The merge writes its outbox record before the DNG, which takes minutes,
and a drain that ran in the meantime took whatever lay at the record's
name: a sync pass that fired mid-merge uploaded the first part of the
composite and cleared the record. The file is now written as x.dng.part
and renamed into place once the last strip is in; the drain skips a
record whose payload does not exist yet.
dtourolle
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