A copied fsentry kept its source's thumbnail pointer verbatim, so both
rows shared one S3 thumbnail object. fs.remove.node deletes the
pointed-to object, so the first removal among the sharers — e.g. the
remove performed by a replace-on-copy — broke every other sharer's
thumbnail. FSService already emits fs.copy.node for exactly this
reason (its doc comment describes the duplication), but the thumbnails
extension never subscribed to it.
Add the missing handler: S3-copy the thumbnail to a freshly minted key
and repoint the copied row, under the same only-keys-we-minted trust
rule as the read/remove paths. If the shared object is already gone
(pre-fix damage), null the pointer instead of leaving the row
advertising a thumbnail it doesn't have.
Nariman Jelveh