A PBS-to-PBS sync that fails mid-snapshot leaves a manifest-less copy of an
already-completed backup on the target datastore. That copy has the same
signature as a snapshot being written right now (no size, no
index.json.blob), so the guest showed Backup Running indefinitely until the
sync was repaired.
An in-flight snapshot only reports the guest as backing up when it is newer
than the guest's last completed backup and within the same 24h cutoff that
already ages out stale running vzdump tasks. A genuine in-progress backup is
always newer than the last completed one; a partial sync copy never is.
Refs #1815
rcourtman
a1336348cb