Two computes decide the owners of the same UUID: the one the VM left
buries them in a tombstone (or a compute delete does), the one it went
to restores them. On PostgreSQL the two ran at once: the restore still
saw the old row of the uncommitted bury, skipped, and the VM ended up
without owners while the tombstone stayed. SQLite was never affected:
its IMMEDIATE transactions take the write lock at the start.
The reconcile and the compute delete now take one transaction-level
advisory lock (pg_advisory_xact_lock) when their database step starts,
so ownership decisions run one after another on PostgreSQL as they do
on SQLite. Reading libvirt stays outside the lock.
