Database Connection Pool Implementation

Implement a simple database connection pool that allows callers to safely acquire and release database connections.

You will be provided with a mock database client, such as one exposing a db.query(connection, sql) function or a similar simulated query API, to interact with connections.

Requirements

• The pool must maintain a fixed maximum number of database connections.
• The acquire() method should behave as follows:
  • If an available connection exists in the pool, return it immediately.
  • If no connection is available but the pool has not yet reached its maximum size, create a new database connection, add it to the pool, and return it.
  • If the pool is at its maximum capacity and all connections are currently busy, acquire() must block and wait until a connection is released and becomes available.
• The release(connection) method must return the specified connection to the pool. If there are any threads waiting for a connection, release() should wake up one of them.
• The implementation must prevent common issues such as invalid releases (e.g., releasing a connection not acquired from the pool), double releases of the same connection, and connection leaks (e.g., connections not being returned to the pool).
• The entire connection pool logic must be safe under concurrent requests from multiple threads or processes.

Discussion Points

Be prepared to discuss edge cases including:

• Handling of query failures.
• Timeouts that may occur while a caller is waiting to acquire a connection.
• Strategies for graceful shutdown of the connection pool.