From 42d566ecb1324fceb257565abdf7984361ae4855 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Sun, 9 Feb 2025 20:29:10 -0800
Subject: [PATCH 01/34] Optimize Promise

---
 .jvmopts                                      |   2 +
 .vscode/settings.json                         |   2 +-
 .../src/main/scala/zio/BenchmarkUtil.scala    |   9 +-
 .../main/scala/zio/PromiseBenchmarks.scala    |  59 +++++-
 .../src/test/scala/zio/FiberRuntimeSpec.scala |  21 ++
 .../zio/ZIOCompanionVersionSpecific.scala     |  26 +--
 .../zio/ZIOCompanionVersionSpecific.scala     |  26 +--
 core/shared/src/main/scala/zio/Promise.scala  | 191 +++++++-----------
 8 files changed, 174 insertions(+), 162 deletions(-)

diff --git a/.jvmopts b/.jvmopts
index 89c8775d9d2d..d8db65d6a4a8 100644
--- a/.jvmopts
+++ b/.jvmopts
@@ -1 +1,3 @@
 -Dcats.effect.stackTracingMode=full
+-Xmx8g
+-Xms8g
\ No newline at end of file
diff --git a/.vscode/settings.json b/.vscode/settings.json
index b4bdcc7fa826..052d27309198 100644
--- a/.vscode/settings.json
+++ b/.vscode/settings.json
@@ -7,5 +7,5 @@
   ],
   "files.watcherExclude": {
     "**/target": true
-  }
+}
 }
\ No newline at end of file
diff --git a/benchmarks/src/main/scala/zio/BenchmarkUtil.scala b/benchmarks/src/main/scala/zio/BenchmarkUtil.scala
index fb5d72171165..45b224363268 100644
--- a/benchmarks/src/main/scala/zio/BenchmarkUtil.scala
+++ b/benchmarks/src/main/scala/zio/BenchmarkUtil.scala
@@ -39,9 +39,12 @@ object BenchmarkUtil extends Runtime[Any] { self =>
     Unsafe.unsafe(implicit unsafe => rt.unsafe.run(zio).getOrThrowFiberFailure())
   }
 
+  override val unsafe = super.unsafe
+
   private object NoFiberRootsRuntime extends Runtime[Any] {
-    val environment  = Runtime.default.environment
-    val fiberRefs    = Runtime.default.fiberRefs
-    val runtimeFlags = RuntimeFlags(RuntimeFlag.CooperativeYielding, RuntimeFlag.Interruption)
+    override val unsafe = super.unsafe
+    val environment     = Runtime.default.environment
+    val fiberRefs       = Runtime.default.fiberRefs
+    val runtimeFlags    = RuntimeFlags(RuntimeFlag.CooperativeYielding, RuntimeFlag.Interruption)
   }
 }
diff --git a/benchmarks/src/main/scala/zio/PromiseBenchmarks.scala b/benchmarks/src/main/scala/zio/PromiseBenchmarks.scala
index 16ab08e815c3..cfb2547b4813 100644
--- a/benchmarks/src/main/scala/zio/PromiseBenchmarks.scala
+++ b/benchmarks/src/main/scala/zio/PromiseBenchmarks.scala
@@ -1,8 +1,11 @@
 package zio
 
 import cats.effect.kernel.Deferred
+import cats.syntax.traverse._
+import cats.instances.list._
 import cats.effect.unsafe.implicits.global
 import cats.effect.{IO => CIO}
+import cats.syntax.foldable._
 import org.openjdk.jmh.annotations.{Scope => JScope, _}
 import zio.BenchmarkUtil._
 
@@ -16,18 +19,19 @@ import java.util.concurrent.TimeUnit
 @Fork(value = 3)
 class PromiseBenchmarks {
 
-  val size = 100000
-
-  val ints: List[Int] = List.range(0, size)
+  val n            = 100000
+  val waiters: Int = 16
 
   @Benchmark
   def zioPromiseAwaitDone(): Unit = {
 
-    val io = ZIO.foreachDiscard(ints) { _ =>
-      Promise.make[Nothing, Unit].flatMap { promise =>
-        promise.succeed(()) *> promise.await
-      }
-    }
+    val io =
+      Promise
+        .make[Nothing, Unit]
+        .flatMap { promise =>
+          promise.succeed(()) *> promise.await
+        }
+        .repeatN(n)
 
     unsafeRun(io)
   }
@@ -35,11 +39,46 @@ class PromiseBenchmarks {
   @Benchmark
   def catsPromiseAwaitDone(): Unit = {
 
-    val io = catsForeachDiscard(List.range(1, size)) { _ =>
+    val io =
       Deferred[CIO, Unit].flatMap { promise =>
         promise.complete(()).flatMap(_ => promise.get)
+      }.replicateA_(n)
+
+    io.unsafeRunSync()
+  }
+
+  @Benchmark
+  def zioPromiseMultiAwaitDone(): Unit = {
+    def createWaiters(promise: Promise[Nothing, Unit]): ZIO[Any, Nothing, Seq[Fiber[Nothing, Unit]]] =
+      ZIO.foreach(Range(0, waiters))(_ => promise.await.forkDaemon)
+
+    val io = Promise
+      .make[Nothing, Unit]
+      .flatMap { promise =>
+        for {
+          fibers <- createWaiters(promise)
+          _      <- promise.done(Exit.unit)
+          _      <- ZIO.foreachDiscard(fibers)(_.join)
+        } yield ()
       }
-    }
+      .repeatN(1023)
+
+    unsafeRun(io)
+  }
+
+  @Benchmark
+  def catsPromiseMultiAwaitDone(): Unit = {
+    def createWaiters(promise: Deferred[CIO, Unit]): CIO[List[cats.effect.Fiber[CIO, Throwable, Unit]]] =
+      List.range(0, waiters).traverse(_ => promise.get.start)
+
+    val io =
+      Deferred[CIO, Unit].flatMap { promise =>
+        for {
+          fibers <- createWaiters(promise)
+          _      <- promise.complete(())
+          _      <- fibers.traverse_(_.join)
+        } yield ()
+      }.replicateA_(1023)
 
     io.unsafeRunSync()
   }
diff --git a/core-tests/shared/src/test/scala/zio/FiberRuntimeSpec.scala b/core-tests/shared/src/test/scala/zio/FiberRuntimeSpec.scala
index be5d6972efcc..ba6823d451d8 100644
--- a/core-tests/shared/src/test/scala/zio/FiberRuntimeSpec.scala
+++ b/core-tests/shared/src/test/scala/zio/FiberRuntimeSpec.scala
@@ -62,6 +62,27 @@ object FiberRuntimeSpec extends ZIOBaseSpec {
             }
         }
       }
+    ),
+    suite("async")(
+      test("async callback after interruption is ignored") {
+        ZIO.suspendSucceed {
+          val cb = Ref.unsafe.make[Option[ZIO[Any, Nothing, Unit] => Unit]](None)
+          val effect =
+            (ZIO.async[Any, Nothing, Unit] { (k: (ZIO[Any, Nothing, Unit] => Unit)) =>
+              cb.unsafe.set(Some(k))
+            } *> ZIO.never)
+
+          for {
+            fiber    <- effect.fork
+            _        <- fiber.interrupt
+            callback <- cb.get.some
+            _        <- ZIO.succeed(callback(ZIO.unit))
+            first    <- fiber.poll
+            _        <- ZIO.succeed(callback(ZIO.unit))
+            second   <- fiber.poll
+          } yield assertTrue(first == second) && assertTrue(first == None)
+        }
+      }
     )
   )
 
diff --git a/core/shared/src/main/scala-2/zio/ZIOCompanionVersionSpecific.scala b/core/shared/src/main/scala-2/zio/ZIOCompanionVersionSpecific.scala
index adcbec9e4fbe..0a0b20ca4879 100644
--- a/core/shared/src/main/scala-2/zio/ZIOCompanionVersionSpecific.scala
+++ b/core/shared/src/main/scala-2/zio/ZIOCompanionVersionSpecific.scala
@@ -4,6 +4,7 @@ import zio.ZIO.Async
 import zio.stacktracer.TracingImplicits.disableAutoTrace
 
 import java.io.IOException
+import java.util.concurrent.atomic.AtomicReference
 
 private[zio] trait ZIOCompanionVersionSpecific {
 
@@ -44,22 +45,15 @@ private[zio] trait ZIOCompanionVersionSpecific {
     blockingOn: => FiberId = FiberId.None
   )(implicit trace: Trace): ZIO[R, E, A] =
     ZIO.suspendSucceed {
-      val cancelerRef = new java.util.concurrent.atomic.AtomicReference[URIO[R, Any]](ZIO.unit)
-
-      ZIO
-        .Async[R, E, A](
-          trace,
-          { k =>
-            val result = register(k(_))
-
-            result match {
-              case Left(canceler) => cancelerRef.set(canceler); null.asInstanceOf[ZIO[R, E, A]]
-              case Right(done)    => done
-            }
-          },
-          () => blockingOn
-        )
-        .onInterrupt(cancelerRef.get())
+      val state = new AtomicReference[URIO[R, Any]](Exit.unit) with ((ZIO[R, E, A] => Unit) => ZIO[R, E, A]) {
+        def apply(k: ZIO[R, E, A] => Unit): ZIO[R, E, A] =
+          register(k(_)) match {
+            case Left(canceler) => set(canceler); null.asInstanceOf[ZIO[R, E, A]]
+            case Right(done)    => done
+          }
+      }
+
+      ZIO.Async[R, E, A](trace, state, () => blockingOn).onInterrupt(state.get())
     }
 
