Advisory locking for protecting TOCTOU (Time-of-Check-Time-of-Use) patterns.

On PostgreSQL, acquires a transaction-scoped advisory lock via pg_advisory_xact_lock(namespace, resource_id). The lock is automatically released when the enclosing Repo.transaction commits or rolls back.

On SQLite, this is a no-op — SQLite serializes all writes at the database level, so advisory locks are unnecessary.

Usage

Always call within a Repo.transaction:

Repo.transaction(fn ->
  AdvisoryLock.lock(:lobby, lobby.id)
  count = count_members(lobby.id)
  if count >= lobby.max_users, do: Repo.rollback(:full)
  do_join(...)
end)

Namespaces

Each resource type uses a distinct integer namespace to avoid collisions:

• :lobby → 1
• :group → 2
• :party → 3

You can also pass an arbitrary string as the namespace. The string is hashed to a stable 32-bit integer via :erlang.phash2/2, so any string (e.g. "word_guessed", "my_rpc") works without pre-registration.

Examples

# Atom namespace (predefined):
AdvisoryLock.lock(:lobby, lobby_id)

# String namespace (ad-hoc):
AdvisoryLock.lock("word_guessed", lobby_id)