How to Drain a Connection Pool Before Exit in Python

import socket
import threading
import time
import random

class ConnectionPool:
    def __init__(self, size=5):
        self.pool = []
        self.lock = threading.Lock()
        self.closed = False
        for _ in range(size):
            self.pool.append(self.create_connection())
    
    def create_connection(self):
        mock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        mock.settimeout(1)
        return mock
    
    def acquire(self):
        with self.lock:
            if self.closed:
                raise RuntimeError("Pool is closed")
            if self.pool:
                return self.pool.pop()
            return self.create_connection()
    
    def release(self, conn):
        with self.lock:
            if not self.closed and conn.fileno() != -1:
                self.pool.append(conn)
            else:
                conn.close()
    
    def drain(self):
        with self.lock:
            self.closed = True
            connections = self.pool
            self.pool = []
        for conn in connections:
            try:
                conn.close()
                print(f"Closed connection fd={conn.fileno()} (now -1)")
            except OSError as e:
                print(f"Error closing: {e}")
        print(f"Drained {len(connections)} connections")

def simulate_work(pool):
    conn = pool.acquire()
    time.sleep(random.uniform(0.01, 0.05))
    pool.release(conn)

if __name__ == "__main__":
    pool = ConnectionPool(3)
    threads = [threading.Thread(target=simulate_work, args=(pool,)) for _ in range(5)]
    for t in threads:
        t.start()
    for t in threads:
        t.join()
    
    pool.drain()
    print("All connections drained before exit")