   /**
diff --git a/core/shared/src/main/scala-3/zio/ZIOCompanionVersionSpecific.scala b/core/shared/src/main/scala-3/zio/ZIOCompanionVersionSpecific.scala
index c9f766ea9953..4783107ec0b6 100644
--- a/core/shared/src/main/scala-3/zio/ZIOCompanionVersionSpecific.scala
+++ b/core/shared/src/main/scala-3/zio/ZIOCompanionVersionSpecific.scala
@@ -51,22 +51,16 @@ private[zio] transparent trait ZIOCompanionVersionSpecific {
     blockingOn: => FiberId = FiberId.None
   )(implicit trace: Trace): ZIO[R, E, A] =
     ZIO.suspendSucceed {
-      val cancelerRef = new java.util.concurrent.atomic.AtomicReference[URIO[R, Any]](ZIO.unit)
-
-      ZIO
-        .Async[R, E, A](
-          trace,
-          { k =>
-            val result = register(using Unsafe)(k(_))
-
-            result match {
-              case Left(canceler) => cancelerRef.set(canceler); null.asInstanceOf[ZIO[R, E, A]]
-              case Right(done)    => done
-            }
-          },
-          () => blockingOn
-        )
-        .onInterrupt(cancelerRef.get())
+      val state = new AtomicReference[URIO[R, Any]](Exit.unit) with ((ZIO[R, E, A] => Unit) => ZIO[R, E, A]) {
+        def apply(k: ZIO[R, E, A] => Unit): ZIO[R, E, A] = {
+          register(using Unsafe)(k(_)) match {
+            case Left(canceler) => set(canceler); null.asInstanceOf[ZIO[R, E, A]]
+            case Right(done)    => done
+          }
+        }
+      }
+
+      ZIO.Async[R, E, A](trace, state, () => blockingOn).onInterrupt(state.get())
     }
 
   /**
diff --git a/core/shared/src/main/scala/zio/Promise.scala b/core/shared/src/main/scala/zio/Promise.scala
index 73b21ec656f1..84d66a9b90c6 100644
--- a/core/shared/src/main/scala/zio/Promise.scala
+++ b/core/shared/src/main/scala/zio/Promise.scala
@@ -18,6 +18,7 @@ package zio
 
 import zio.Promise.internal._
 import zio.stacktracer.TracingImplicits.disableAutoTrace
+import scala.collection.immutable.LongMap
 
 import java.util.concurrent.atomic.AtomicReference
 
@@ -38,10 +39,7 @@ import java.util.concurrent.atomic.AtomicReference
  * } yield value
  * }}}
  */
-final class Promise[E, A] private (
-  private val state: AtomicReference[Promise.internal.State[E, A]],
-  blockingOn: FiberId
-) extends Serializable {
+final class Promise[E, A] private (blockingOn: FiberId) extends Serializable {
 
   /**
    * Retrieves the value of the promise, suspending the fiber running the action
@@ -50,32 +48,19 @@ final class Promise[E, A] private (
   def await(implicit trace: Trace): IO[E, A] =
     ZIO.suspendSucceed {
       state.get match {
-        case Done(value) =>
-          value
-        case _ =>
-          ZIO.asyncInterrupt[Any, E, A](
+        case Done(value) => value
+        case pending =>
+          ZIO.async[Any, E, A]( // intentionally never remove the callback, interrupted fibers won't be resumed
             k => {
-              var result = null.asInstanceOf[Either[UIO[Any], IO[E, A]]]
-              var retry  = true
-
-              while (retry) {
-                val oldState = state.get
-
-                val newState = oldState match {
-                  case Pending(joiners) =>
-                    result = Left(interruptJoiner(k))
-
-                    Pending(k :: joiners)
-                  case s @ Done(value) =>
-                    result = Right(value)
-
-                    s
+              @annotation.tailrec
+              def loop(current: State[E, A]): Unit =
+                current match {
+                  case pending: Pending[?, ?] =>
+                    if (state.compareAndSet(pending, pending.add(k))) ()
+                    else loop(state.get)
+                  case Done(value) => k(value)
                 }
-
-                retry = !state.compareAndSet(oldState, newState)
-              }
-
-              result
+              loop(pending)
             },
             blockingOn
           )
@@ -176,24 +161,6 @@ final class Promise[E, A] private (
   def succeed(a: A)(implicit trace: Trace): UIO[Boolean] =
     ZIO.succeed(unsafe.succeed(a)(trace, Unsafe.unsafe))
 
-  private def interruptJoiner(joiner: IO[E, A] => Any)(implicit trace: Trace): UIO[Any] = ZIO.succeed {
-    var retry = true
-
-    while (retry) {
-      val oldState = state.get
-
-      val newState = oldState match {
-        case Pending(joiners) =>
-          Pending(joiners.filter(j => !j.eq(joiner)))
-
-        case _ =>
-          oldState
-      }
-
-      retry = !state.compareAndSet(oldState, newState)
-    }
-  }
-
   private[zio] trait UnsafeAPI extends Serializable {
     def completeWith(io: IO[E, A])(implicit unsafe: Unsafe): Boolean
     def die(e: Throwable)(implicit trace: Trace, unsafe: Unsafe): Boolean
@@ -207,104 +174,96 @@ final class Promise[E, A] private (
     def succeed(a: A)(implicit trace: Trace, unsafe: Unsafe): Boolean
   }
 
-  private[zio] val unsafe: UnsafeAPI =
-    new UnsafeAPI {
-      def completeWith(io: IO[E, A])(implicit unsafe: Unsafe): Boolean = {
-        var action: () => Boolean = null.asInstanceOf[() => Boolean]
-        var retry                 = true
-
-        while (retry) {
-          val oldState = state.get
-
-          val newState = oldState match {
-            case Pending(joiners) =>
-              action = () => { joiners.foreach(_(io)); true }
-
-              Done(io)
-
-            case _ =>
-              action = Promise.ConstFalse
-
-              oldState
-          }
-
-          retry = !state.compareAndSet(oldState, newState)
+  @deprecated("Kept for binary compatibility only. Do not use", "2.1.15")
+  private[zio] def state: AtomicReference[Promise.internal.State[E, A]] =
+    unsafe.asInstanceOf[AtomicReference[Promise.internal.State[E, A]]]
+  private[zio] val unsafe: UnsafeAPI = new AtomicReference(Promise.internal.State.empty[E, A]) with UnsafeAPI { state =>
+    def completeWith(io: IO[E, A])(implicit unsafe: Unsafe): Boolean = {
+      @annotation.tailrec
+      def loop(): Boolean =
+        state.get match {
+          case pending: Pending[?, ?] =>
+            if (state.compareAndSet(pending, Done(io))) {
+              pending.complete(io)
+              true
+            } else {
+              loop()
+            }
+          case _: Done[?, ?] => false
         }
+      loop()
+    }
 
-        action()
-      }
+    def die(e: Throwable)(implicit trace: Trace, unsafe: Unsafe): Boolean =
+      completeWith(ZIO.die(e))
 
-      def die(e: Throwable)(implicit trace: Trace, unsafe: Unsafe): Boolean =
-        completeWith(ZIO.die(e))
+    def done(io: IO[E, A])(implicit unsafe: Unsafe): Unit = completeWith(io)
 
-      def done(io: IO[E, A])(implicit unsafe: Unsafe): Unit = {
-        var retry: Boolean                 = true
-        var joiners: List[IO[E, A] => Any] = null
+    def fail(e: E)(implicit trace: Trace, unsafe: Unsafe): Boolean =
+      completeWith(ZIO.fail(e))
 
-        while (retry) {
-          val oldState = state.get
+    def failCause(e: Cause[E])(implicit trace: Trace, unsafe: Unsafe): Boolean =
+      completeWith(ZIO.failCause(e))
 
-          val newState = oldState match {
-            case Pending(js) =>
-              joiners = js
-              Done(io)
-            case _ => oldState
-          }
+    def interruptAs(fiberId: FiberId)(implicit trace: Trace, unsafe: Unsafe): Boolean =
+      completeWith(ZIO.interruptAs(fiberId))
 
-          retry = !state.compareAndSet(oldState, newState)
-        }
+    def isDone(implicit unsafe: Unsafe): Boolean =
+      state.get().isInstanceOf[Done[?, ?]]
 
-        if (joiners ne null) joiners.foreach(_(io))
+    def poll(implicit unsafe: Unsafe): Option[IO[E, A]] =
+      state.get() match {
+        case _: Pending[?, ?] => None
+        case Done(value)      => Some(value)
       }
 
-      def fail(e: E)(implicit trace: Trace, unsafe: Unsafe): Boolean =
-        completeWith(ZIO.fail(e))
-
-      def failCause(e: Cause[E])(implicit trace: Trace, unsafe: Unsafe): Boolean =
-        completeWith(ZIO.failCause(e))
-
-      def interruptAs(fiberId: FiberId)(implicit trace: Trace, unsafe: Unsafe): Boolean =
-        completeWith(ZIO.interruptAs(fiberId))
-
-      def isDone(implicit unsafe: Unsafe): Boolean =
-        state.get().isInstanceOf[Done[?, ?]]
-
-      def poll(implicit unsafe: Unsafe): Option[IO[E, A]] =
-        state.get() match {
-          case _: Pending[?, ?] => None
-          case Done(io)         => Some(io)
-        }
-
-      def refailCause(e: Cause[E])(implicit trace: Trace, unsafe: Unsafe): Boolean =
-        completeWith(Exit.failCause(e))
+    def refailCause(e: Cause[E])(implicit trace: Trace, unsafe: Unsafe): Boolean =
+      completeWith(Exit.failCause(e))
 
-      def succeed(a: A)(implicit trace: Trace, unsafe: Unsafe): Boolean =
-        completeWith(Exit.succeed(a))
-    }
+    def succeed(a: A)(implicit trace: Trace, unsafe: Unsafe): Boolean =
+      completeWith(Exit.succeed(a))
+  }
 
 }
 object Promise {
   private val ConstFalse: () => Boolean = () => false
 
   private[zio] object internal {
-    sealed abstract class State[E, A]                              extends Serializable with Product
-    final case class Pending[E, A](joiners: List[IO[E, A] => Any]) extends State[E, A]
-    final case class Done[E, A](value: IO[E, A])                   extends State[E, A]
+    sealed abstract class State[E, A]
+    final case class Done[E, A](val value: IO[E, A]) extends State[E, A]
+    sealed abstract class Pending[E, A] extends State[E, A] { self =>
+      @annotation.tailrec
+      final def complete(io: IO[E, A]): Unit =
+        self match {
+          case Chain(j, js) =>
+            j(io)
+            js.complete(io)
+          case _: Empty.type => ()
+        }
+      def add(joiner: IO[E, A] => Any): Pending[E, A] = new Chain(joiner, self)
+    }
+
+    final case class Chain[E, A](j: IO[E, A] => Any, js: Pending[E, A]) extends Pending[E, A]
+    case object Empty                                                   extends Pending[Nothing, Nothing]
+
+    object State {
+      def empty[E, A]: State[E, A] = Empty.asInstanceOf[State[E, A]]
+    }
   }
 
