MM-17071 Add mention counting when marking a post as unread (#11966)

* Add different types for different mentions

* Remove redundant THREAD_ANY and THREAD_ROOT constants

* Make PostStore.Get return thread in order

* MM-17071 Add initial version of countMentionsFromPost

* MM-17071 Refactor comment mention counting

* MM-17071 Use mention counting when marking post as unread

* Fix shadowing in tests

* Remove repeated check of user count

* Refactor code using MentionType

* Update comments around -1 return value

* Move inner functions out of countMentionsFromPost

* Remove preconditions check as separate test case

* Update comments

* Add User.GetMentionKeys

* Revert "Make PostStore.Get return thread in order"

This reverts commit 22aa010cee359655fe75e1dc899cf0ffb0943c2a.

* Fix tests

* Fix merge conflict

* Add store.MentionAllPosts

app/channel_test.go

@@ -1128,7 +1128,6 @@ func TestMarkChannelAsUnreadFromPost(t *testing.T) {
 		unread, err := th.App.GetChannelUnread(c1.Id, u2.Id)
 		require.Nil(t, err)
 		assert.Equal(t, int64(0), unread.MsgCount)
-
 	})
 
 	t.Run("Unread on a private channel", func(t *testing.T) {
@@ -1150,6 +1149,48 @@ func TestMarkChannelAsUnreadFromPost(t *testing.T) {
 		assert.Equal(t, pp2.CreateAt-1, response.LastViewedAt)
 	})
 
+	t.Run("Unread with mentions", func(t *testing.T) {
+		c2 := th.CreateChannel(th.BasicTeam)
+		_, err := th.App.AddUserToChannel(u2, c2)
+		require.Nil(t, err)
+
+		p4, err := th.App.CreatePost(&model.Post{
+			UserId:    u2.Id,
+			ChannelId: c2.Id,
+			Message:   "@" + u1.Username,
+		}, c2, false)
+		require.Nil(t, err)
+		th.CreatePost(c2)
+
+		response, err := th.App.MarkChannelAsUnreadFromPost(p4.Id, u1.Id)
+		assert.Nil(t, err)
+		assert.Equal(t, int64(1), response.MsgCount)
+		assert.Equal(t, int64(1), response.MentionCount)
+
+		unread, err := th.App.GetChannelUnread(c2.Id, u1.Id)
+		require.Nil(t, err)
+		assert.Equal(t, int64(1), unread.MsgCount)
+		assert.Equal(t, int64(1), unread.MentionCount)
+	})
+
+	t.Run("Unread on a DM channel", func(t *testing.T) {
+		dc := th.CreateDmChannel(u2)
+
+		dm1 := th.CreatePost(dc)
+		th.CreatePost(dc)
+		th.CreatePost(dc)
+
+		response, err := th.App.MarkChannelAsUnreadFromPost(dm1.Id, u1.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)
+		require.Nil(t, err)
+		assert.Equal(t, int64(3), unread.MsgCount)
+		assert.Equal(t, int64(3), unread.MentionCount)
+	})
+
 	t.Run("Can't unread an imaginary post", func(t *testing.T) {
 		response, err := th.App.MarkChannelAsUnreadFromPost("invalid4ofngungryquinj976y", u1.Id)
 		assert.NotNil(t, err)