Resource sharing with semaphore and mutex in kotlin

ResourcePool.kt

/**
 * Thread safe shared resources.
 *
 * ```kotlin
 * // await 1s with the resource
 * fun run(pool: ResourcePool<*>) = launch { pool { delay(1.seconds) } }
 *
 * // Pool of 2 usable elements at the same time
 * val pool = ResourcePool<Unit>(2) { Unit }
 * (0 ..< 120).map { run(pool) }.joinAll() // elapsed time ~60s
 *
 * // Pool of 4 usable elements at the same time
 * val pool = ResourcePool<Unit>(4) { Unit }
 * (0 ..< 120).map { run(pool) }.joinAll() // elapsed time ~30s
 * ```
 */
class ResourcePool<out T>(count: Int, factory: (Int) -> T) : Closeable {

    private val mutex = Mutex()
    private val semaphore = Semaphore(count)
    private val resources = MutableList(count, factory)

    suspend operator fun <R> invoke(handler: suspend (T) -> R): R {
        semaphore.withPermit {
            val borrowed = mutex.withLock { resources.removeLast() }
            try {
                return handler(borrowed)
            } finally {
                mutex.withLock { resources.add(borrowed) }
            }
        }
    }

    override fun close() {
        resources.clear()
    }
}