fix(coderd): serialize chat model config default election with advisory lock - #27114
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Can the provider be used to choose to promote a model config to default, but during concurrent creation the first non-default creation that acquires the lock promotes itself resulting in an error creating the one we want as default?
…ry lock Replaces the unique-violation retry loop: the election transactions now take pg_advisory_xact_lock(LockIDChatModelConfigDefault) so only one election runs at a time and the single-default index is never contended. The index and the 409 mapping stay as the schema-level backstop.
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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_defaultrejects the loser as a spurious 409. The coderd Terraform provider hits this routinely, since a singleterraform applycreates 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_lockon a dedicatedLockIDChatModelConfigDefault, 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_idFK, 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.