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Ethan
535c775f2a
Closes CODAGT-736 Concurrent chat model config writes on a deployment with no default all elect themselves default: at READ COMMITTED neither transaction sees the other's uncommitted default, so both self-promote and `idx_chat_model_configs_single_default` rejects the loser as a spurious 409. The coderd Terraform provider hits this routinely, since a single `terraform apply` creates or deletes many configs in parallel by design. The fix serializes the election with a transaction-scoped advisory lock: the create, update, and delete handlers run their default election inside a transaction that first takes `pg_advisory_xact_lock` on a dedicated `LockIDChatModelConfigDefault`, so elections run one at a time and the index is never contended. The partial unique index stays in place as the schema-level invariant, and the existing 409 mapping remains as a backstop for any writer that bypasses the lock. We considered a singleton pointer table (one row holding a `model_config_id` FK, making a second default unrepresentable), which would remove the race outright, but it needs a migration, new queries, dbauthz rules, and handler/read-path rework. Not proportionate for an experimental endpoint.