[yordis]

• The advisory lock decides who runs a subscription, but nothing in the database records who may write its progress, so a node that lost the lock without knowing yet can still overwrite the position of the node that took over.
• Recording a lock generation on the subscription row when the lock is acquired gives every progress write something to be checked against, and returns the committed position under the same lock so the new owner resumes from where the last owner actually got to rather than from a value read before the lock was held.
• The database must be migrated before nodes running this version start, as described in the upgrade guide.

---

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PR Summary

High Risk
Changes subscription ownership and progress semantics with a required DB migration; incorrect rollout or fencing integration could corrupt subscription positions in multi-node deployments.

Overview
Adds subscription lock claiming so acquiring a subscription advisory lock also bumps lock_generation on the subscription row and returns a lease (FencingToken, committed last_seen, session) for fencing progress writes.

try_subscription_lock/3 on EventStore.AdvisoryLocks wraps storage claim_subscription_lock with the same monitor/release semantics as generic advisory locks. New types: FencingToken, Storage.Lock.Lease, and AdvisoryLocks.Lease.

Schema 1.5.0 adds subscriptions.lock_generation (default 0) via migration and fresh installs; operators must migrate before rolling out this version. A compatibility test confirms pre-1.5 nodes still ack and lock against the new column.

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Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

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