   /**
    * Makes a new promise to be completed by the fiber creating the promise.
    */
-  def make[E, A](implicit trace: Trace): UIO[Promise[E, A]] = ZIO.fiberIdWith(makeAs(_))
+  def make[E, A](implicit trace: Trace): UIO[Promise[E, A]] =
+    ZIO.fiberIdWith(id => Exit.succeed(unsafe.make(id)(Unsafe)))
 
   /**
    * Makes a new promise to be completed by the fiber with the specified id.
    */
   def makeAs[E, A](fiberId: => FiberId)(implicit trace: Trace): UIO[Promise[E, A]] =
-    ZIO.succeed(unsafe.make(fiberId)(Unsafe.unsafe))
+    ZIO.succeed(unsafe.make(fiberId)(Unsafe))
 
   object unsafe {
-    def make[E, A](fiberId: FiberId)(implicit unsafe: Unsafe): Promise[E, A] =
-      new Promise[E, A](new AtomicReference[State[E, A]](new internal.Pending[E, A](Nil)), fiberId)
+    def make[E, A](fiberId: FiberId)(implicit unsafe: Unsafe): Promise[E, A] = new Promise[E, A](fiberId)
   }
 }

From a95ae72c151e6a1844721b2e90d52f81fb22e05c Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Sun, 9 Feb 2025 20:44:44 -0800
Subject: [PATCH 02/34] missing import

---
 .../src/main/scala-3/zio/ZIOCompanionVersionSpecific.scala       | 1 +
 1 file changed, 1 insertion(+)

diff --git a/core/shared/src/main/scala-3/zio/ZIOCompanionVersionSpecific.scala b/core/shared/src/main/scala-3/zio/ZIOCompanionVersionSpecific.scala
index 4783107ec0b6..26572af6ca7f 100644
--- a/core/shared/src/main/scala-3/zio/ZIOCompanionVersionSpecific.scala
+++ b/core/shared/src/main/scala-3/zio/ZIOCompanionVersionSpecific.scala
@@ -4,6 +4,7 @@ import zio.ZIO.Async
 import zio.stacktracer.TracingImplicits.disableAutoTrace
 
 import java.io.IOException
+import java.util.concurrent.atomic.AtomicReference
 import scala.annotation.targetName
 
 private[zio] transparent trait ZIOCompanionVersionSpecific {

From a2b54e98b45db74d76fd9987f89d80cc19793e71 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Tue, 4 Mar 2025 18:33:05 -0800
Subject: [PATCH 03/34] fix merge/address feedback

---
 .../main/scala/zio/PromiseBenchmarks.scala    |  4 +--
 core/shared/src/main/scala/zio/Promise.scala  | 30 ++++---------------
 2 files changed, 8 insertions(+), 26 deletions(-)

diff --git a/benchmarks/src/main/scala/zio/PromiseBenchmarks.scala b/benchmarks/src/main/scala/zio/PromiseBenchmarks.scala
index cfb2547b4813..1caa514ef435 100644
--- a/benchmarks/src/main/scala/zio/PromiseBenchmarks.scala
+++ b/benchmarks/src/main/scala/zio/PromiseBenchmarks.scala
@@ -29,7 +29,7 @@ class PromiseBenchmarks {
       Promise
         .make[Nothing, Unit]
         .flatMap { promise =>
-          promise.succeed(()) *> promise.await
+          promise.done(Exit.unit) *> promise.await
         }
         .repeatN(n)
 
@@ -58,7 +58,7 @@ class PromiseBenchmarks {
         for {
           fibers <- createWaiters(promise)
           _      <- promise.done(Exit.unit)
-          _      <- ZIO.foreachDiscard(fibers)(_.join)
+          _      <- ZIO.foreachDiscard(fibers)(_.await)
         } yield ()
       }
       .repeatN(1023)
diff --git a/core/shared/src/main/scala/zio/Promise.scala b/core/shared/src/main/scala/zio/Promise.scala
index bacbbf60d946..cd38ba23d730 100644
--- a/core/shared/src/main/scala/zio/Promise.scala
+++ b/core/shared/src/main/scala/zio/Promise.scala
@@ -170,24 +170,6 @@ final class Promise[E, A] private (blockingOn: FiberId) extends Serializable {
   private[zio] def succeedUnit(implicit ev0: A =:= Unit, trace: Trace): UIO[Boolean] =
     ZIO.succeed(unsafe.succeedUnit(ev0, trace, Unsafe))
 
-  private def interruptJoiner(joiner: IO[E, A] => Any)(implicit trace: Trace): UIO[Any] = ZIO.succeed {
-    var retry = true
-
-    while (retry) {
-      val oldState = state.get
-
-      val newState = oldState match {
-        case Pending(joiners) =>
-          Pending(joiners.filter(j => !j.eq(joiner)))
-
-        case _ =>
-          oldState
-      }
-
-      retry = !state.compareAndSet(oldState, newState)
-    }
-  }
-
   private[zio] trait UnsafeAPI extends Serializable {
     def completeWith(io: IO[E, A])(implicit unsafe: Unsafe): Boolean
     def die(e: Throwable)(implicit trace: Trace, unsafe: Unsafe): Boolean
@@ -217,7 +199,7 @@ final class Promise[E, A] private (blockingOn: FiberId) extends Serializable {
             } else {
               loop()
             }
-          case _: Done[?, ?] => false
+          case _ => false
         }
       loop()
     }
@@ -241,8 +223,8 @@ final class Promise[E, A] private (blockingOn: FiberId) extends Serializable {
 
     def poll(implicit unsafe: Unsafe): Option[IO[E, A]] =
       state.get() match {
-        case _: Pending[?, ?] => None
-        case Done(value)      => Some(value)
+        case Done(value) => Some(value)
+        case _           => None
       }
 
     def refailCause(e: Cause[E])(implicit trace: Trace, unsafe: Unsafe): Boolean =
@@ -251,8 +233,8 @@ final class Promise[E, A] private (blockingOn: FiberId) extends Serializable {
     def succeed(a: A)(implicit trace: Trace, unsafe: Unsafe): Boolean =
       completeWith(Exit.succeed(a))
 
-      override def succeedUnit(implicit ev0: A =:= Unit, trace: Trace, unsafe: Unsafe): Boolean =
-        completeWith(Exit.unit.asInstanceOf[IO[E, A]])
+    override def succeedUnit(implicit ev0: A =:= Unit, trace: Trace, unsafe: Unsafe): Boolean =
+      completeWith(Exit.unit.asInstanceOf[IO[E, A]])
   }
 
 }
@@ -269,7 +251,7 @@ object Promise {
           case Chain(j, js) =>
             j(io)
             js.complete(io)
-          case _: Empty.type => ()
+          case _ => ()
         }
       def add(joiner: IO[E, A] => Any): Pending[E, A] = new Chain(joiner, self)
     }

From 6859183b271cd4f3e410008e4f60a13c8df23de4 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Tue, 4 Mar 2025 19:54:49 -0800
Subject: [PATCH 04/34] Invoke waiters in order

---
 .../src/test/scala/zio/PromiseSpec.scala      | 10 ++++++-
 core/shared/src/main/scala/zio/Promise.scala  | 26 +++++--------------
 2 files changed, 16 insertions(+), 20 deletions(-)

diff --git a/core-tests/shared/src/test/scala/zio/PromiseSpec.scala b/core-tests/shared/src/test/scala/zio/PromiseSpec.scala
index 04cf82c1f682..8b55ea9c55f3 100644
--- a/core-tests/shared/src/test/scala/zio/PromiseSpec.scala
+++ b/core-tests/shared/src/test/scala/zio/PromiseSpec.scala
@@ -120,6 +120,14 @@ object PromiseSpec extends ZIOBaseSpec {
         _ <- p.fail("failure")
         d <- p.isDone
       } yield assert(d)(isTrue)
-    } @@ zioTag(errors)
+    } @@ zioTag(errors),
+    test("waiter stack safety") {
+      for {
+        p      <- Promise.make[Nothing, Unit]
+        fibers <- ZIO.foreach(1 to 100_000)(_ => p.await.forkDaemon)
+        _      <- p.complete(Exit.unit)
+        _      <- ZIO.foreach(fibers)(_.await)
+      } yield assertCompletes
+    },
   )
 }
diff --git a/core/shared/src/main/scala/zio/Promise.scala b/core/shared/src/main/scala/zio/Promise.scala
index cd38ba23d730..006056ea2d7d 100644
--- a/core/shared/src/main/scala/zio/Promise.scala
+++ b/core/shared/src/main/scala/zio/Promise.scala
@@ -16,7 +16,6 @@
 
 package zio
 
-import zio.Promise.internal._
 import zio.stacktracer.TracingImplicits.disableAutoTrace
 import scala.collection.immutable.LongMap
 
@@ -40,6 +39,7 @@ import java.util.concurrent.atomic.AtomicReference
  * }}}
  */
 final class Promise[E, A] private (blockingOn: FiberId) extends Serializable {
+  import Promise.internal._
 
   /**
    * Retrieves the value of the promise, suspending the fiber running the action
@@ -239,27 +239,15 @@ final class Promise[E, A] private (blockingOn: FiberId) extends Serializable {
 
 }
 object Promise {
-  private val ConstFalse: () => Boolean = () => false
-
-  private[zio] object internal {
-    sealed abstract class State[E, A]
+  private[Promise] object internal {
+    sealed abstract class State[E, A] extends Serializable
     final case class Done[E, A](val value: IO[E, A]) extends State[E, A]
-    sealed abstract class Pending[E, A] extends State[E, A] { self =>
-      @annotation.tailrec
-      final def complete(io: IO[E, A]): Unit =
-        self match {
-          case Chain(j, js) =>
-            j(io)
-            js.complete(io)
-          case _ => ()
-        }
-      def add(joiner: IO[E, A] => Any): Pending[E, A] = new Chain(joiner, self)
+    final class Pending[E, A](waiters: LongMap[IO[E, A] => Any], next: Long) extends State[E, A] { self =>
+      def complete(io: IO[E, A]): Unit = waiters.valuesIterator.foreach(_(io))
+      def add(joiner: IO[E, A] => Any): Pending[E, A] = new Pending[E, A](waiters.updated(next, joiner), next + 1)
     }
-
-    final case class Chain[E, A](j: IO[E, A] => Any, js: Pending[E, A]) extends Pending[E, A]
-    case object Empty                                                   extends Pending[Nothing, Nothing]
-
     object State {
+      private val Empty = new Pending[Nothing, Nothing](LongMap.empty, 1)
       def empty[E, A]: State[E, A] = Empty.asInstanceOf[State[E, A]]
     }
   }

