MM-40880: Sentry crash: MarkChannelAsUnread (#19370)

Use the correct error variable to prevent crash

The errors.As check has been removed because ThreadStore.Get
returns a nil error if no thread is found and that's how
the application logic is written. So the check was redundant.

```release-note
NONE
```

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

app/channel.go

@@ -2552,8 +2552,8 @@ func (a *App) MarkChannelAsUnreadFromPost(postID string, userID string, collapse
 				return nil, model.NewAppError("MarkChannelAsUnreadFromPost", "app.channel.update_last_viewed_at_post.app_error", nil, storeErr.Error(), http.StatusInternalServerError)
 			}
 			threadData, storeErr2 := a.Srv().Store.Thread().Get(threadId)
-			if storeErr2 != nil && !errors.As(storeErr2, &nfErr) {
+			if storeErr2 != nil {
-				return nil, model.NewAppError("MarkChannelAsUnreadFromPost", "app.channel.update_last_viewed_at_post.app_error", nil, storeErr.Error(), http.StatusInternalServerError)
+				return nil, model.NewAppError("MarkChannelAsUnreadFromPost", "app.channel.update_last_viewed_at_post.app_error", nil, storeErr2.Error(), http.StatusInternalServerError)
 			}
 			if threadData != nil && threadMembership != nil && threadMembership.Following {
 				channel, nErr := a.Srv().Store.Channel().Get(post.ChannelId, true)

app/channel_test.go

@@ -5,6 +5,7 @@ package app
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"net/http"
 	"os"
@@ -1992,6 +1993,78 @@ func TestMarkChannelsAsViewedPanic(t *testing.T) {
 	require.Nil(t, appErr)
 }
 
+func TestMarkChannelAsUnreadFromPostPanic(t *testing.T) {
+	th := SetupWithStoreMock(t)
+	defer th.TearDown()
+
+	mockStore := th.App.Srv().Store.(*mocks.Store)
+	mockUserStore := mocks.UserStore{}
+	mockUserStore.On("Get", context.Background(), "userID").Return(&model.User{Id: "userID"}, nil)
+	mockUserStore.On("Count", mock.Anything).Return(int64(10), nil)
+
+	mockChannelStore := mocks.ChannelStore{}
+	mockChannelStore.On("Get", "channelID", true).Return(&model.Channel{Id: "channelID"}, nil)
+	mockChannelStore.On("GetMember", context.Background(), "channelID", "userID").Return(&model.ChannelMember{
+		NotifyProps: model.StringMap{
+			model.PushNotifyProp: model.ChannelNotifyDefault,
+		}}, nil)
+
+	mockSessionStore := mocks.SessionStore{}
+	mockOAuthStore := mocks.OAuthStore{}
+	var err error
+	th.App.ch.srv.userService, err = users.New(users.ServiceConfig{
+		UserStore:    &mockUserStore,
+		SessionStore: &mockSessionStore,
+		OAuthStore:   &mockOAuthStore,
+		ConfigFn:     th.App.ch.srv.Config,
+		LicenseFn:    th.App.ch.srv.License,
+	})
+	require.NoError(t, err)
+
+	mockPreferenceStore := mocks.PreferenceStore{}
+	mockPreferenceStore.On("Get", "userID", model.PreferenceCategoryDisplaySettings, model.PreferenceNameCollapsedThreadsEnabled).Return(&model.Preference{Value: "on"}, nil)
+
+	mockThreadStore := mocks.ThreadStore{}
+	mockThreadStore.On("MarkAllAsReadInChannels", "userID", []string{"channelID"}).Return(nil)
+	mockThreadStore.On("GetMembershipForUser", "userID", "rootID").Return(nil, nil)
+	mockThreadStore.On("MaintainMembership", "userID", "rootID", mock.AnythingOfType("store.ThreadMembershipOpts")).Return(&model.ThreadMembership{}, nil)
+	mockThreadStore.On("Get", "rootID").Return(nil, errors.New("bad error")) // Returning an error from here causes the panic
+
+	mockPostStore := mocks.PostStore{}
+	mockPostStore.On("GetMaxPostSize").Return(65535, nil)
+	mockPostStore.On("Get", context.Background(), "postID", false, false, false, "userID").Return(&model.PostList{}, nil)
+	mockPostStore.On("GetPostsAfter", mock.AnythingOfType("model.GetPostsOptions")).Return(&model.PostList{}, nil)
+	mockPostStore.On("GetSingle", "postID", false).Return(&model.Post{
+		Id:        "postID",
+		RootId:    "rootID",
+		ChannelId: "channelID",
+	}, nil)
+
+	mockSystemStore := mocks.SystemStore{}
+	mockSystemStore.On("GetByName", "UpgradedFromTE").Return(&model.System{Name: "UpgradedFromTE", Value: "false"}, nil)
+	mockSystemStore.On("GetByName", "InstallationDate").Return(&model.System{Name: "InstallationDate", Value: "10"}, nil)
+	mockSystemStore.On("GetByName", "FirstServerRunTimestamp").Return(&model.System{Name: "FirstServerRunTimestamp", Value: "10"}, nil)
+	mockLicenseStore := mocks.LicenseStore{}
+	mockLicenseStore.On("Get", "").Return(&model.LicenseRecord{}, nil)
+
+	mockStore.On("Channel").Return(&mockChannelStore)
+	mockStore.On("Preference").Return(&mockPreferenceStore)
+	mockStore.On("Thread").Return(&mockThreadStore)
+	mockStore.On("Post").Return(&mockPostStore)
+	mockStore.On("User").Return(&mockUserStore)
+	mockStore.On("System").Return(&mockSystemStore)
+	mockStore.On("License").Return(&mockLicenseStore)
+
+	th.App.UpdateConfig(func(cfg *model.Config) {
+		*cfg.ServiceSettings.ThreadAutoFollow = true
+		*cfg.ServiceSettings.CollapsedThreads = model.CollapsedThreadsDefaultOn
+	})
+
+	require.NotPanics(t, func() {
+		th.App.MarkChannelAsUnreadFromPost("postID", "userID", true, true)
+	}, "unexpected panic from MarkChannelAsUnreadFromPost")
+}
+
 func TestClearChannelMembersCache(t *testing.T) {
 	th := SetupWithStoreMock(t)
 	defer th.TearDown()