/*
 * Copyright (C) 2009-2021 Lightbend Inc. <https://www.lightbend.com>
 */

package akka.dispatch

import akka.annotation.InternalApi

import java.util.ArrayDeque
import java.util.concurrent.Executor
import scala.annotation.tailrec
import scala.concurrent._

/**
 * INTERNAL API
 *
 * All Batchables are automatically batched when submitted to a BatchingExecutor
 */
@InternalApi
private[akka] trait Batchable extends Runnable {
  def isBatchable: Boolean
}

/**
 * INTERNAL API
 *
 * Mixin trait for an Executor
 * which groups multiple nested `Runnable.run()` calls
 * into a single Runnable passed to the original
 * Executor. This can be a useful optimization
 * because it bypasses the original context's task
 * queue and keeps related (nested) code on a single
 * thread which may improve CPU affinity. However,
 * if tasks passed to the Executor are blocking
 * or expensive, this optimization can prevent work-stealing
 * and make performance worse. Also, some ExecutionContext
 * may be fast enough natively that this optimization just
 * adds overhead.
 * The default ExecutionContext.global is already batching
 * or fast enough not to benefit from it; while
 * `fromExecutor` and `fromExecutorService` do NOT add
 * this optimization since they don't know whether the underlying
 * executor will benefit from it.
 * A batching executor can create deadlocks if code does
 * not use `scala.concurrent.blocking` when it should,
 * because tasks created within other tasks will block
 * on the outer task completing.
 * This executor may run tasks in any order, including LIFO order.
 * There are no ordering guarantees.
 *
 * WARNING: The underlying Executor's execute-method must not execute the submitted Runnable
 * in the calling thread synchronously. It must enqueue/handoff the Runnable.
 */
@InternalApi
private[akka] trait BatchingExecutor extends Executor {

  // invariant: if "_tasksLocal.get ne null" then we are inside Batch.run; if it is null, we are outside
  private[this] val _tasksLocal = new ThreadLocal[AbstractBatch]()

  private[this] abstract class AbstractBatch extends ArrayDeque[Runnable](4) with Runnable {
    @tailrec final def processBatch(batch: AbstractBatch): Unit =
      if ((batch eq this) && !batch.isEmpty) {
        batch.poll().run()
        processBatch(_tasksLocal.get) // If this is null, then we have been using managed blocking, so bail out
      }

    protected final def resubmitUnbatched(): Boolean = {
      val current = _tasksLocal.get()
      _tasksLocal.remove()
      if ((current eq this) && !current.isEmpty) { // Resubmit ourselves if something bad happened and we still have work to do
        unbatchedExecute(current) //TODO what if this submission fails?
        true
      } else false
    }
  }

  private[this] final class Batch extends AbstractBatch {
    override final def run: Unit = {
      require(_tasksLocal.get eq null)
      _tasksLocal.set(this) // Install ourselves as the current batch
      try processBatch(this)
      catch {
        case t: Throwable =>
          resubmitUnbatched()
          throw t
      } finally _tasksLocal.remove()
    }
  }

  private[this] val _blockContext = new ThreadLocal[BlockContext]()

  private[this] final class BlockableBatch extends AbstractBatch with BlockContext {
    // this method runs in the delegate ExecutionContext's thread
    override final def run(): Unit = {
      require(_tasksLocal.get eq null)
      _tasksLocal.set(this) // Install ourselves as the current batch
      val firstInvocation = _blockContext.get eq null
      if (firstInvocation) _blockContext.set(BlockContext.current)
      BlockContext.withBlockContext(this) {
        try processBatch(this)
        catch {
          case t: Throwable =>
            resubmitUnbatched()
            throw t
        } finally {
          _tasksLocal.remove()
          if (firstInvocation) _blockContext.remove()
        }
      }
    }

    override def blockOn[T](thunk: => T)(implicit permission: CanAwait): T = {
      // if we know there will be blocking, we don't want to keep tasks queued up because it could deadlock.
      resubmitUnbatched()
      // now delegate the blocking to the previous BC
      _blockContext.get.blockOn(thunk)
    }
  }

  protected def unbatchedExecute(r: Runnable): Unit

  protected def resubmitOnBlock: Boolean

  override def execute(runnable: Runnable): Unit = {
    if (batchable(runnable)) { // If we can batch the runnable
      _tasksLocal.get match {
        case null =>
          val newBatch: AbstractBatch = if (resubmitOnBlock) new BlockableBatch() else new Batch()
          newBatch.add(runnable)
          unbatchedExecute(newBatch) // If we aren't in batching mode yet, enqueue batch
        case batch => batch.add(runnable) // If we are already in batching mode, add to batch
      }
    } else unbatchedExecute(runnable) // If not batchable, just delegate to underlying
  }

  /** Override this to define which runnables will be batched. */
  def batchable(runnable: Runnable): Boolean = akka.dispatch.internal.ScalaBatchable.isBatchable(runnable)
}