From 4dc3766599c4ebfe0d5dda6b529149440a1e3fa0 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Tue, 4 Mar 2025 19:56:48 -0800
Subject: [PATCH 05/34] format

---
 core-tests/shared/src/test/scala/zio/PromiseSpec.scala | 2 +-
 core/shared/src/main/scala/zio/Promise.scala           | 6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/core-tests/shared/src/test/scala/zio/PromiseSpec.scala b/core-tests/shared/src/test/scala/zio/PromiseSpec.scala
index 8b55ea9c55f3..7e25d7546cf7 100644
--- a/core-tests/shared/src/test/scala/zio/PromiseSpec.scala
+++ b/core-tests/shared/src/test/scala/zio/PromiseSpec.scala
@@ -128,6 +128,6 @@ object PromiseSpec extends ZIOBaseSpec {
         _      <- p.complete(Exit.unit)
         _      <- ZIO.foreach(fibers)(_.await)
       } yield assertCompletes
-    },
+    }
   )
 }
diff --git a/core/shared/src/main/scala/zio/Promise.scala b/core/shared/src/main/scala/zio/Promise.scala
index 006056ea2d7d..a232a938c6a1 100644
--- a/core/shared/src/main/scala/zio/Promise.scala
+++ b/core/shared/src/main/scala/zio/Promise.scala
@@ -240,14 +240,14 @@ final class Promise[E, A] private (blockingOn: FiberId) extends Serializable {
 }
 object Promise {
   private[Promise] object internal {
-    sealed abstract class State[E, A] extends Serializable
+    sealed abstract class State[E, A]                extends Serializable
     final case class Done[E, A](val value: IO[E, A]) extends State[E, A]
     final class Pending[E, A](waiters: LongMap[IO[E, A] => Any], next: Long) extends State[E, A] { self =>
-      def complete(io: IO[E, A]): Unit = waiters.valuesIterator.foreach(_(io))
+      def complete(io: IO[E, A]): Unit                = waiters.valuesIterator.foreach(_(io))
       def add(joiner: IO[E, A] => Any): Pending[E, A] = new Pending[E, A](waiters.updated(next, joiner), next + 1)
     }
     object State {
-      private val Empty = new Pending[Nothing, Nothing](LongMap.empty, 1)
+      private val Empty            = new Pending[Nothing, Nothing](LongMap.empty, 1)
       def empty[E, A]: State[E, A] = Empty.asInstanceOf[State[E, A]]
     }
   }

From 0842795e2b884cea33cb6cb572bf7be271905988 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Tue, 4 Mar 2025 20:00:54 -0800
Subject: [PATCH 06/34] fix version

---
 core/shared/src/main/scala/zio/Promise.scala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/core/shared/src/main/scala/zio/Promise.scala b/core/shared/src/main/scala/zio/Promise.scala
index a232a938c6a1..1d10bfd59765 100644
--- a/core/shared/src/main/scala/zio/Promise.scala
+++ b/core/shared/src/main/scala/zio/Promise.scala
@@ -184,7 +184,7 @@ final class Promise[E, A] private (blockingOn: FiberId) extends Serializable {
     def succeedUnit(implicit ev0: A =:= Unit, trace: Trace, unsafe: Unsafe): Boolean
   }
 
-  @deprecated("Kept for binary compatibility only. Do not use", "2.1.15")
+  @deprecated("Kept for binary compatibility only. Do not use", "2.1.16")
   private[zio] def state: AtomicReference[Promise.internal.State[E, A]] =
     unsafe.asInstanceOf[AtomicReference[Promise.internal.State[E, A]]]
   private[zio] val unsafe: UnsafeAPI = new AtomicReference(Promise.internal.State.empty[E, A]) with UnsafeAPI { state =>

From f3f7a945cb78de59da4152e41e5c221a0e558a99 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Tue, 4 Mar 2025 20:10:12 -0800
Subject: [PATCH 07/34] fix PromiseSpec on 212

---
 core-tests/shared/src/test/scala/zio/PromiseSpec.scala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/core-tests/shared/src/test/scala/zio/PromiseSpec.scala b/core-tests/shared/src/test/scala/zio/PromiseSpec.scala
index 7e25d7546cf7..ed3e1a8949d9 100644
--- a/core-tests/shared/src/test/scala/zio/PromiseSpec.scala
+++ b/core-tests/shared/src/test/scala/zio/PromiseSpec.scala
@@ -124,7 +124,7 @@ object PromiseSpec extends ZIOBaseSpec {
     test("waiter stack safety") {
       for {
         p      <- Promise.make[Nothing, Unit]
-        fibers <- ZIO.foreach(1 to 100_000)(_ => p.await.forkDaemon)
+        fibers <- ZIO.foreach(1 to 100000)(_ => p.await.forkDaemon)
         _      <- p.complete(Exit.unit)
         _      <- ZIO.foreach(fibers)(_.await)
       } yield assertCompletes

From 78f163152a32a8ad0668b5de5eb2eae849d83730 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Tue, 4 Mar 2025 20:36:55 -0800
Subject: [PATCH 08/34] fix PromiseBenchmarks on 212

---
 benchmarks/src/main/scala/zio/PromiseBenchmarks.scala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/benchmarks/src/main/scala/zio/PromiseBenchmarks.scala b/benchmarks/src/main/scala/zio/PromiseBenchmarks.scala
index 1caa514ef435..43c92a50ee3a 100644
--- a/benchmarks/src/main/scala/zio/PromiseBenchmarks.scala
+++ b/benchmarks/src/main/scala/zio/PromiseBenchmarks.scala
@@ -50,7 +50,7 @@ class PromiseBenchmarks {
   @Benchmark
   def zioPromiseMultiAwaitDone(): Unit = {
     def createWaiters(promise: Promise[Nothing, Unit]): ZIO[Any, Nothing, Seq[Fiber[Nothing, Unit]]] =
-      ZIO.foreach(Range(0, waiters))(_ => promise.await.forkDaemon)
+      ZIO.foreach(Vector.range(0, waiters))(_ => promise.await.forkDaemon)
 
     val io = Promise
       .make[Nothing, Unit]

From e56d50923897104562153d72a0861043dada40ec Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Wed, 5 Mar 2025 20:07:44 -0800
Subject: [PATCH 09/34] fix jvmopts

---
 .jvmopts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.jvmopts b/.jvmopts
index d8db65d6a4a8..c49340da9834 100644
--- a/.jvmopts
+++ b/.jvmopts
@@ -1,3 +1,3 @@
 -Dcats.effect.stackTracingMode=full
 -Xmx8g
--Xms8g
\ No newline at end of file
+-Xms2g
\ No newline at end of file

From f62ab661de86959cc55127c0580b3155204a125d Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Wed, 5 Mar 2025 21:11:07 -0800
Subject: [PATCH 10/34] optimize Single/Zero waiter

---
 core/shared/src/main/scala/zio/Promise.scala | 9 ++++++++-
 project/MimaSettings.scala                   | 3 ++-
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/core/shared/src/main/scala/zio/Promise.scala b/core/shared/src/main/scala/zio/Promise.scala
index 1d10bfd59765..d7e3ded8c44b 100644
--- a/core/shared/src/main/scala/zio/Promise.scala
+++ b/core/shared/src/main/scala/zio/Promise.scala
@@ -21,6 +21,8 @@ import scala.collection.immutable.LongMap
 
 import java.util.concurrent.atomic.AtomicReference
 
+import scala.annotation.switch
+
 /**
  * A promise represents an asynchronous variable, of [[zio.ZIO]] type, that can
  * be set exactly once, with the ability for an arbitrary number of fibers to
@@ -243,7 +245,12 @@ object Promise {
     sealed abstract class State[E, A]                extends Serializable
     final case class Done[E, A](val value: IO[E, A]) extends State[E, A]
     final class Pending[E, A](waiters: LongMap[IO[E, A] => Any], next: Long) extends State[E, A] { self =>
-      def complete(io: IO[E, A]): Unit                = waiters.valuesIterator.foreach(_(io))
+      def complete(io: IO[E, A]): Unit                =
+        (next: @switch) match {
+          case 1 => ()
+          case 2 => waiters(2L)(io)
+          case _ => waiters.valuesIterator.foreach(_(io))
+        }
       def add(joiner: IO[E, A] => Any): Pending[E, A] = new Pending[E, A](waiters.updated(next, joiner), next + 1)
     }
     object State {
diff --git a/project/MimaSettings.scala b/project/MimaSettings.scala
index 4900181b8287..07d3d7e781b0 100644
--- a/project/MimaSettings.scala
+++ b/project/MimaSettings.scala
@@ -32,7 +32,8 @@ object MimaSettings {
         exclude[NewMixinForwarderProblem]("zio.Exit.mapBoth"),
         exclude[NewMixinForwarderProblem]("zio.Exit.mapError"),
         exclude[NewMixinForwarderProblem]("zio.Exit.mapErrorCause"),
-        exclude[NewMixinForwarderProblem]("zio.Exit.unit")
+        exclude[NewMixinForwarderProblem]("zio.Exit.unit"),
+        exclude[Problem]("zio.Promise#internal*"),
       ),
       mimaFailOnProblem := failOnProblem
     )

From 201d77437c2e78b9c2523e17383a9e6cac268862 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Wed, 5 Mar 2025 21:25:29 -0800
Subject: [PATCH 11/34] mima

