Release every resource you open

How to do it

Open the resource in a block that closes it on exit. Examples are with in Python, try-with-resources in Java, using in C#, defer in Go and try with finally in JavaScript. Never rely on the garbage collector to close it. Give pooled connections back the same way.

Example

# Bad: an error in parse() leaves the file open
file = open("orders.csv")
orders = parse(file)
file.close()

# Good: the file closes even when parse() raises
with open("orders.csv") as file:
    orders = parse(file)

// Good: try-with-resources closes the connection on every path
try (Connection connection = dataSource.getConnection()) {
    return orderRepository.load(connection, orderId);
}

// Good: finally gives the client back even when the query throws
const client = await pool.connect();
try {
  return await client.query('SELECT total FROM orders WHERE id = $1', [orderId]);
} finally {
  client.release();
}

How to check

A linter fails on resources opened outside a scoped block. Examples are Pylint consider-using-with, Ruff SIM115, PMD CloseResource for Java and the .NET analyzer CA2000. A test that throws inside the block finds the resource closed.