pekko/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphInterpreter.scala

/**
 * Copyright (C) 2015-2016 Lightbend Inc. <http://www.lightbend.com>
 */
package akka.stream.impl.fusing

import java.util.Arrays
import akka.actor.ActorRef
import akka.event.LoggingAdapter
import akka.stream.stage._
import scala.annotation.tailrec
import scala.collection.immutable
import akka.stream._
import akka.stream.impl.StreamLayout._
import java.util.concurrent.ThreadLocalRandom
import scala.util.control.NonFatal
import java.{ util ⇒ ju }
import akka.stream.impl.fusing.GraphStages.MaterializedValueSource

/**
 * INTERNAL API
 *
 * (See the class for the documentation of the internals)
 */
object GraphInterpreter {
  /**
   * Compile time constant, enable it for debug logging to the console.
   */
  final val Debug = false

  final val NoEvent = null
  final val Boundary = -1

  final val InReady = 1
  final val Pulling = 2
  final val Pushing = 4
  final val OutReady = 8

  final val InClosed = 16
  final val OutClosed = 32
  final val InFailed = 64

  final val PullStartFlip = 3 // 0011
  final val PullEndFlip = 10 // 1010
  final val PushStartFlip = 12 //1100
  final val PushEndFlip = 5 //0101

  final val KeepGoingFlag = 0x4000000
  final val KeepGoingMask = 0x3ffffff

  /**
   * Marker object that indicates that a port holds no element since it was already grabbed. The port is still pullable,
   * but there is no more element to grab.
   */
  case object Empty
  final case class Failed(ex: Throwable, previousElem: Any)

  abstract class UpstreamBoundaryStageLogic[T] extends GraphStageLogic(inCount = 0, outCount = 1) {
    def out: Outlet[T]
  }

  abstract class DownstreamBoundaryStageLogic[T] extends GraphStageLogic(inCount = 1, outCount = 0) {
    def in: Inlet[T]
  }

  val singleNoAttribute: Array[Attributes] = Array(Attributes.none)

  /**
   * INERNAL API
   *
   * Contains all the necessary information for the GraphInterpreter to be able to implement a connection
   * between an output and input ports.
   *
   * @param id Identifier of the connection. Corresponds to the array slot in the [[GraphAssembly]]
   * @param inOwnerId Identifier of the owner of the input side of the connection. Corresponds to the array slot in
   *                  the [[GraphAssembly]]
   * @param inOwner The stage logic that corresponds to the input side of the connection.
   * @param outOwnerId Identifier of the owner of the output side of the connection. Corresponds to the array slot
   *                   in the [[GraphAssembly]]
   * @param outOwner The stage logic that corresponds to the output side of the connection.
   * @param inHandler The handler that contains the callback for input events.
   * @param outHandler The handler that contains the callback for output events.
   */
  final class Connection(
    val id:         Int,
    val inOwnerId:  Int,
    val inOwner:    GraphStageLogic,
    val outOwnerId: Int,
    val outOwner:   GraphStageLogic,
    var inHandler:  InHandler,
    var outHandler: OutHandler
  ) {
    var portState: Int = InReady
    var slot: Any = Empty

    override def toString = s"Connection($id, $portState, $slot, $inHandler, $outHandler)"
  }

  /**
   * INTERNAL API
   *
   * A GraphAssembly represents a small stream processing graph to be executed by the interpreter. Instances of this
   * class **must not** be mutated after construction.
   *
   * The array ``originalAttributes`` may contain the attribute information of the original atomic module, otherwise
   * it must contain a none (otherwise the enclosing module could not overwrite attributes defined in this array).
   *
   * The arrays [[ins]] and [[outs]] correspond to the notion of a *connection* in the [[GraphInterpreter]]. Each slot
   * *i* contains the input and output port corresponding to connection *i*. Slots where the graph is not closed (i.e.
   * ports are exposed to the external world) are marked with *null* values. For example if an input port *p* is
   * exposed, then outs(p) will contain a *null*.
   *
   * The arrays [[inOwners]] and [[outOwners]] are lookup tables from a connection id (the index of the slot)
   * to a slot in the [[stages]] array, indicating which stage is the owner of the given input or output port.
   * Slots which would correspond to non-existent stages (where the corresponding port is null since it represents
   * the currently unknown external context) contain the value [[GraphInterpreter#Boundary]].
   *
   * The current assumption by the infrastructure is that the layout of these arrays looks like this:
   *
   *            +---------------------------------------+-----------------+
   * inOwners:  | index to stages array                 | Boundary (-1)   |
   *            +----------------+----------------------+-----------------+
   * ins:       | exposed inputs | internal connections | nulls           |
   *            +----------------+----------------------+-----------------+
   * outs:      | nulls          | internal connections | exposed outputs |
   *            +----------------+----------------------+-----------------+
   * outOwners: | Boundary (-1)  | index to stages array                  |
   *            +----------------+----------------------------------------+
   *
   * In addition, it is also assumed by the infrastructure that the order of exposed inputs and outputs in the
   * corresponding segments of these arrays matches the exact same order of the ports in the [[Shape]].
   *
   */
  final class GraphAssembly(
    val stages:             Array[GraphStageWithMaterializedValue[Shape, Any]],
    val originalAttributes: Array[Attributes],
    val ins:                Array[Inlet[_]],
    val inOwners:           Array[Int],
    val outs:               Array[Outlet[_]],
    val outOwners:          Array[Int]) {
    require(ins.length == inOwners.length && inOwners.length == outs.length && outs.length == outOwners.length)

