/pl/5rpoy4o3nbgwjm7gs4cm71h6ho"
+		timestamp := int64(1547510400000)
+
+		_, _, _, err := th.App.getLinkMetadata(requestURL, timestamp, true, "")
+		assert.Error(t, err)
+	})
 }
 
 func TestResolveMetadataURL(t *testing.T) {