core/kotlinx-coroutines-core/src/main/kotlin/kotlinx/coroutines/experimental/CommonPool.kt

/*
 * Copyright 2016-2017 JetBrains s.r.o.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package kotlinx.coroutines.experimental

import kotlinx.coroutines.experimental.timeunit.TimeUnit
import java.util.concurrent.*
import java.util.concurrent.atomic.AtomicInteger
import kotlin.coroutines.experimental.CoroutineContext

/**
 * Represents common pool of shared threads as coroutine dispatcher for compute-intensive tasks.
 *
 * If there isn't a SecurityManager present it uses [java.util.concurrent.ForkJoinPool] when available, which implements
 * efficient work-stealing algorithm for its queues, so every coroutine resumption is dispatched as a separate task even
 * when it already executes inside the pool. When available, it wraps `ForkJoinPool.commonPool` and provides a similar
 * shared pool where not.
 * 
 * If there is a SecurityManager present (as would be if running inside a Java Web Start context) then a plain thread
 * pool is created. This is to work around the fact that ForkJoinPool creates threads that cannot perform
 * privileged actions.
 */
object CommonPool : CoroutineDispatcher() {
    private var usePrivatePool = false

    @Volatile
    private var _pool: Executor? = null

    private inline fun <T> Try(block: () -> T) = try { block() } catch (e: Throwable) { null }

    private fun createPool(): ExecutorService {
        if (System.getSecurityManager() != null) return createPlainPool()
        val fjpClass = Try { Class.forName("java.util.concurrent.ForkJoinPool") }
            ?: return createPlainPool()
        if (!usePrivatePool) {
            Try { fjpClass.getMethod("commonPool")?.invoke(null) as? ExecutorService }
                ?.let { return it }
        }
        Try { fjpClass.getConstructor(Int::class.java).newInstance(defaultParallelism()) as? ExecutorService }
            ?. let { return it }
        return createPlainPool()
    }

    private fun createPlainPool(): ExecutorService {
        val threadId = AtomicInteger()
        return Executors.newFixedThreadPool(defaultParallelism()) {
            Thread(it, "CommonPool-worker-${threadId.incrementAndGet()}").apply { isDaemon = true }
        }
    }

    private fun defaultParallelism() = (Runtime.getRuntime().availableProcessors() - 1).coerceAtLeast(1)

    @Synchronized
    private fun getOrCreatePoolSync(): Executor =
        _pool ?: createPool().also { _pool = it }

    override fun dispatch(context: CoroutineContext, block: Runnable) =
        try { (_pool ?: getOrCreatePoolSync()).execute(timeSource.trackTask(block)) }
        catch (e: RejectedExecutionException) {
            timeSource.unTrackTask()
            DefaultExecutor.execute(block)
        }

    // used for tests
    @Synchronized
    internal fun usePrivatePool() {
        shutdown(0)
        usePrivatePool = true
        _pool = null
    }

    // used for tests
    @Synchronized
    internal fun shutdown(timeout: Long) {
        (_pool as? ExecutorService)?.apply {
            shutdown()
            if (timeout > 0)
                awaitTermination(timeout, TimeUnit.MILLISECONDS)
            shutdownNow().forEach { DefaultExecutor.execute(it) }
        }
        _pool = Executor { throw RejectedExecutionException("CommonPool was shutdown") }
    }

    // used for tests
    @Synchronized
    internal fun restore() {
        shutdown(0)
        usePrivatePool = false
        _pool = null
    }

    override fun toString(): String = "CommonPool"
}