The user-namespace prefix that lets an unprivileged restore put back a file's
original owner was only wired into restic. borg extract and kopia snapshot
restore run as the same backup user with the same lack of CAP_CHOWN, so both
lost <container-uid>:<backup-user> exactly the way restic did — an app whose
data comes back owned by the backup user cannot write it, which is how grafana
kept dying with "attempt to write a readonly database".
borg is quieter about it than restic: it does not print an "ignoring error"
line at all, so there was nothing to notice.
Move the prefix to engine_dispatch.sh as backupUsernsPrefix — it was never
restic-specific — and use it from all three engines.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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