    def connectionCount: Int = ins.length

    /**
     * Takes an interpreter and returns three arrays required by the interpreter containing the input, output port
     * handlers and the stage logic instances.
     *
     * Returns a tuple of
     *  - lookup table for Connections
     *  - array of the logics
     */
    def materialize(
      inheritedAttributes: Attributes,
      copiedModules:       Array[Module],
      matVal:              ju.Map[Module, Any],
      register:            MaterializedValueSource[Any] ⇒ Unit): (Array[Connection], Array[GraphStageLogic]) = {
      val logics = Array.ofDim[GraphStageLogic](stages.length)

      var i = 0
      while (i < stages.length) {
        // Port initialization loops, these must come first
        val shape = stages(i).shape

        var idx = 0
        val inletItr = shape.inlets.iterator
        while (inletItr.hasNext) {
          val inlet = inletItr.next()
          require(inlet.id == -1 || inlet.id == idx, s"Inlet $inlet was shared among multiple stages. This is illegal.")
          inlet.id = idx
          idx += 1
        }

        idx = 0
        val outletItr = shape.outlets.iterator
        while (outletItr.hasNext) {
          val outlet = outletItr.next()
          require(outlet.id == -1 || outlet.id == idx, s"Outlet $outlet was shared among multiple stages. This is illegal.")
          outlet.id = idx
          idx += 1
        }

        val stage = stages(i) match {
          case mv: MaterializedValueSource[_] ⇒
            val copy = mv.copySrc.asInstanceOf[MaterializedValueSource[Any]]
            register(copy)
            copy
          case x ⇒ x
        }

        val logicAndMat = stage.createLogicAndMaterializedValue(inheritedAttributes and originalAttributes(i))
        matVal.put(copiedModules(i), logicAndMat._2)

        logics(i) = logicAndMat._1
        i += 1
      }

      val connections = Array.ofDim[Connection](connectionCount)

      i = 0
      while (i < connectionCount) {
        connections(i) = new Connection(
          id = i,
          inOwner = if (inOwners(i) == Boundary) null else logics(inOwners(i)),
          inOwnerId = inOwners(i),
          outOwner = if (outOwners(i) == Boundary) null else logics(outOwners(i)),
          outOwnerId = outOwners(i),
          inHandler = null,
          outHandler = null
        )

        if (ins(i) ne null) {
          val logic = logics(inOwners(i))
          logic.handlers(ins(i).id) match {
            case null ⇒ throw new IllegalStateException(s"no handler defined in stage $logic for port ${ins(i)}")
            case h: InHandler ⇒
              connections(i).inHandler = h
          }
          logics(inOwners(i)).portToConn(ins(i).id) = connections(i)
        }
        if (outs(i) ne null) {
          val logic = logics(outOwners(i))
          val inCount = logic.inCount
          logic.handlers(outs(i).id + inCount) match {
            case null ⇒ throw new IllegalStateException(s"no handler defined in stage $logic for port ${outs(i)}")
            case h: OutHandler ⇒
              connections(i).outHandler = h
          }
          logic.portToConn(outs(i).id + inCount) = connections(i)
        }
        i += 1
      }

