The filer deleted a pending entry's source after copying it on a later
rename, but that rename locks only its own paths: another rename can
copy onto the source key before its entry is visible, and the cleanup
then deleted that copy. And deleting a pending entry never released its
source at all.
The filer now leaves marked sources alone. remote.sync releases one
when the pending entry is deleted, or when a later rename was copied
right away, and only while the live object is still the one the
entry's stamp describes and no remote-only entry holds the path: a
copy that landed there since has a different ETag and is kept.