MM-20649 Fix incorrect mention count when marking a DM channel as unread (#13245)

* MM-20649 Fix incorrect mention count when marking a DM channel as unread

* Satisfy govet

* Update missed test

app/channel_test.go

@@ -1132,12 +1132,12 @@ func TestMarkChannelAsUnreadFromPost(t *testing.T) {
 		th.CreatePost(dc)
 		th.CreatePost(dc)
 
-		response, err := th.App.MarkChannelAsUnreadFromPost(dm1.Id, u1.Id)
+		response, err := th.App.MarkChannelAsUnreadFromPost(dm1.Id, u2.Id)
 		assert.Nil(t, err)
 		assert.Equal(t, int64(0), response.MsgCount)
 		assert.Equal(t, int64(3), response.MentionCount)
 
-		unread, err := th.App.GetChannelUnread(dc.Id, u1.Id)
+		unread, err := th.App.GetChannelUnread(dc.Id, u2.Id)
 		require.Nil(t, err)
 		assert.Equal(t, int64(3), unread.MsgCount)
 		assert.Equal(t, int64(3), unread.MentionCount)

app/post.go

@@ -1180,7 +1180,7 @@ func (a *App) MaxPostSize() int {
 }
 
 // countMentionsFromPost returns the number of posts in the post's channel that mention the user after and including the
-// given post. Returns the number of mentions or store.MentionAllPosts if the post is in a direct message channel.
+// given post.
 func (a *App) countMentionsFromPost(user *model.User, post *model.Post) (int, *model.AppError) {
 	channel, err := a.GetChannel(post.ChannelId)
 	if err != nil {
@@ -1188,7 +1188,13 @@ func (a *App) countMentionsFromPost(user *model.User, post *model.Post) (int, *m
 	}
 
 	if channel.Type == model.CHANNEL_DIRECT {
-		return store.MentionAllPosts, nil
+		// In a DM channel, every post made by the other user is a mention
+		count, countErr := a.Srv.Store.Channel().CountPostsAfter(post.ChannelId, post.CreateAt-1, channel.GetOtherUserIdForDM(user.Id))
+		if countErr != nil {
+			return 0, countErr
+		}
+
+		return int(count), countErr
 	}
 
 	channelMember, err := a.GetChannelMember(channel.Id, user.Id)

app/post_test.go

@@ -16,7 +16,6 @@ import (
 	"github.com/mattermost/mattermost-server/v5/einterfaces/mocks"
 	"github.com/mattermost/mattermost-server/v5/model"
 	"github.com/mattermost/mattermost-server/v5/plugin/plugintest/mock"
-	"github.com/mattermost/mattermost-server/v5/store"
 	"github.com/mattermost/mattermost-server/v5/store/storetest"
 )
 
@@ -1261,7 +1260,7 @@ func TestCountMentionsFromPost(t *testing.T) {
 		assert.Equal(t, 2, count)
 	})
 
-	t.Run("should return store.MentionAllPosts for a direct channel", func(t *testing.T) {
+	t.Run("should return the number of posts made by the other user for a direct channel", func(t *testing.T) {
 		th := Setup(t).InitBasic()
 		defer th.TearDown()
 
@@ -1278,10 +1277,22 @@ func TestCountMentionsFromPost(t *testing.T) {
 		}, channel, false)
 		require.Nil(t, err)
 
+		_, err = th.App.CreatePost(&model.Post{
+			UserId:    user1.Id,
+			ChannelId: channel.Id,
+			Message:   "test2",
+		}, channel, false)
+		require.Nil(t, err)
+
 		count, err := th.App.countMentionsFromPost(user2, post1)
 
 		assert.Nil(t, err)
-		assert.Equal(t, store.MentionAllPosts, count)
+		assert.Equal(t, 2, count)
+
+		count, err = th.App.countMentionsFromPost(user1, post1)
+
+		assert.Nil(t, err)
+		assert.Equal(t, 0, count)
 	})
 
 	t.Run("should not count mentions from the before the given post", func(t *testing.T) {