      (connections, logics)
    }

    override def toString: String = {
      val stageList = stages.iterator.zip(originalAttributes.iterator).map {
        case (stage, attr) ⇒ s"${stage.module}    [${attr.attributeList.mkString(", ")}]"
      }
      "GraphAssembly\n  " +
        stageList.mkString("[ ", "\n    ", "\n  ]") + "\n  " +
        ins.mkString("[", ",", "]") + "\n  " +
        inOwners.mkString("[", ",", "]") + "\n  " +
        outs.mkString("[", ",", "]") + "\n  " +
        outOwners.mkString("[", ",", "]")
    }
  }

  object GraphAssembly {
    /**
     * INTERNAL API
     */
    final def apply(
      inlets:  immutable.Seq[Inlet[_]],
      outlets: immutable.Seq[Outlet[_]],
      stages:  GraphStageWithMaterializedValue[Shape, _]*): GraphAssembly = {
      // add the contents of an iterator to an array starting at idx
      @tailrec def add[T](i: Iterator[T], a: Array[T], idx: Int): Array[T] =
        if (i.hasNext) {
          a(idx) = i.next()
          add(i, a, idx + 1)
        } else a

      // fill array slots with Boundary
      def markBoundary(owners: Array[Int], from: Int, to: Int): Array[Int] = {
        Arrays.fill(owners, from, to, Boundary)
        owners
      }

      val inletsSize = inlets.size
      val outletsSize = outlets.size
      val connectionCount = inletsSize + outletsSize
      require(connectionCount > 0, s"sum of inlets ({$inletsSize}) & outlets ({$outletsSize}) must be > 0")

      val assembly = new GraphAssembly(
        stages.toArray,
        GraphInterpreter.singleNoAttribute,
        add(inlets.iterator, Array.ofDim(connectionCount), 0),
        markBoundary(Array.ofDim(connectionCount), inletsSize, connectionCount),
        add(outlets.iterator, Array.ofDim(connectionCount), inletsSize),
        markBoundary(Array.ofDim(connectionCount), 0, inletsSize))

      assembly
    }
  }

  /**
   * INTERNAL API
   */
  private val _currentInterpreter = new ThreadLocal[Array[AnyRef]] {
    /*
     * Using an Object-array avoids holding on to the GraphInterpreter class
     * when this accidentally leaks onto threads that are not stopped when this
     * class should be unloaded.
     */
    override def initialValue = new Array(1)
  }

  /**
   * INTERNAL API
   */
  private[akka] def currentInterpreter: GraphInterpreter =
    _currentInterpreter.get()(0).asInstanceOf[GraphInterpreter].nonNull
  // nonNull is just a debug helper to find nulls more timely

  /**
   * INTERNAL API
   */
  private[akka] def currentInterpreterOrNull: GraphInterpreter =
    _currentInterpreter.get()(0).asInstanceOf[GraphInterpreter]