---
 project/MimaSettings.scala | 1 +
 1 file changed, 1 insertion(+)

diff --git a/project/MimaSettings.scala b/project/MimaSettings.scala
index 07d3d7e781b0..8e4a43c33018 100644
--- a/project/MimaSettings.scala
+++ b/project/MimaSettings.scala
@@ -34,6 +34,7 @@ object MimaSettings {
         exclude[NewMixinForwarderProblem]("zio.Exit.mapErrorCause"),
         exclude[NewMixinForwarderProblem]("zio.Exit.unit"),
         exclude[Problem]("zio.Promise#internal*"),
+        exclude[Problem]("zio.Promise$internal*"),
       ),
       mimaFailOnProblem := failOnProblem
     )

From 1f98962396bdb2ffca31511d3a06820a99b76a83 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Wed, 5 Mar 2025 21:45:06 -0800
Subject: [PATCH 12/34] format

---
 project/MimaSettings.scala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/project/MimaSettings.scala b/project/MimaSettings.scala
index 8e4a43c33018..09381a66efb9 100644
--- a/project/MimaSettings.scala
+++ b/project/MimaSettings.scala
@@ -34,7 +34,7 @@ object MimaSettings {
         exclude[NewMixinForwarderProblem]("zio.Exit.mapErrorCause"),
         exclude[NewMixinForwarderProblem]("zio.Exit.unit"),
         exclude[Problem]("zio.Promise#internal*"),
-        exclude[Problem]("zio.Promise$internal*"),
+        exclude[Problem]("zio.Promise$internal*")
       ),
       mimaFailOnProblem := failOnProblem
     )

From 511d5734ec78dae9fc44458a1683b1c7e9791787 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Thu, 6 Mar 2025 13:47:14 -0800
Subject: [PATCH 13/34] attempt to improve test

---
 .../src/test/scala/zio/FiberRuntimeSpec.scala | 42 ++++++++++++-------
 core/shared/src/main/scala/zio/Promise.scala  |  2 +-
 2 files changed, 28 insertions(+), 16 deletions(-)

diff --git a/core-tests/shared/src/test/scala/zio/FiberRuntimeSpec.scala b/core-tests/shared/src/test/scala/zio/FiberRuntimeSpec.scala
index ba6823d451d8..65763f4cfc98 100644
--- a/core-tests/shared/src/test/scala/zio/FiberRuntimeSpec.scala
+++ b/core-tests/shared/src/test/scala/zio/FiberRuntimeSpec.scala
@@ -66,23 +66,35 @@ object FiberRuntimeSpec extends ZIOBaseSpec {
     suite("async")(
       test("async callback after interruption is ignored") {
         ZIO.suspendSucceed {
-          val cb = Ref.unsafe.make[Option[ZIO[Any, Nothing, Unit] => Unit]](None)
-          val effect =
-            (ZIO.async[Any, Nothing, Unit] { (k: (ZIO[Any, Nothing, Unit] => Unit)) =>
-              cb.unsafe.set(Some(k))
-            } *> ZIO.never)
-
+          val executed = Ref.unsafe.make(0)
+          val cb       = Ref.unsafe.make[Option[ZIO[Any, Nothing, Unit] => Unit]](None)
+          val latch    = Promise.unsafe.make[Nothing, Unit](FiberId.None)
+          val async = ZIO.async[Any, Nothing, Unit] { k =>
+            cb.unsafe.set(Some(k))
+            latch.unsafe.done(Exit.unit)
+          }
+          val increment = executed.update(_ + 1)
           for {
-            fiber    <- effect.fork
-            _        <- fiber.interrupt
-            callback <- cb.get.some
-            _        <- ZIO.succeed(callback(ZIO.unit))
-            first    <- fiber.poll
-            _        <- ZIO.succeed(callback(ZIO.unit))
-            second   <- fiber.poll
-          } yield assertTrue(first == second) && assertTrue(first == None)
+            fiber          <- async.fork
+            _              <- latch.await
+            exit           <- fiber.interrupt
+            callback       <- cb.get.some
+            state1         <- fiber.poll
+            _              <- ZIO.succeed(callback(increment))
+            state2         <- fiber.poll
+            executedBefore <- executed.get
+            _              <- ZIO.succeed(callback(increment))
+            state3         <- fiber.poll
+            executedAfter  <- executed.get
+          } yield assertTrue(
+            state1 == state2,
+            state1 == state3,
+            executedBefore == 0,
+            executedAfter == 0,
+            state1.exists(_.isInterrupted)
+          )
         }
-      }
+      } @@ TestAspect.nonFlaky(10)
     )
   )
 
diff --git a/core/shared/src/main/scala/zio/Promise.scala b/core/shared/src/main/scala/zio/Promise.scala
index d7e3ded8c44b..0b789ece9bee 100644
--- a/core/shared/src/main/scala/zio/Promise.scala
+++ b/core/shared/src/main/scala/zio/Promise.scala
@@ -245,7 +245,7 @@ object Promise {
     sealed abstract class State[E, A]                extends Serializable
     final case class Done[E, A](val value: IO[E, A]) extends State[E, A]
     final class Pending[E, A](waiters: LongMap[IO[E, A] => Any], next: Long) extends State[E, A] { self =>
-      def complete(io: IO[E, A]): Unit                =
+      def complete(io: IO[E, A]): Unit =
         (next: @switch) match {
           case 1 => ()
           case 2 => waiters(2L)(io)

From ee30a743b9c3247955ec01feeb2a510b71029fc5 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Thu, 6 Mar 2025 20:11:34 -0800
Subject: [PATCH 14/34] update test

---
 .../shared/src/test/scala/zio/FiberRuntimeSpec.scala       | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/core-tests/shared/src/test/scala/zio/FiberRuntimeSpec.scala b/core-tests/shared/src/test/scala/zio/FiberRuntimeSpec.scala
index 65763f4cfc98..b80fd89649c3 100644
--- a/core-tests/shared/src/test/scala/zio/FiberRuntimeSpec.scala
+++ b/core-tests/shared/src/test/scala/zio/FiberRuntimeSpec.scala
@@ -87,11 +87,12 @@ object FiberRuntimeSpec extends ZIOBaseSpec {
             state3         <- fiber.poll
             executedAfter  <- executed.get
           } yield assertTrue(
-            state1 == state2,
-            state1 == state3,
+            state1 == Some(exit),
+            state2 == Some(exit),
+            state3 == Some(exit),
             executedBefore == 0,
             executedAfter == 0,
-            state1.exists(_.isInterrupted)
+            exit.isInterrupted
           )
         }
       } @@ TestAspect.nonFlaky(10)

From 349202dca34fd51445b7b90652424a59c3413943 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Sun, 9 Mar 2025 13:29:23 -0700
Subject: [PATCH 15/34] fix: complete

---
 core/shared/src/main/scala/zio/Promise.scala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/core/shared/src/main/scala/zio/Promise.scala b/core/shared/src/main/scala/zio/Promise.scala
index 0b789ece9bee..658e99750d40 100644
--- a/core/shared/src/main/scala/zio/Promise.scala
+++ b/core/shared/src/main/scala/zio/Promise.scala
@@ -248,7 +248,7 @@ object Promise {
       def complete(io: IO[E, A]): Unit =
         (next: @switch) match {
           case 1 => ()
-          case 2 => waiters(2L)(io)
+          case 2 => waiters(1L)(io)
           case _ => waiters.valuesIterator.foreach(_(io))
         }
       def add(joiner: IO[E, A] => Any): Pending[E, A] = new Pending[E, A](waiters.updated(next, joiner), next + 1)

From 071bfbbbbc53e24c8f62c6d5e016626644968dd4 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Sun, 9 Mar 2025 17:27:27 -0700
Subject: [PATCH 16/34] PromiseBench: 16 -> 8 waiters

---
 benchmarks/src/main/scala/zio/PromiseBenchmarks.scala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/benchmarks/src/main/scala/zio/PromiseBenchmarks.scala b/benchmarks/src/main/scala/zio/PromiseBenchmarks.scala
index 43c92a50ee3a..7f7ea224431f 100644
--- a/benchmarks/src/main/scala/zio/PromiseBenchmarks.scala
+++ b/benchmarks/src/main/scala/zio/PromiseBenchmarks.scala
@@ -20,7 +20,7 @@ import java.util.concurrent.TimeUnit
 class PromiseBenchmarks {
 
   val n            = 100000
-  val waiters: Int = 16
+  val waiters: Int = 8
 
   @Benchmark
   def zioPromiseAwaitDone(): Unit = {

From fad1bf8bedc578accc6bdcf2e189f7e3538115d1 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Sun, 9 Mar 2025 21:26:36 -0700
Subject: [PATCH 17/34] bring back linked list

---
 core/shared/src/main/scala/zio/Promise.scala | 41 ++++++++++++++------
 1 file changed, 30 insertions(+), 11 deletions(-)

diff --git a/core/shared/src/main/scala/zio/Promise.scala b/core/shared/src/main/scala/zio/Promise.scala
index 658e99750d40..f8e42bac14e9 100644
--- a/core/shared/src/main/scala/zio/Promise.scala
+++ b/core/shared/src/main/scala/zio/Promise.scala
@@ -242,19 +242,38 @@ final class Promise[E, A] private (blockingOn: FiberId) extends Serializable {
 }
 object Promise {
   private[Promise] object internal {
-    sealed abstract class State[E, A]                extends Serializable
-    final case class Done[E, A](val value: IO[E, A]) extends State[E, A]
-    final class Pending[E, A](waiters: LongMap[IO[E, A] => Any], next: Long) extends State[E, A] { self =>
-      def complete(io: IO[E, A]): Unit =
-        (next: @switch) match {
-          case 1 => ()
-          case 2 => waiters(1L)(io)
-          case _ => waiters.valuesIterator.foreach(_(io))
-        }
-      def add(joiner: IO[E, A] => Any): Pending[E, A] = new Pending[E, A](waiters.updated(next, joiner), next + 1)
+    sealed abstract class State[E, A]            extends Serializable
+    final case class Done[E, A](value: IO[E, A]) extends State[E, A]
+    sealed abstract class Pending[E, A] extends State[E, A] { self =>
+      def complete(io: IO[E, A]): Unit = {
+        val size = self.size
+        val arr = new Array[IO[E, A] => Any](size)
+        @annotation.tailrec
+        def fill(pending: Pending[E, A], i: Int): Unit =
+          pending match {
+            case link: Link[?, ?] =>
+              arr(i) = link.waiter
+              fill(link.ws, i - 1)
+            case _ => () // Empty
+          }
+        fill(self, size - 1)
+        arr.foreach(_(io))
+      }
+      def add(waiter: IO[E, A] => Any): Pending[E, A]
+      def size: Int
+    }
+    private case object Empty extends Pending[Nothing, Nothing] { self =>
+      override def complete(io: IO[Nothing, Nothing]): Unit = ()
+      override def add(waiter: IO[Nothing, Nothing] => Any): Pending[Nothing, Nothing] = new Link[Nothing, Nothing](waiter, self, 1) {
+        override def complete(io: IO[Nothing, Nothing]): Unit = waiter(io)
+      }
+      def size = 0
+    }
+    private sealed class Link[E, A](val waiter: IO[E, A] => Any, val ws: Pending[E, A], val size: Int) extends Pending[E, A] { self =>
+      def add(waiter: IO[E, A] => Any): Pending[E, A] = new Link(waiter, self, size + 1)
     }
+
     object State {
-      private val Empty            = new Pending[Nothing, Nothing](LongMap.empty, 1)
       def empty[E, A]: State[E, A] = Empty.asInstanceOf[State[E, A]]
     }
   }

