chore: use AtomicReferenceArray to keep track of worker threads

For the most part the array of worker threads is read-only, however it
may be updated from any thread when a worker thread transitions to
blocking or back.

Previously, a separate atomic variable was used to introduce memory
barriers when accessing the array. This is discarded in favour of
AtomicReferenceArray that specifically manages access to elements of the
array.

The `getWorkerThreads` method is replaced by a more limited
`getWorkerThread(index: Int)` method. This is used by `WorkerThread`
when checking for blocked threads, and the new method can be used to
specifically access a single worker thread.

core/jvm/src/main/scala/cats/effect/unsafe/WorkStealingThreadPool.scala

@@ -42,7 +42,13 @@ import java.time.Instant
 import java.time.temporal.ChronoField
 import java.util.Comparator
 import java.util.concurrent.{ConcurrentSkipListSet, ThreadLocalRandom}
-import java.util.concurrent.atomic.{AtomicBoolean, AtomicInteger, AtomicLong, AtomicReference}
+import java.util.concurrent.atomic.{
+  AtomicBoolean,
+  AtomicInteger,
+  AtomicLong,
+  AtomicReference,
+  AtomicReferenceArray
+}
 
 import WorkStealingThreadPool._
 
@@ -84,7 +90,8 @@ private[effect] final class WorkStealingThreadPool[P <: AnyRef](
   /**
    * References to worker threads and their local queues.
    */
-  private[this] val workerThreads: Array[WorkerThread[P]] = new Array(threadCount)
+  private[this] val workerThreads: AtomicReferenceArray[WorkerThread[P]] =
+    new AtomicReferenceArray(threadCount)
   private[unsafe] val localQueues: Array[LocalQueue] = new Array(threadCount)
   private[unsafe] val sleepers: Array[TimerHeap] = new Array(threadCount)
   private[unsafe] val parkedSignals: Array[AtomicBoolean] = new Array(threadCount)
@@ -114,15 +121,6 @@ private[effect] final class WorkStealingThreadPool[P <: AnyRef](
     } else false
   }
 
-  /**
-   * Atomic variable for used for publishing changes to the references in the `workerThreads`
-   * array. Worker threads can be changed whenever blocking code is encountered on the pool.
-   * When a worker thread is about to block, it spawns a new worker thread that would replace
-   * it, transfers the local queue to it and proceeds to run the blocking code, after which it
-   * exits.
-   */
-  private[this] val workerThreadPublisher: AtomicBoolean = new AtomicBoolean(false)
-
   private[this] val externalQueue: ScalQueue[AnyRef] =
     new ScalQueue(threadCount << 2)
 
@@ -173,22 +171,19 @@ private[effect] final class WorkStealingThreadPool[P <: AnyRef](
           poller,
           this)
 
-      workerThreads(i) = thread
+      workerThreads.set(i, thread)
       i += 1
     }
 
-    // Publish the worker threads.
-    workerThreadPublisher.set(true)
-
     // Start the worker threads.
     i = 0
     while (i < threadCount) {
-      workerThreads(i).start()
+      workerThreads.get(i).start()
       i += 1
     }
   }
 
-  private[unsafe] def getWorkerThreads: Array[WorkerThread[P]] = workerThreads
+  private[unsafe] def getWorkerThread(index: Int): WorkerThread[P] = workerThreads.get(index)
 
   /**
    * Tries to steal work from other worker threads. This method does a linear search of the
@@ -321,8 +316,7 @@ private[effect] final class WorkStealingThreadPool[P <: AnyRef](
         // can only be replaced right before executing blocking code, at which
         // point it is already unparked and entering this code region is thus
         // impossible.
-        workerThreadPublisher.get()
-        val worker = workerThreads(index)
+        val worker = workerThreads.get(index)
         system.interrupt(worker, pollers(index))
         return true
       }
@@ -460,8 +454,7 @@ private[effect] final class WorkStealingThreadPool[P <: AnyRef](
    *   the new worker thread instance to be installed at the provided index
    */
   private[unsafe] def replaceWorker(index: Int, newWorker: WorkerThread[P]): Unit = {
-    workerThreads(index) = newWorker
-    workerThreadPublisher.lazySet(true)
+    workerThreads.lazySet(index, newWorker)
   }
 
   /**
@@ -563,7 +556,7 @@ private[effect] final class WorkStealingThreadPool[P <: AnyRef](
     var i = 0
     while (i < threadCount) {
       val localFibers = localQueues(i).snapshot().iterator.flatMap(r => captureTrace(r)).toMap
-      val worker = workerThreads(i)
+      val worker = workerThreads.get(i)
       val _ = parkedSignals(i).get()
       val active = Option(worker.active)
       map += (worker -> ((
@@ -707,12 +700,10 @@ private[effect] final class WorkStealingThreadPool[P <: AnyRef](
       // executed mostly in situations where the thread pool is shutting down in
       // the face of unhandled exceptions or as part of the whole JVM exiting.
 
-      workerThreadPublisher.get()
-
       // Send an interrupt signal to each of the worker threads.
       var i = 0
       while (i < threadCount) {
-        val workerThread = workerThreads(i)
+        val workerThread = workerThreads.get(i)
         if (workerThread ne currentThread) {
           workerThread.interrupt()
         }
@@ -725,7 +716,7 @@ private[effect] final class WorkStealingThreadPool[P <: AnyRef](
         case d => d.toNanos
       }
       while (i < threadCount && joinTimeout > 0) {
-        val workerThread = workerThreads(i)
+        val workerThread = workerThreads.get(i)
         if (workerThread ne currentThread) {
           val now = System.nanoTime()
           workerThread.join(joinTimeout / 1000000, (joinTimeout % 1000000).toInt)
@@ -738,7 +729,7 @@ private[effect] final class WorkStealingThreadPool[P <: AnyRef](
       i = 0
       var allClosed = true
       while (i < threadCount) {
-        val workerThread = workerThreads(i)
+        val workerThread = workerThreads.get(i)
         // only close the poller if it is safe to do so, leak otherwise ...
         if ((workerThread eq currentThread) || !workerThread.isAlive()) {
           system.closePoller(pollers(i))
@@ -838,8 +829,15 @@ private[effect] final class WorkStealingThreadPool[P <: AnyRef](
    * @return
    *   the number of asynchronously suspended fibers
    */
-  private[unsafe] def getSuspendedFiberCount(): Long =
-    workerThreads.map(_.getSuspendedFiberCount().toLong).sum
+  private[unsafe] def getSuspendedFiberCount(): Long = {
+    var sum = 0L
+    var i = 0
+    while (i < threadCount) {
+      sum += workerThreads.get(i).getSuspendedFiberCount().toLong
+      i += 1
+    }
+    sum
+  }
 }
 
 private object WorkStealingThreadPool {

core/jvm/src/main/scala/cats/effect/unsafe/WorkerThread.scala

@@ -553,7 +553,7 @@ private[effect] final class WorkerThread[P <: AnyRef](
             val idx = index
             val threadCount = pool.getWorkerThreadCount()
             val otherIdx = (idx + random.nextInt(threadCount - 1)) % threadCount
-            val thread = pool.getWorkerThreads(otherIdx)
+            val thread = pool.getWorkerThread(otherIdx)
             val state = thread.getState()
             val parked = thread.parked