}

/**
 * INTERNAL API
 *
 * From an external viewpoint, the GraphInterpreter takes an assembly of graph processing stages encoded as a
 * [[GraphInterpreter#GraphAssembly]] object and provides facilities to execute and interact with this assembly.
 * The lifecylce of the Interpreter is roughly the following:
 *  - Boundary logics are attached via [[attachDownstreamBoundary()]] and [[attachUpstreamBoundary()]]
 *  - [[init()]] is called
 *  - [[execute()]] is called whenever there is need for execution, providing an upper limit on the processed events
 *  - [[finish()]] is called before the interpreter is disposed, preferably after [[isCompleted]] returned true, although
 *    in abort cases this is not strictly necessary
 *
 * The [[execute()]] method of the interpreter accepts an upper bound on the events it will process. After this limit
 * is reached or there are no more pending events to be processed, the call returns. It is possible to inspect
 * if there are unprocessed events left via the [[isSuspended]] method. [[isCompleted]] returns true once all stages
 * reported completion inside the interpreter.
 *
 * The internal architecture of the interpreter is based on the usage of arrays and optimized for reducing allocations
 * on the hot paths.
 *
 * One of the basic abstractions inside the interpreter is the [[akka.stream.impl.fusing.GraphInterpreter.Connection]].
 * A connection represents an output-input port pair (an analogue for a connected RS Publisher-Subscriber pair).
 * The Connection object contains all the necessary data for the interpreter to pass elements, demand, completion
 * or errors across the Connection.
 * In particular
 *  - portStates contains a bitfield that tracks the states of the ports (output-input) corresponding to this
 *    connection. This bitfield is used to decode the event that is in-flight.
 *  - connectionSlot contains a potential element or exception that accompanies the
 *    event encoded in the portStates bitfield
 *  - inHandler contains the [[InHandler]] instance that handles the events corresponding
 *    to the input port of the connection
 *  - outHandler contains the [[OutHandler]] instance that handles the events corresponding
 *    to the output port of the connection
 *
 * On top of the Connection table there is an eventQueue, represented as a circular buffer of Connections. The queue
 * contains the Connections that have pending events to be processed. The pending event itself is encoded
 * in the portState bitfield of the Connection. This implies that there can be only one event in flight for a given
 * Connection, which is true in almost all cases, except a complete-after-push or fail-after-push which has to
 * be decoded accordingly.
 *
 * The layout of the portState bitfield is the following:
 *
 *             |- state machn.-| Only one bit is hot among these bits
 *  64  32  16 | 8   4   2   1 |
 * +---+---+---|---+---+---+---|
 *   |   |   |   |   |   |   |
 *   |   |   |   |   |   |   |  From the following flags only one is active in any given time. These bits encode
 *   |   |   |   |   |   |   |  state machine states, and they are "moved" around using XOR masks to keep other bits
 *   |   |   |   |   |   |   |  intact.
 *   |   |   |   |   |   |   |
 *   |   |   |   |   |   |   +- InReady:  The input port is ready to be pulled
 *   |   |   |   |   |   +----- Pulling:  A pull is active, but have not arrived yet (queued)
 *   |   |   |   |   +--------- Pushing:  A push is active, but have not arrived yet (queued)
 *   |   |   |   +------------- OutReady: The output port is ready to be pushed
 *   |   |   |
 *   |   |   +----------------- InClosed:  The input port is closed and will not receive any events.
 *   |   |                                 A push might be still in flight which will be then processed first.
 *   |   +--------------------- OutClosed: The output port is closed and will not receive any events.
 *   +------------------------- InFailed:  Always set in conjunction with InClosed. Indicates that the close event
 *                                         is a failure
 *
 * Sending an event is usually the following sequence:
 *  - An action is requested by a stage logic (push, pull, complete, etc.)
 *  - the state machine in portStates is transitioned from a ready state to a pending event
 *  - the affected Connection is enqueued
 *
 * Receiving an event is usually the following sequence:
 *  - the Connection to be processed is dequeued
 *  - the type of the event is determined from the bits set on portStates
 *  - the state machine in portStates is transitioned to a ready state
 *  - using the inHandlers/outHandlers table the corresponding callback is called on the stage logic.
 *
 * Because of the FIFO construction of the queue the interpreter is fair, i.e. a pending event is always executed
 * after a bounded number of other events. This property, together with suspendability means that even infinite cycles can
 * be modeled, or even dissolved (if preempted and a "stealing" external event is injected; for example the non-cycle
 * edge of a balance is pulled, dissolving the original cycle).
 */
final class GraphInterpreter(
  private val assembly: GraphInterpreter.GraphAssembly,
  val materializer:     Materializer,
  val log:              LoggingAdapter,
  val logics:           Array[GraphStageLogic], // Array of stage logics
  val connections:      Array[GraphInterpreter.Connection],
  val onAsyncInput:     (GraphStageLogic, Any, (Any) ⇒ Unit) ⇒ Unit,
  val fuzzingMode:      Boolean,
  val context:          ActorRef) {
  import GraphInterpreter._

  private[this] val ChaseLimit = if (fuzzingMode) 0 else 16

  /**
   * INTERNAL API
   */
  private[stream] var activeStage: GraphStageLogic = _

  // The number of currently running stages. Once this counter reaches zero, the interpreter is considered to be
  // completed
  private[this] var runningStages = assembly.stages.length

  // Counts how many active connections a stage has. Once it reaches zero, the stage is automatically stopped.
  private[this] val shutdownCounter = Array.tabulate(assembly.stages.length) { i ⇒
    val shape = assembly.stages(i).shape
    shape.inlets.size + shape.outlets.size
  }

  private[this] var _subFusingMaterializer: Materializer = _
  def subFusingMaterializer: Materializer = _subFusingMaterializer

  // An event queue implemented as a circular buffer
  // FIXME: This calculates the maximum size ever needed, but most assemblies can run on a smaller queue
  private[this] val eventQueue = Array.ofDim[Connection](1 << (32 - Integer.numberOfLeadingZeros(assembly.connectionCount - 1)))
  private[this] val mask = eventQueue.length - 1
  private[this] var queueHead: Int = 0
  private[this] var queueTail: Int = 0

  private[this] var chaseCounter = 0 // the first events in preStart blocks should be not chased
  private[this] var chasedPush: Connection = NoEvent
  private[this] var chasedPull: Connection = NoEvent

  private def queueStatus: String = {
    val contents = (queueHead until queueTail).map(idx ⇒ {
      val conn = eventQueue(idx & mask)
      conn
    })
    s"(${eventQueue.length}, $queueHead, $queueTail)(${contents.mkString(", ")})"
  }
  private[this] var _Name: String = _
  def Name: String =
    if (_Name eq null) {
      _Name = f"${System.identityHashCode(this)}%08X"
      _Name
    } else _Name

