and can theoretically result in a migration that never ends.

The issue is caused by yielding here:

https://github.com/mongodb/mongo/blob/84223317a67211cf19d9ef1e944ef3fe12a819ac/src/mongo/db/s/migration_chunk_cloner_source_legacy.cpp#L421

Here, we break early and if the iterator didn't move then the erase here:

https://github.com/mongodb/mongo/blob/84223317a67211cf19d9ef1e944ef3fe12a819ac/src/mongo/db/s/migration_chunk_cloner_source_legacy.cpp#L426

will end up erasing nothing. A potential fix is to move the yield check at top to make sure that the iterator is not equal to the begin iterator when the command added a document to the response buffer (with a check that it is not equal to the begin iterator, otherwise, we can end up with a different problem of not returning any document).