From aa485075e5563aa8864088125714818afeb0d5b6 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Sun, 9 Mar 2025 21:28:54 -0700
Subject: [PATCH 18/34] formatting

---
 core/shared/src/main/scala/zio/Promise.scala | 27 +++++++++++---------
 1 file changed, 15 insertions(+), 12 deletions(-)

diff --git a/core/shared/src/main/scala/zio/Promise.scala b/core/shared/src/main/scala/zio/Promise.scala
index f8e42bac14e9..08443d386ffd 100644
--- a/core/shared/src/main/scala/zio/Promise.scala
+++ b/core/shared/src/main/scala/zio/Promise.scala
@@ -245,9 +245,23 @@ object Promise {
     sealed abstract class State[E, A]            extends Serializable
     final case class Done[E, A](value: IO[E, A]) extends State[E, A]
     sealed abstract class Pending[E, A] extends State[E, A] { self =>
+      def complete(io: IO[E, A]): Unit
+      def add(waiter: IO[E, A] => Any): Pending[E, A]
+      def size: Int
+    }
+    private case object Empty extends Pending[Nothing, Nothing] { self =>
+      override def complete(io: IO[Nothing, Nothing]): Unit = ()
+      override def add(waiter: IO[Nothing, Nothing] => Any): Pending[Nothing, Nothing] =
+        new Link[Nothing, Nothing](waiter, self, 1) {
+          override def complete(io: IO[Nothing, Nothing]): Unit = waiter(io)
+        }
+      def size = 0
+    }
+    private sealed class Link[E, A](val waiter: IO[E, A] => Any, val ws: Pending[E, A], val size: Int)
+        extends Pending[E, A] { self =>
       def complete(io: IO[E, A]): Unit = {
         val size = self.size
-        val arr = new Array[IO[E, A] => Any](size)
+        val arr  = new Array[IO[E, A] => Any](size)
         @annotation.tailrec
         def fill(pending: Pending[E, A], i: Int): Unit =
           pending match {
@@ -259,17 +273,6 @@ object Promise {
         fill(self, size - 1)
         arr.foreach(_(io))
       }
-      def add(waiter: IO[E, A] => Any): Pending[E, A]
-      def size: Int
-    }
-    private case object Empty extends Pending[Nothing, Nothing] { self =>
-      override def complete(io: IO[Nothing, Nothing]): Unit = ()
-      override def add(waiter: IO[Nothing, Nothing] => Any): Pending[Nothing, Nothing] = new Link[Nothing, Nothing](waiter, self, 1) {
-        override def complete(io: IO[Nothing, Nothing]): Unit = waiter(io)
-      }
-      def size = 0
-    }
-    private sealed class Link[E, A](val waiter: IO[E, A] => Any, val ws: Pending[E, A], val size: Int) extends Pending[E, A] { self =>
       def add(waiter: IO[E, A] => Any): Pending[E, A] = new Link(waiter, self, size + 1)
     }
 

From 9265c1b1c81a2147f1a13bebced51062510f5d6b Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Sun, 9 Mar 2025 21:30:41 -0700
Subject: [PATCH 19/34] remove unused imports

---
 core/shared/src/main/scala/zio/Promise.scala | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/core/shared/src/main/scala/zio/Promise.scala b/core/shared/src/main/scala/zio/Promise.scala
index 08443d386ffd..8d9f3a4b8350 100644
--- a/core/shared/src/main/scala/zio/Promise.scala
+++ b/core/shared/src/main/scala/zio/Promise.scala
@@ -17,12 +17,9 @@
 package zio
 
 import zio.stacktracer.TracingImplicits.disableAutoTrace
-import scala.collection.immutable.LongMap
 
 import java.util.concurrent.atomic.AtomicReference
 
-import scala.annotation.switch
-
 /**
  * A promise represents an asynchronous variable, of [[zio.ZIO]] type, that can
  * be set exactly once, with the ability for an arbitrary number of fibers to

From a4fe187bf041cf273996291ee0e5f15fd4ad0c86 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Sun, 9 Mar 2025 21:46:42 -0700
Subject: [PATCH 20/34] Use Unsafe directly

---
 core/shared/src/main/scala/zio/Promise.scala | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/core/shared/src/main/scala/zio/Promise.scala b/core/shared/src/main/scala/zio/Promise.scala
index 8d9f3a4b8350..f1fb80d28cda 100644
--- a/core/shared/src/main/scala/zio/Promise.scala
+++ b/core/shared/src/main/scala/zio/Promise.scala
@@ -71,14 +71,14 @@ final class Promise[E, A] private (blockingOn: FiberId) extends Serializable {
    * fibers waiting on the value of the promise.
    */
   def die(e: Throwable)(implicit trace: Trace): UIO[Boolean] =
-    ZIO.succeed(unsafe.die(e)(trace, Unsafe.unsafe))
+    ZIO.succeed(unsafe.die(e)(trace, Unsafe))
 
   /**
    * Exits the promise with the specified exit, which will be propagated to all
    * fibers waiting on the value of the promise.
    */
   def done(e: Exit[E, A])(implicit trace: Trace): UIO[Boolean] =
-    ZIO.succeed(unsafe.completeWith(e)(Unsafe.unsafe))
+    ZIO.succeed(unsafe.completeWith(e)(Unsafe))
 
   /**
    * Completes the promise with the result of the specified effect. If the
@@ -102,21 +102,21 @@ final class Promise[E, A] private (blockingOn: FiberId) extends Serializable {
    * promise with the result of an effect see [[Promise.complete]].
    */
   def completeWith(io: IO[E, A])(implicit trace: Trace): UIO[Boolean] =
-    ZIO.succeed(unsafe.completeWith(io)(Unsafe.unsafe))
+    ZIO.succeed(unsafe.completeWith(io)(Unsafe))
 
   /**
    * Fails the promise with the specified error, which will be propagated to all
    * fibers waiting on the value of the promise.
    */
   def fail(e: E)(implicit trace: Trace): UIO[Boolean] =
-    ZIO.succeed(unsafe.fail(e)(trace, Unsafe.unsafe))
+    ZIO.succeed(unsafe.fail(e)(trace, Unsafe))
 
   /**
    * Fails the promise with the specified cause, which will be propagated to all
    * fibers waiting on the value of the promise.
    */
   def failCause(e: Cause[E])(implicit trace: Trace): UIO[Boolean] =
-    ZIO.succeed(unsafe.failCause(e)(trace, Unsafe.unsafe))
+    ZIO.succeed(unsafe.failCause(e)(trace, Unsafe))
 
   /**
    * Completes the promise with interruption. This will interrupt all fibers
@@ -130,21 +130,21 @@ final class Promise[E, A] private (blockingOn: FiberId) extends Serializable {
    * waiting on the value of the promise as by the specified fiber.
    */
   def interruptAs(fiberId: FiberId)(implicit trace: Trace): UIO[Boolean] =
-    ZIO.succeed(unsafe.interruptAs(fiberId)(trace, Unsafe.unsafe))
+    ZIO.succeed(unsafe.interruptAs(fiberId)(trace, Unsafe))
 
   /**
    * Checks for completion of this Promise. Produces true if this promise has
    * already been completed with a value or an error and false otherwise.
    */
   def isDone(implicit trace: Trace): UIO[Boolean] =
-    ZIO.succeed(unsafe.isDone(Unsafe.unsafe))
+    ZIO.succeed(unsafe.isDone(Unsafe))
 
   /**
    * Checks for completion of this Promise. Returns the result effect if this
    * promise has already been completed or a `None` otherwise.
    */
   def poll(implicit trace: Trace): UIO[Option[IO[E, A]]] =
-    ZIO.succeed(unsafe.poll(Unsafe.unsafe))
+    ZIO.succeed(unsafe.poll(Unsafe))
 
   /**
    * Fails the promise with the specified cause, which will be propagated to all
@@ -152,13 +152,13 @@ final class Promise[E, A] private (blockingOn: FiberId) extends Serializable {
    * to the cause.
    */
   def refailCause(e: Cause[E])(implicit trace: Trace): UIO[Boolean] =
-    ZIO.succeed(unsafe.refailCause(e)(trace, Unsafe.unsafe))
+    ZIO.succeed(unsafe.refailCause(e)(trace, Unsafe))
 
   /**
    * Completes the promise with the specified value.
    */
   def succeed(a: A)(implicit trace: Trace): UIO[Boolean] =
-    ZIO.succeed(unsafe.succeed(a)(trace, Unsafe.unsafe))
+    ZIO.succeed(unsafe.succeed(a)(trace, Unsafe))
 
   /**
    * Internally, you can use this method instead of calling

From e6cdde377a38dc858ccad2e8fe7465e5e7b7cf6c Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Sun, 9 Mar 2025 21:47:22 -0700
Subject: [PATCH 21/34] fix scala3

