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fix: keep lazy dispatch state on VarHandle #3116

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Merged
He-Pin merged 1 commit into from
Jun 21, 2026
Merged

fix: keep lazy dispatch state on VarHandle #3116

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18 changes: 7 additions & 11 deletions stream/src/main/scala/org/apache/pekko/stream/stage/GraphStage.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -13,8 +13,8 @@

package org.apache.pekko.stream.stage

import java.util.Spliterator
import java.lang.invoke.{ MethodHandles, VarHandle }
import java.util.Spliterator
import java.util.concurrent.{ CompletionStage, ConcurrentHashMap }
import java.util.concurrent.atomic.AtomicReference

Expand Down Expand Up @@ -359,9 +359,8 @@ object GraphStageLogic {
* object (one allocation per StageActor, one fewer field deref on the producer hot path).
* - Implements `Any => Unit` directly — serves as both the producer callback and the drain callback,
* eliminating the separate `drainCallback` lambda allocation.
* - `state` is a plain `@volatile var Int` driven by a static `VarHandle` in the companion object
* (via `MethodHandles.privateLookupIn`), same pattern as `AbstractNodeQueue` itself;
* avoids per-instance `AtomicInteger`.
* - `state` is a plain `@volatile var Int` driven by a static `VarHandle`; volatile state access
* keeps the election protocol conservative and avoids per-instance `AtomicInteger`.
* - `drainBatchSize` is read once into a stack-local at the top of `drain` so the JIT can treat the loop
* bound as a constant.
* - Per-tell allocation = 1 Node (`AbstractNodeQueue.Node`, ~24 bytes) + 1 Tuple2 (~24 bytes). The
Expand All @@ -378,18 +377,14 @@ object GraphStageLogic {
extends AbstractNodeQueue[(ActorRef, Any)]
with (Any => Unit) {

// IDLE/SCHEDULED election state. VarHandle avoids per-instance AtomicInteger;
// the handle lives in the companion object (true JVM static), same pattern as
// AbstractNodeQueue._tailDoNotCallMeDirectly.
// IDLE/SCHEDULED election state. VarHandle avoids per-instance AtomicInteger.
@volatile var state: Int = SchedStateIdle

// Typed local witnesses the Int-returning signature-polymorphic VarHandle.get overload
// (required for Scala 3 cross-compile; on Scala 2 this is a no-op).
private def getState(): Int = {
val v: Int = LazyDispatch.stateHandle.get(this)
val v: Int = LazyDispatch.stateHandle.getVolatile(this)
v
}
private def setState(v: Int): Unit = LazyDispatch.stateHandle.set(this, v)
private def setState(v: Int): Unit = LazyDispatch.stateHandle.setVolatile(this, v)
private def casState(expect: Int, update: Int): Boolean =
LazyDispatch.stateHandle.compareAndSet(this, expect, update)

Expand Down Expand Up @@ -448,6 +443,7 @@ object GraphStageLogic {
lookup.findVarHandle(classOf[LazyDispatch], "state", Integer.TYPE)
}
}

}

/**
Expand Down