  /**
   * INTERNAL API
   */
  private[stream] def nonNull: GraphInterpreter = this

  /**
   * Assign the boundary logic to a given connection. This will serve as the interface to the external world
   * (outside the interpreter) to process and inject events.
   */
  def attachUpstreamBoundary(connection: Connection, logic: UpstreamBoundaryStageLogic[_]): Unit = {
    logic.portToConn(logic.out.id + logic.inCount) = connection
    logic.interpreter = this
    connection.outHandler = logic.handlers(0).asInstanceOf[OutHandler]
  }

  def attachUpstreamBoundary(connection: Int, logic: UpstreamBoundaryStageLogic[_]): Unit =
    attachUpstreamBoundary(connections(connection), logic)

  /**
   * Assign the boundary logic to a given connection. This will serve as the interface to the external world
   * (outside the interpreter) to process and inject events.
   */
  def attachDownstreamBoundary(connection: Connection, logic: DownstreamBoundaryStageLogic[_]): Unit = {
    logic.portToConn(logic.in.id) = connection
    logic.interpreter = this
    connection.inHandler = logic.handlers(0).asInstanceOf[InHandler]
  }

  def attachDownstreamBoundary(connection: Int, logic: DownstreamBoundaryStageLogic[_]): Unit =
    attachDownstreamBoundary(connections(connection), logic)

  /**
   * Dynamic handler changes are communicated from a GraphStageLogic by this method.
   */
  def setHandler(connection: Connection, handler: InHandler): Unit = {
    if (Debug) println(s"$Name SETHANDLER ${inOwnerName(connection)} (in) $handler")
    connection.inHandler = handler
  }

  /**
   * Dynamic handler changes are communicated from a GraphStageLogic by this method.
   */
  def setHandler(connection: Connection, handler: OutHandler): Unit = {
    if (Debug) println(s"$Name SETHANDLER ${outOwnerName(connection)} (out) $handler")
    connection.outHandler = handler
  }

  /**
   * Returns true if there are pending unprocessed events in the event queue.
   */
  def isSuspended: Boolean = queueHead != queueTail

  /**
   * Returns true if there are no more running stages and pending events.
   */
  def isCompleted: Boolean = runningStages == 0 && !isSuspended

  /**
   * Initializes the states of all the stage logics by calling preStart().
   * The passed-in materializer is intended to be a SubFusingActorMaterializer
   * that avoids creating new Actors when stages materialize sub-flows. If no
   * such materializer is available, passing in `null` will reuse the normal
   * materializer for the GraphInterpreter—fusing is only an optimization.
   */
  def init(subMat: Materializer): Unit = {
    _subFusingMaterializer = if (subMat == null) materializer else subMat
    var i = 0
    while (i < logics.length) {
      val logic = logics(i)
      logic.stageId = i
      logic.interpreter = this
      try {
        logic.beforePreStart()
        logic.preStart()
      } catch {
        case NonFatal(e) ⇒
          log.error(e, "Error during preStart in [{}]", assembly.stages(logic.stageId))
          logic.failStage(e)
      }
      afterStageHasRun(logic)
      i += 1
    }
  }

  /**
   * Finalizes the state of all stages by calling postStop() (if necessary).
   */
  def finish(): Unit = {
    var i = 0
    while (i < logics.length) {
      val logic = logics(i)
      if (!isStageCompleted(logic)) finalizeStage(logic)
      i += 1
    }
  }

  // Debug name for a connections input part
  private def inOwnerName(connection: Connection): String =
    assembly.inOwners(connection.id) match {
      case Boundary ⇒ "DownstreamBoundary"
      case owner    ⇒ assembly.stages(owner).toString
    }

  // Debug name for a connections output part
  private def outOwnerName(connection: Connection): String =
    assembly.outOwners(connection.id) match {
      case Boundary ⇒ "UpstreamBoundary"
      case owner    ⇒ assembly.stages(owner).toString
    }

  // Debug name for a connections input part
  private def inLogicName(connection: Connection): String =
    assembly.inOwners(connection.id) match {
      case Boundary ⇒ "DownstreamBoundary"
      case owner    ⇒ logics(owner).toString
    }

  // Debug name for a connections output part
  private def outLogicName(connection: Connection): String =
    assembly.outOwners(connection.id) match {
      case Boundary ⇒ "UpstreamBoundary"
      case owner    ⇒ logics(owner).toString
    }

  private def shutdownCounters: String =
    shutdownCounter.map(x ⇒ if (x >= KeepGoingFlag) s"${x & KeepGoingMask}(KeepGoing)" else x.toString).mkString(",")

