fix: restore volatile reads/writes downgraded by the VarHandle migration

[He-Pin]

Motivation

The migration from sun.misc.Unsafe to VarHandle (#1990, and #1894 for Mailbox) replaced the producer writes correctly (compareAndSwapObject → compareAndSet, putOrderedObject → setRelease) but silently downgraded the corresponding reads (and two writes) from volatile to plain access:

• Unsafe.getObjectVolatile / getIntVolatile / getLongVolatile → VarHandle.get (plain)
• putObjectVolatile / putIntVolatile → VarHandle.set (plain)

VarHandle.get / set have plain memory semantics even when the field is declared volatile, so these accesses lost their happens-before guarantees with the concurrent compareAndSet / setRelease publications. On weakly-ordered hardware (AArch64) a reader can observe a stale value, and inside tight loops the plain read may be hoisted by the JIT. On x86-64 plain loads happen to have acquire semantics, which is why this largely went unnoticed.

Modification

Restore the original ordered semantics on every downgraded access:

• Mailbox: currentStatus / setStatus and systemQueueGet (mailbox status and system-message queue head).
• ActorCell: mailbox (Dispatch), childrenRefs / functionRefs reads and the setTerminated write (Children).
• RepointableActorRef: underlying / lookup cell reads.
• CircuitBreaker: currentState / currentResetTimeout reads.
• PromiseActorRef (AskSupport): state / watchedBy reads and the setState write.
• MessageDispatcher: inhabitants / shutdownSchedule reads.
• Artery Association: associationState read.

CAS-published fields use getVolatile / setVolatile, restoring the exact getObjectVolatile / putObjectVolatile semantics. For a load, getVolatile compiles to the same instruction as a plain load on x86-64 (MOV) and to LDAR on AArch64 — i.e. exactly what the original code emitted — so this restores the prior semantics at no extra cost on the read side. The two restored setVolatile writes carry a StoreLoad fence as they did before the migration.

The lock-free node-queue spin reads (AbstractNodeQueue / AbstractBoundedNodeQueue) are handled in a separate PR, as they pair with release stores and use getAcquire.

Result

All concurrently-accessed fields that were volatile before the migration are volatile again, closing latent visibility races — most importantly the mailbox status / system-message queue and the actor cell/mailbox references. Method signatures are unchanged, so MiMa is unaffected.

Tests

• sbt actor/compile remote/compile succeed.

References

• ForkJoinPool compensation-thread regression (JDK-8300995) causes test flakiness on JDK 21+ #2870
• nightly tests (CI): java 25 runs have a lot of stream test failures #2573

[Copilot]

Pull request overview

This PR restores the original volatile (JMM-ordered) read/write semantics that were unintentionally downgraded to plain accesses during the Unsafe → VarHandle migrations. This closes latent visibility races on weakly-ordered hardware by switching affected reads to getVolatile and two writes to setVolatile, matching the pre-migration get*Volatile / put*Volatile behavior.

Changes:

• Replace several VarHandle.get reads with VarHandle.getVolatile for fields published via compareAndSet / getAndAdd.
• Replace two VarHandle.set writes with VarHandle.setVolatile to restore prior volatile-store ordering.
• Add inline comments documenting the publication mechanism and why volatile access is required.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
remote/src/main/scala/org/apache/pekko/remote/artery/Association.scala	Restore volatile read of association shared state via getVolatile.
actor/src/main/scala/org/apache/pekko/pattern/CircuitBreaker.scala	Restore volatile reads of breaker state and reset timeout.
actor/src/main/scala/org/apache/pekko/pattern/AskSupport.scala	Restore volatile reads of PromiseActorRef state/watchers and a volatile state write.
actor/src/main/scala/org/apache/pekko/dispatch/Mailbox.scala	Restore volatile mailbox status read/write and system-message queue head read.
actor/src/main/scala/org/apache/pekko/dispatch/AbstractDispatcher.scala	Restore volatile reads of dispatcher inhabitants and shutdown schedule.
actor/src/main/scala/org/apache/pekko/actor/RepointableActorRef.scala	Restore volatile reads of cell / lookup references.
actor/src/main/scala/org/apache/pekko/actor/dungeon/Dispatch.scala	Restore volatile read of ActorCell mailbox reference.
actor/src/main/scala/org/apache/pekko/actor/dungeon/Children.scala	Restore volatile reads of children/function refs and a volatile terminal-state write.

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[pjfanning]

lgtm