Implement a Database Connection Pool

Implement a simple database connection pool. You are given a mock database client with a function such as `db.query(connection, sql)` or an equivalent simulated query API. Complete the connection-pool logic so callers can safely acquire and release database connections. Requirements: - The pool has a fixed maximum number of database connections. - `acquire()` returns an available connection if one exists. - If no connection is available but the pool has not reached its maximum size, create a new connection and return it. - If the pool is at capacity and all connections are busy, `acquire()` should wait until a connection is released. - `release(connection)` returns a connection to the pool and wakes one pending waiter, if any. - Prevent invalid releases, double releases, and connection leaks. - The implementation should be safe under concurrent requests. Discuss edge cases such as query failures, timeouts while waiting for a connection, and graceful shutdown of the pool.