  /**
   * Executes pending events until the given limit is met. If there were remaining events, isSuspended will return
   * true.
   */
  def execute(eventLimit: Int): Int = {
    if (Debug) println(s"$Name ---------------- EXECUTE $queueStatus (running=$runningStages, shutdown=$shutdownCounters)")
    val currentInterpreterHolder = _currentInterpreter.get()
    val previousInterpreter = currentInterpreterHolder(0)
    currentInterpreterHolder(0) = this
    var eventsRemaining = eventLimit
    try {
      while (eventsRemaining > 0 && queueTail != queueHead) {
        val connection = dequeue()
        eventsRemaining -= 1
        chaseCounter = math.min(ChaseLimit, eventsRemaining)

        def reportStageError(e: Throwable): Unit = {
          if (activeStage == null) throw e
          else {
            val stage = assembly.stages(activeStage.stageId)

            log.error(e, "Error in stage [{}]: {}", stage, e.getMessage)
            activeStage.failStage(e)

            // Abort chasing
            chaseCounter = 0
            if (chasedPush ne NoEvent) {
              enqueue(chasedPush)
              chasedPush = NoEvent
            }
            if (chasedPull ne NoEvent) {
              enqueue(chasedPull)
              chasedPull = NoEvent
            }
          }
        }

        /*
         * This is the "normal" event processing code which dequeues directly from the internal event queue. Since
         * most execution paths tend to produce either a Push that will be propagated along a longer chain we take
         * extra steps below to make this more efficient.
         */
        try processEvent(connection)
        catch {
          case NonFatal(e) ⇒ reportStageError(e)
        }
        afterStageHasRun(activeStage)

        /*
         * "Event chasing" optimization follows from here. This optimization works under the assumption that a Push or
         * Pull is very likely immediately followed by another Push/Pull. The difference from the "normal" event
         * dispatch is that chased events are never touching the event queue, they use a "streamlined" execution path
         * instead. Looking at the scenario of a Push, the following events will happen.
         *  - "normal" dispatch executes an onPush event
         *  - stage eventually calls push()
         *  - code inside the push() method checks the validity of the call, and also if it can be safely ignored
         *    (because the target stage already completed we just have not been notified yet)
         *  - if the upper limit of ChaseLimit has not been reached, then the Connection is put into the chasedPush
         *    variable
         *  - the loop below immediately captures this push and dispatches it
         *
         * What is saved by this optimization is three steps:
         *  - no need to enqueue the Connection in the queue (array), it ends up in a simple variable, reducing
         *    pressure on array load-store
         *  - no need to dequeue the Connection from the queue, similar to above
         *  - no need to decode the event, we know it is a Push already
         *  - no need to check for validity of the event because we already checked at the push() call, and there
         *    can be no concurrent events interleaved unlike with the normal dispatch (think about a cancel() that is
         *    called in the target stage just before the onPush() arrives). This avoids unnecessary branching.
         */

        // Chasing PUSH events
        while (chasedPush != NoEvent) {
          val connection = chasedPush
          chasedPush = NoEvent
          try processPush(connection)
          catch {
            case NonFatal(e) ⇒ reportStageError(e)
          }
          afterStageHasRun(activeStage)
        }

        // Chasing PULL events
        while (chasedPull != NoEvent) {
          val connection = chasedPull
          chasedPull = NoEvent
          try processPull(connection)
          catch {
            case NonFatal(e) ⇒ reportStageError(e)
          }
          afterStageHasRun(activeStage)
        }

        if (chasedPush != NoEvent) {
          enqueue(chasedPush)
          chasedPush = NoEvent
        }

      }
      // Event *must* be enqueued while not in the execute loop (events enqueued from external, possibly async events)
      chaseCounter = 0
    } finally {
      currentInterpreterHolder(0) = previousInterpreter
    }
    if (Debug) println(s"$Name ---------------- $queueStatus (running=$runningStages, shutdown=$shutdownCounters)")
    // TODO: deadlock detection
    eventsRemaining
  }

  def runAsyncInput(logic: GraphStageLogic, evt: Any, handler: (Any) ⇒ Unit): Unit =
    if (!isStageCompleted(logic)) {
      if (GraphInterpreter.Debug) println(s"$Name ASYNC $evt ($handler) [$logic]")
      val currentInterpreterHolder = _currentInterpreter.get()
      val previousInterpreter = currentInterpreterHolder(0)
      currentInterpreterHolder(0) = this
      try {
        activeStage = logic
        try handler(evt)
        catch {
          case NonFatal(ex) ⇒ logic.failStage(ex)
        }
        afterStageHasRun(logic)
      } finally currentInterpreterHolder(0) = previousInterpreter
    }

