Add exclusive table lock manager - #100
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Introduce transaction-owned table leases with strict two-phase locking, FIFO wait queues, direct ownership handoff, and wait-for deadlock detection. Locks remain held until commit or rollback finalization completes. Add typed lifecycle and invariant errors, database-wide DDL admission, and focused tests for contention, cancellation, deadlocks, and lease behavior.
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This PR introduces a lock manager. The locks are per-table and exclusive-only. This means that each table can be locked by at most one transaction, whether the transaction is reading or writing. We use strict two-phase locking, where all held locks are released only after a transaction has been committed. For detecting deadlocks between transaction, we maintain a directed graph where an edge
(T1,T2)represents transactionT1waiting on a table lock held by transactionT2. A cycle in this graph is a deadlock. A FIFO queue of waiting transactions is maintained for each table in order to ensure that transactions are granted table locks in the expected order.We use Loom for testing for correctness under concurrency.
This PR only implements this lock manager module, but does not integrate it with the rest of the database system.