Provided that it should not be common to build an index over a sharded collection while the latter is being renamed, it is technically possible to do so.

The check for concurrent index builds happens in the local catalog rename code, called by rename participants after blocking CRUDs. If the two operations overlap, an assertion is thrown and the participant indefinitely retries the rename operation as long as the build is not finished.

As a consequence, if an index build happens during a rename it is possible that CRUDs are blocked during the whole time of the build.