---
 core/shared/src/main/scala/zio/Promise.scala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/core/shared/src/main/scala/zio/Promise.scala b/core/shared/src/main/scala/zio/Promise.scala
index f1fb80d28cda..5c6c56f6489a 100644
--- a/core/shared/src/main/scala/zio/Promise.scala
+++ b/core/shared/src/main/scala/zio/Promise.scala
@@ -250,7 +250,7 @@ object Promise {
       override def complete(io: IO[Nothing, Nothing]): Unit = ()
       override def add(waiter: IO[Nothing, Nothing] => Any): Pending[Nothing, Nothing] =
         new Link[Nothing, Nothing](waiter, self, 1) {
-          override def complete(io: IO[Nothing, Nothing]): Unit = waiter(io)
+          override def complete(io: IO[Nothing, Nothing]): Unit = this.waiter(io)
         }
       def size = 0
     }

From 4689adf610daaec7f8c58ca3c94ebe14cdf16682 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Sun, 30 Mar 2025 16:25:44 -0700
Subject: [PATCH 22/34] fix formatting settings.json

Co-authored-by: Jules Ivanic <jules.ivanic@gmail.com>
---
 .vscode/settings.json | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.vscode/settings.json b/.vscode/settings.json
index 052d27309198..b4bdcc7fa826 100644
--- a/.vscode/settings.json
+++ b/.vscode/settings.json
@@ -7,5 +7,5 @@
   ],
   "files.watcherExclude": {
     "**/target": true
-}
+  }
 }
\ No newline at end of file

From 63bfeb96e37b9a52c71613faf6e479ef113146c9 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Thu, 13 Mar 2025 17:01:02 -0700
Subject: [PATCH 23/34] update benches

---
 .../main/scala/zio/PromiseBenchmarks.scala    | 62 +++++++++++++++----
 1 file changed, 50 insertions(+), 12 deletions(-)

diff --git a/benchmarks/src/main/scala/zio/PromiseBenchmarks.scala b/benchmarks/src/main/scala/zio/PromiseBenchmarks.scala
index 7f7ea224431f..41a0ae690d58 100644
--- a/benchmarks/src/main/scala/zio/PromiseBenchmarks.scala
+++ b/benchmarks/src/main/scala/zio/PromiseBenchmarks.scala
@@ -14,16 +14,22 @@ import java.util.concurrent.TimeUnit
 @State(JScope.Thread)
 @BenchmarkMode(Array(Mode.Throughput))
 @OutputTimeUnit(TimeUnit.SECONDS)
-@Measurement(iterations = 5, timeUnit = TimeUnit.SECONDS, time = 3)
-@Warmup(iterations = 5, timeUnit = TimeUnit.SECONDS, time = 3)
+@Measurement(iterations = 5, timeUnit = TimeUnit.SECONDS, time = 10)
+@Warmup(iterations = 5, timeUnit = TimeUnit.SECONDS, time = 10)
 @Fork(value = 3)
 class PromiseBenchmarks {
 
   val n            = 100000
   val waiters: Int = 8
 
+  def createWaitersZIO(promise: Promise[Nothing, Unit]): ZIO[Any, Nothing, Seq[Fiber[Nothing, Unit]]] =
+    ZIO.foreach(Vector.range(0, waiters))(_ => promise.await.forkDaemon)
+
+  def createWaitersCats(promise: Deferred[CIO, Unit]) =
+    List.range(0, waiters).traverse(_ => promise.get.start)
+
   @Benchmark
-  def zioPromiseAwaitDone(): Unit = {
+  def zioPromiseDoneAwait(): Unit = {
 
     val io =
       Promise
@@ -37,7 +43,7 @@ class PromiseBenchmarks {
   }
 
   @Benchmark
-  def catsPromiseAwaitDone(): Unit = {
+  def catsPromiseDoneAwait(): Unit = {
 
     val io =
       Deferred[CIO, Unit].flatMap { promise =>
@@ -49,14 +55,11 @@ class PromiseBenchmarks {
 
   @Benchmark
   def zioPromiseMultiAwaitDone(): Unit = {
-    def createWaiters(promise: Promise[Nothing, Unit]): ZIO[Any, Nothing, Seq[Fiber[Nothing, Unit]]] =
-      ZIO.foreach(Vector.range(0, waiters))(_ => promise.await.forkDaemon)
-
     val io = Promise
       .make[Nothing, Unit]
       .flatMap { promise =>
         for {
-          fibers <- createWaiters(promise)
+          fibers <- createWaitersZIO(promise)
           _      <- promise.done(Exit.unit)
           _      <- ZIO.foreachDiscard(fibers)(_.await)
         } yield ()
@@ -68,13 +71,10 @@ class PromiseBenchmarks {
 
   @Benchmark
   def catsPromiseMultiAwaitDone(): Unit = {
-    def createWaiters(promise: Deferred[CIO, Unit]): CIO[List[cats.effect.Fiber[CIO, Throwable, Unit]]] =
-      List.range(0, waiters).traverse(_ => promise.get.start)
-
     val io =
       Deferred[CIO, Unit].flatMap { promise =>
         for {
-          fibers <- createWaiters(promise)
+          fibers <- createWaitersCats(promise)
           _      <- promise.complete(())
           _      <- fibers.traverse_(_.join)
         } yield ()
@@ -82,4 +82,42 @@ class PromiseBenchmarks {
 
     io.unsafeRunSync()
   }
+
+  @Benchmark
+  def zioPromiseMultiAwaitMultiDone(): Unit = {
+    def createCompleters(promise: Promise[Nothing, Unit], latch: Promise[Nothing, Unit]) =
+      ZIO.foreach(Vector.range(0, waiters))(_ => (latch.await *> promise.done(Exit.unit)).forkDaemon)
+
+    val io = {
+      for {
+        latch   <- Promise.make[Nothing, Unit]
+        promise <- Promise.make[Nothing, Unit]
+        waiters <- createWaitersZIO(promise)
+        fibers  <- createCompleters(promise, latch)
+        _       <- latch.done(Exit.unit)
+        result  <- promise.await
+      } yield result
+    }.repeatN(1023)
+
+    unsafeRun(io)
+  }
+
+  @Benchmark
+  def catsPromiseMultiAwaitMultiDone(): Unit = {
+    def createCompleters(promise: Deferred[CIO, Unit], latch: Deferred[CIO, Unit]) =
+      List.range(0, waiters).traverse(_ => (latch.get *> promise.complete(())).start)
+
+    val io = {
+      for {
+        latch   <- Deferred[CIO, Unit]
+        promise <- Deferred[CIO, Unit]
+        waiters <- createWaitersCats(promise)
+        fibers  <- createCompleters(promise, latch)
+        _       <- latch.complete(())
+        result  <- promise.get
+      } yield result
+    }.replicateA_(1023)
+
+    io.unsafeRunSync()
+  }
 }

From 4f05b1ee8791e499b2a93c1c55a72c193bb1b937 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Sun, 30 Mar 2025 19:01:17 -0700
Subject: [PATCH 24/34] Revert "fix formatting settings.json"

This reverts commit 4689adf610daaec7f8c58ca3c94ebe14cdf16682.
---
 .vscode/settings.json | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.vscode/settings.json b/.vscode/settings.json
index b4bdcc7fa826..052d27309198 100644
--- a/.vscode/settings.json
+++ b/.vscode/settings.json
@@ -7,5 +7,5 @@
   ],
   "files.watcherExclude": {
     "**/target": true
-  }
+}
 }
\ No newline at end of file

From e70a5a362327018c08dda56b9d5b8bf0043ca36b Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Sun, 30 Mar 2025 19:04:44 -0700
Subject: [PATCH 25/34] fix scala 3 formatting

---
 .../src/main/scala-3/zio/ZIOCompanionVersionSpecific.scala     | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/core/shared/src/main/scala-3/zio/ZIOCompanionVersionSpecific.scala b/core/shared/src/main/scala-3/zio/ZIOCompanionVersionSpecific.scala
index 26572af6ca7f..070c8198dce9 100644
--- a/core/shared/src/main/scala-3/zio/ZIOCompanionVersionSpecific.scala
+++ b/core/shared/src/main/scala-3/zio/ZIOCompanionVersionSpecific.scala
@@ -53,12 +53,11 @@ private[zio] transparent trait ZIOCompanionVersionSpecific {
   )(implicit trace: Trace): ZIO[R, E, A] =
     ZIO.suspendSucceed {
       val state = new AtomicReference[URIO[R, Any]](Exit.unit) with ((ZIO[R, E, A] => Unit) => ZIO[R, E, A]) {
-        def apply(k: ZIO[R, E, A] => Unit): ZIO[R, E, A] = {
+        def apply(k: ZIO[R, E, A] => Unit): ZIO[R, E, A] =
           register(using Unsafe)(k(_)) match {
             case Left(canceler) => set(canceler); null.asInstanceOf[ZIO[R, E, A]]
             case Right(done)    => done
           }
-        }
       }
 
       ZIO.Async[R, E, A](trace, state, () => blockingOn).onInterrupt(state.get())

From a34e5d81de9d4058b138631929579746a4cc13de Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Sat, 5 Apr 2025 12:36:19 -0700
Subject: [PATCH 26/34] updates

