MM-43780: Adjust getPostThread API for easier pagination (#20172)

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

store/storetest/post_store.go

@@ -567,7 +567,7 @@ func testPostStoreGetForThread(t *testing.T, ss store.Store) {
 		require.NoError(t, err)
 		_, err = ss.Post().Save(&model.Post{ChannelId: o1.ChannelId, UserId: model.NewId(), Message: NewTestId(), RootId: o1.Id})
 		require.NoError(t, err)
-		_, err = ss.Post().Save(&model.Post{ChannelId: o1.ChannelId, UserId: model.NewId(), Message: NewTestId(), RootId: o1.Id})
+		m1, err := ss.Post().Save(&model.Post{ChannelId: o1.ChannelId, UserId: model.NewId(), Message: NewTestId(), RootId: o1.Id})
 		require.NoError(t, err)
 		_, err = ss.Post().Save(&model.Post{ChannelId: o1.ChannelId, UserId: model.NewId(), Message: NewTestId(), RootId: o1.Id})
 		require.NoError(t, err)
@@ -615,6 +615,20 @@ func testPostStoreGetForThread(t *testing.T, ss store.Store) {
 		assert.LessOrEqual(t, r1.Posts[r1.Order[1]].CreateAt, firstPostCreateAt)
 		assert.False(t, r1.HasNext)
 
+		// Only with CreateAt
+		opts = model.GetPostsOptions{
+			CollapsedThreads: false,
+			PerPage:          1,
+			Direction:        "up",
+			FromCreateAt:     m1.CreateAt,
+			SkipFetchThreads: false,
+		}
+		r1, err = ss.Post().Get(context.Background(), o1.Id, opts, o1.UserId)
+		require.NoError(t, err)
+		assert.Len(t, r1.Order, 2) // including the root post
+		assert.LessOrEqual(t, r1.Posts[r1.Order[1]].CreateAt, m1.CreateAt)
+		assert.True(t, r1.HasNext)
+
 		// Non-CRT mode
 		opts = model.GetPostsOptions{
 			CollapsedThreads: false,
@@ -659,6 +673,20 @@ func testPostStoreGetForThread(t *testing.T, ss store.Store) {
 		assert.Len(t, r1.Order, 3) // including the root post
 		assert.LessOrEqual(t, r1.Posts[r1.Order[1]].CreateAt, firstPostCreateAt)
 		assert.False(t, r1.HasNext)
+
+		// Only with CreateAt
+		opts = model.GetPostsOptions{
+			CollapsedThreads: false,
+			PerPage:          1,
+			Direction:        "down",
+			FromCreateAt:     m1.CreateAt,
+			SkipFetchThreads: false,
+		}
+		r1, err = ss.Post().Get(context.Background(), o1.Id, opts, o1.UserId)
+		require.NoError(t, err)
+		assert.Len(t, r1.Order, 2) // including the root post
+		assert.GreaterOrEqual(t, r1.Posts[r1.Order[1]].CreateAt, m1.CreateAt)
+		assert.True(t, r1.HasNext)
 	})
 }