DB pool behaves incorrectly when I close the same connection multiple times

[cbxp]

Case:

I close DB connection multiple times

Expected result:

it's totally fine. The first close() call closes the connection, and the following close() calls are just ignored.

Actual result:

1. The connection is added to pool.available multiple times.
2. As a result, subsequent calls to pool.getConnection() return the same connection multiple times.

Failing test:

class PooledDataSourceTest {

  @Test fun `close same connection multiple times`() {
    val conn = pool.connection
    expect(pool.used.keys).toHaveSize(1)
    expect(pool.available).toHaveSize(0)

    conn.close()
    conn.close()
    conn.close()

    expect(pool.used.keys).toHaveSize(0)
    expect(pool.available).toHaveSize(1)  // Fails: pool.available.size is 3

    val conn2 = pool.connection
    expect(pool.connection).notToEqual(conn2) // Fails: it's the same connection
    expect(pool.connection).notToEqual(conn2)
  }
}