  // Decodes and processes a single event for the given connection
  private def processEvent(connection: Connection): Unit = {

    // this must be the state after returning without delivering any signals, to avoid double-finalization of some unlucky stage
    // (this can happen if a stage completes voluntarily while connection close events are still queued)
    activeStage = null
    val code = connection.portState

    // Manual fast decoding, fast paths are PUSH and PULL
    //   PUSH
    if ((code & (Pushing | InClosed | OutClosed)) == Pushing) {
      processPush(connection)

      // PULL
    } else if ((code & (Pulling | OutClosed | InClosed)) == Pulling) {
      processPull(connection)

      // CANCEL
    } else if ((code & (OutClosed | InClosed)) == InClosed) {
      activeStage = connection.outOwner
      if (Debug) println(s"$Name CANCEL ${inOwnerName(connection)} -> ${outOwnerName(connection)} (${connection.outHandler}) [${outLogicName(connection)}]")
      connection.portState |= OutClosed
      completeConnection(connection.outOwnerId)
      connection.outHandler.onDownstreamFinish()
    } else if ((code & (OutClosed | InClosed)) == OutClosed) {
      // COMPLETIONS

      if ((code & Pushing) == 0) {
        // Normal completion (no push pending)
        if (Debug) println(s"$Name COMPLETE ${outOwnerName(connection)} -> ${inOwnerName(connection)} (${connection.inHandler}) [${inLogicName(connection)}]")
        connection.portState |= InClosed
        activeStage = connection.inOwner
        completeConnection(connection.inOwnerId)
        if ((connection.portState & InFailed) == 0) connection.inHandler.onUpstreamFinish()
        else connection.inHandler.onUpstreamFailure(connection.slot.asInstanceOf[Failed].ex)
      } else {
        // Push is pending, first process push, then re-enqueue closing event
        processPush(connection)
        enqueue(connection)
      }

    }
  }

  private def processPush(connection: Connection): Unit = {
    if (Debug) println(s"$Name PUSH ${outOwnerName(connection)} -> ${inOwnerName(connection)}, ${connection.slot} (${connection.inHandler}) [${inLogicName(connection)}]")
    activeStage = connection.inOwner
    connection.portState ^= PushEndFlip
    connection.inHandler.onPush()
  }

  private def processPull(connection: Connection): Unit = {
    if (Debug) println(s"$Name PULL ${inOwnerName(connection)} -> ${outOwnerName(connection)} (${connection.outHandler}) [${outLogicName(connection)}]")
    activeStage = connection.outOwner
    connection.portState ^= PullEndFlip
    connection.outHandler.onPull()
  }

  private def dequeue(): Connection = {
    val idx = queueHead & mask
    if (fuzzingMode) {
      val swapWith = (ThreadLocalRandom.current.nextInt(queueTail - queueHead) + queueHead) & mask
      val ev = eventQueue(swapWith)
      eventQueue(swapWith) = eventQueue(idx)
      eventQueue(idx) = ev
    }
    val elem = eventQueue(idx)
    eventQueue(idx) = NoEvent
    queueHead += 1
    elem
  }

  def enqueue(connection: Connection): Unit = {
    if (Debug) if (queueTail - queueHead > mask) new Exception(s"$Name internal queue full ($queueStatus) + $connection").printStackTrace()
    eventQueue(queueTail & mask) = connection
    queueTail += 1
  }

  def afterStageHasRun(logic: GraphStageLogic): Unit =
    if (isStageCompleted(logic)) {
      runningStages -= 1
      finalizeStage(logic)
    }

  // Returns true if the given stage is already completed
  def isStageCompleted(stage: GraphStageLogic): Boolean = stage != null && shutdownCounter(stage.stageId) == 0

