Chunk migrations replicate retryable findAndModify information by transferring the noop oplog entries that represent the pre/post images.
With retryable images being saved into an implicitly replicated collection, those images are no longer naturally discovered to be transferred.
When a chunk migration source is about to transfer an oplog entry that requires a pre/post image written in the new format, we will:
- perform the lookup on image collection
- forge a noop oplog entry
This effectively downconverts the new retryable findAndModify format to the old format for chunk migrations.