  // Register that a connection in which the given stage participated has been completed and therefore the stage
  // itself might stop, too.
  private def completeConnection(stageId: Int): Unit = {
    if (stageId != Boundary) {
      val activeConnections = shutdownCounter(stageId)
      if (activeConnections > 0) shutdownCounter(stageId) = activeConnections - 1
    }
  }

  private[stream] def setKeepGoing(logic: GraphStageLogic, enabled: Boolean): Unit =
    if (enabled) shutdownCounter(logic.stageId) |= KeepGoingFlag
    else shutdownCounter(logic.stageId) &= KeepGoingMask

  private def finalizeStage(logic: GraphStageLogic): Unit = {
    try {
      logic.postStop()
      logic.afterPostStop()
    } catch {
      case NonFatal(e) ⇒
        log.error(e, s"Error during postStop in [{}]: {}", assembly.stages(logic.stageId), e.getMessage)
    }
  }

  private[stream] def chasePush(connection: Connection): Unit = {
    if (chaseCounter > 0 && chasedPush == NoEvent) {
      chaseCounter -= 1
      chasedPush = connection
    } else enqueue(connection)
  }

  private[stream] def chasePull(connection: Connection): Unit = {
    if (chaseCounter > 0 && chasedPull == NoEvent) {
      chaseCounter -= 1
      chasedPull = connection
    } else enqueue(connection)
  }

  private[stream] def complete(connection: Connection): Unit = {
    val currentState = connection.portState
    if (Debug) println(s"$Name   complete($connection) [$currentState]")
    connection.portState = currentState | OutClosed

    // Push-Close needs special treatment, cannot be chased, convert back to ordinary event
    if (chasedPush == connection) {
      chasedPush = NoEvent
      enqueue(connection)
    } else if ((currentState & (InClosed | Pushing | Pulling | OutClosed)) == 0) enqueue(connection)

    if ((currentState & OutClosed) == 0) completeConnection(connection.outOwnerId)
  }

  private[stream] def fail(connection: Connection, ex: Throwable): Unit = {
    val currentState = connection.portState
    if (Debug) println(s"$Name   fail($connection, $ex) [$currentState]")
    connection.portState = currentState | OutClosed
    if ((currentState & (InClosed | OutClosed)) == 0) {
      connection.portState = currentState | (OutClosed | InFailed)
      connection.slot = Failed(ex, connection.slot)
      if ((currentState & (Pulling | Pushing)) == 0) enqueue(connection)
      else if (chasedPush eq connection) {
        // Abort chasing so Failure is not lost (chasing does NOT decode the event but assumes it to be a PUSH
        // but we just changed the event!)
        chasedPush = NoEvent
        enqueue(connection)
      }
    }
    if ((currentState & OutClosed) == 0) completeConnection(connection.outOwnerId)
  }

  private[stream] def cancel(connection: Connection): Unit = {
    val currentState = connection.portState
    if (Debug) println(s"$Name   cancel($connection) [$currentState]")
    connection.portState = currentState | InClosed
    if ((currentState & OutClosed) == 0) {
      connection.slot = Empty
      if ((currentState & (Pulling | Pushing | InClosed)) == 0) enqueue(connection)
      else if (chasedPull eq connection) {
        // Abort chasing so Cancel is not lost (chasing does NOT decode the event but assumes it to be a PULL
        // but we just changed the event!)
        chasedPull = NoEvent
        enqueue(connection)
      }
    }
    if ((currentState & InClosed) == 0) completeConnection(connection.inOwnerId)
  }

  /**
   * Debug utility to dump the "waits-on" relationships in DOT format to the console for analysis of deadlocks.
   *
   * Only invoke this after the interpreter completely settled, otherwise the results might be off. This is a very
   * simplistic tool, make sure you are understanding what you are doing and then it will serve you well.
   */
  def dumpWaits(): Unit = println(toString)

  override def toString: String = {
    val builder = new StringBuilder("digraph waits {\n")

    for (i ← assembly.stages.indices)
      builder.append(s"""N$i [label="${assembly.stages(i)}"]""" + "\n")

    def nameIn(port: Int): String = {
      val owner = assembly.inOwners(port)
      if (owner == Boundary) "Out" + port
      else "N" + owner
    }

    def nameOut(port: Int): String = {
      val owner = assembly.outOwners(port)
      if (owner == Boundary) "In" + port
      else "N" + owner
    }

    for (i ← connections.indices) {
      connections(i).portState match {
        case InReady ⇒
          builder.append(s"""  ${nameIn(i)} -> ${nameOut(i)} [label=shouldPull; color=blue]""")
        case OutReady ⇒
          builder.append(s"""  ${nameOut(i)} -> ${nameIn(i)} [label=shouldPush; color=red];""")
        case x if (x | InClosed | OutClosed) == (InClosed | OutClosed) ⇒
          builder.append(s"""  ${nameIn(i)} -> ${nameOut(i)} [style=dotted; label=closed dir=both];""")
        case _ ⇒
      }
      builder.append("\n")
    }

    builder.append("}\n")
    builder.append(s"// $queueStatus (running=$runningStages, shutdown=${shutdownCounter.mkString(",")})")
    builder.toString()
  }
}