---
 .../src/test/scala/zio/PromiseSpec.scala      | 65 ++++++++++++++++-
 core/shared/src/main/scala/zio/Promise.scala  | 73 ++++++++++++++-----
 2 files changed, 117 insertions(+), 21 deletions(-)

diff --git a/core-tests/shared/src/test/scala/zio/PromiseSpec.scala b/core-tests/shared/src/test/scala/zio/PromiseSpec.scala
index ed3e1a8949d9..6cd344b57cbb 100644
--- a/core-tests/shared/src/test/scala/zio/PromiseSpec.scala
+++ b/core-tests/shared/src/test/scala/zio/PromiseSpec.scala
@@ -7,6 +7,8 @@ object PromiseSpec extends ZIOBaseSpec {
 
   import ZIOTag._
 
+  private def empty[E, A]: Promise.internal.Pending[E, A] = Promise.internal.State.empty[E, A].asInstanceOf[Promise.internal.Pending[E, A]]
+
   def spec: Spec[Any, TestFailure[Any]] = suite("PromiseSpec")(
     test("complete a promise using succeed") {
       for {
@@ -128,6 +130,67 @@ object PromiseSpec extends ZIOBaseSpec {
         _      <- p.complete(Exit.unit)
         _      <- ZIO.foreach(fibers)(_.await)
       } yield assertCompletes
-    }
+    },
+    suite("State")(
+      suite("add")(
+        test("stack safety") {
+          (0 to 100000).foldLeft(empty[Nothing, Unit])((acc, _) => acc.add(_ => ()))
+          assertCompletes
+        }
+      ),
+      suite("complete")(
+        test("one") {
+          var increment = 0
+          val state     = empty[Nothing, Unit].add(_ => increment += 1)
+          state.complete(ZIO.unit)
+          assert(increment)(equalTo(1))
+        },
+        test("multiple") {
+          val n         = 10
+          var increment = 0
+          val state     = (0 until n).foldLeft(empty[Nothing, Unit])((acc, _) => acc.add(_ => increment += 1))
+          state.complete(ZIO.unit)
+          assert(increment)(equalTo(n))
+        }
+      ),
+      suite("remove")(
+        test("one") {
+          var increment = 0
+          val cb        = (_: IO[Nothing, Unit]) => increment += 1
+          val state     = empty[Nothing, Unit].add(cb)
+          val removed   = state.remove(cb)
+          removed.complete(ZIO.unit)
+          assert(removed)(equalTo(empty[Nothing, Unit])) &&
+          assert(increment)(equalTo(0))
+        },
+        test("multiple") {
+          val n     = 10
+          var fired = 0
+          val cb    = (_: IO[Nothing, Unit]) => ()
+          val toRemove = (_: IO[Nothing, Unit]) => fired += 1
+          val state =
+            (0 until n).foldLeft(empty[Nothing, Unit])((acc, i) => if (i < 5) acc.add(cb) else acc.add(toRemove))
+          val removed = state.remove(toRemove)
+          removed.complete(ZIO.unit)
+          assert(removed.size)(equalTo(5)) &&
+          assert(fired)(equalTo(0))
+        }
+      ),
+      suite("complete")(
+        test("one") {
+          var completed = 0
+          val state = empty[Nothing, Unit].add(_ => completed += 1)
+          state.complete(ZIO.unit)
+          assert(completed)(equalTo(1))
+        },
+        test("multiple") {
+          val n     = 10
+          var completed = Seq.empty[Int]
+          val state = (0 until n).foldLeft(empty[Nothing, Unit])((acc, i) => acc.add(_ => completed = completed :+ i))
+          state.complete(ZIO.unit)
+          assert(completed)(equalTo(0 until n))
+        }
+      )
+    )
   )
 }
diff --git a/core/shared/src/main/scala/zio/Promise.scala b/core/shared/src/main/scala/zio/Promise.scala
index 5c6c56f6489a..2521939dace3 100644
--- a/core/shared/src/main/scala/zio/Promise.scala
+++ b/core/shared/src/main/scala/zio/Promise.scala
@@ -49,7 +49,7 @@ final class Promise[E, A] private (blockingOn: FiberId) extends Serializable {
       state.get match {
         case Done(value) => value
         case pending =>
-          ZIO.async[Any, E, A]( // intentionally never remove the callback, interrupted fibers won't be resumed
+          ZIO.asyncInterrupt[Any, E, A](
             k => {
               @annotation.tailrec
               def loop(current: State[E, A]): Unit =
@@ -60,6 +60,11 @@ final class Promise[E, A] private (blockingOn: FiberId) extends Serializable {
                   case Done(value) => k(value)
                 }
               loop(pending)
+
+              Left(ZIO.succeed(state.updateAndGet {
+                case pending: Pending[?, ?] => pending.remove(k)
+                case completed              => completed
+              }))
             },
             blockingOn
           )
@@ -238,39 +243,67 @@ final class Promise[E, A] private (blockingOn: FiberId) extends Serializable {
 
 }
 object Promise {
-  private[Promise] object internal {
+  private[zio] object internal {
     sealed abstract class State[E, A]            extends Serializable
     final case class Done[E, A](value: IO[E, A]) extends State[E, A]
     sealed abstract class Pending[E, A] extends State[E, A] { self =>
       def complete(io: IO[E, A]): Unit
       def add(waiter: IO[E, A] => Any): Pending[E, A]
+      def remove(waiter: IO[E, A] => Any): Pending[E, A]
       def size: Int
     }
     private case object Empty extends Pending[Nothing, Nothing] { self =>
       override def complete(io: IO[Nothing, Nothing]): Unit = ()
-      override def add(waiter: IO[Nothing, Nothing] => Any): Pending[Nothing, Nothing] =
-        new Link[Nothing, Nothing](waiter, self, 1) {
-          override def complete(io: IO[Nothing, Nothing]): Unit = this.waiter(io)
+      def size                                              = 0
+      def add(waiter: IO[Nothing, Nothing] => Any): Pending[Nothing, Nothing]     = new Link[Nothing, Nothing](waiter, self) {
+        override def size = 1
+      }
+      def remove(waiter: IO[Nothing, Nothing] => Any): Pending[Nothing, Nothing]     = self
+    }
+    private sealed abstract class Link[E, A](val waiter: IO[E, A] => Any, val ws: Pending[E, A]) extends Pending[E, A] { self =>
+      final def add(waiter: IO[E, A] => Any): Pending[E, A] = new Link(waiter, self) {
+        override val size = self.size + 1
+        override val ws   = self
+      }
+      final def complete(io: IO[E, A]): Unit =
+        if (size == 1) waiter(io)
+        else Link.materialize(self, size).foreach(_(io))
+
+      final def remove(waiter: IO[E, A] => Any): Pending[E, A] =
+        if (size == 1 && (waiter eq self.waiter)) ws
+        else {
+          val arr = Link.materialize(self, size)
+
+          @annotation.tailrec
+          def tabulate(i: Int, acc: Pending[E, A]): Pending[E, A] =
+            if (i >= 0) {
+              if (arr(i) ne waiter)
+                tabulate(i - 1, acc.add(arr(i)))
+              else
+                tabulate(i - 1, acc)
+            } else acc
+
+          tabulate(size - 1, Empty.asInstanceOf[Pending[E, A]])
         }
-      def size = 0
     }
-    private sealed class Link[E, A](val waiter: IO[E, A] => Any, val ws: Pending[E, A], val size: Int)
-        extends Pending[E, A] { self =>
-      def complete(io: IO[E, A]): Unit = {
-        val size = self.size
-        val arr  = new Array[IO[E, A] => Any](size)
+
+    private object Link {
+      def materialize[E, A](pending: Pending[E, A], size: Int): Array[IO[E, A] => Any] = {
+        val array = new Array[IO[E, A] => Any](size)
+
         @annotation.tailrec
         def fill(pending: Pending[E, A], i: Int): Unit =
-          pending match {
-            case link: Link[?, ?] =>
-              arr(i) = link.waiter
-              fill(link.ws, i - 1)
-            case _ => () // Empty
-          }
-        fill(self, size - 1)
-        arr.foreach(_(io))
+          if (i >= 0)
+            pending match {
+              case link: Link[?, ?] =>
+                array(i) = link.waiter
+                fill(link.ws, i - 1)
+              case _ => () // Empty
+            }
+
+        fill(pending, size - 1)
+        array
       }
-      def add(waiter: IO[E, A] => Any): Pending[E, A] = new Link(waiter, self, size + 1)
     }
 
     object State {

From c85456279a48465a95448773f8ec8471d972e3d4 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Sat, 5 Apr 2025 12:36:42 -0700
Subject: [PATCH 27/34] fmt

---
 .../shared/src/test/scala/zio/PromiseSpec.scala   | 15 ++++++++-------
 core/shared/src/main/scala/zio/Promise.scala      | 12 +++++++-----
 2 files changed, 15 insertions(+), 12 deletions(-)

diff --git a/core-tests/shared/src/test/scala/zio/PromiseSpec.scala b/core-tests/shared/src/test/scala/zio/PromiseSpec.scala
index 6cd344b57cbb..e62a093563e0 100644
--- a/core-tests/shared/src/test/scala/zio/PromiseSpec.scala
+++ b/core-tests/shared/src/test/scala/zio/PromiseSpec.scala
@@ -7,7 +7,8 @@ object PromiseSpec extends ZIOBaseSpec {
 
   import ZIOTag._
 
-  private def empty[E, A]: Promise.internal.Pending[E, A] = Promise.internal.State.empty[E, A].asInstanceOf[Promise.internal.Pending[E, A]]
+  private def empty[E, A]: Promise.internal.Pending[E, A] =
+    Promise.internal.State.empty[E, A].asInstanceOf[Promise.internal.Pending[E, A]]
 
   def spec: Spec[Any, TestFailure[Any]] = suite("PromiseSpec")(
     test("complete a promise using succeed") {
@@ -164,9 +165,9 @@ object PromiseSpec extends ZIOBaseSpec {
           assert(increment)(equalTo(0))
         },
         test("multiple") {
-          val n     = 10
-          var fired = 0
-          val cb    = (_: IO[Nothing, Unit]) => ()
+          val n        = 10
+          var fired    = 0
+          val cb       = (_: IO[Nothing, Unit]) => ()
           val toRemove = (_: IO[Nothing, Unit]) => fired += 1
           val state =
             (0 until n).foldLeft(empty[Nothing, Unit])((acc, i) => if (i < 5) acc.add(cb) else acc.add(toRemove))
@@ -179,14 +180,14 @@ object PromiseSpec extends ZIOBaseSpec {
       suite("complete")(
         test("one") {
           var completed = 0
-          val state = empty[Nothing, Unit].add(_ => completed += 1)
+          val state     = empty[Nothing, Unit].add(_ => completed += 1)
           state.complete(ZIO.unit)
           assert(completed)(equalTo(1))
         },
         test("multiple") {
-          val n     = 10
+          val n         = 10
           var completed = Seq.empty[Int]
-          val state = (0 until n).foldLeft(empty[Nothing, Unit])((acc, i) => acc.add(_ => completed = completed :+ i))
+          val state     = (0 until n).foldLeft(empty[Nothing, Unit])((acc, i) => acc.add(_ => completed = completed :+ i))
           state.complete(ZIO.unit)
           assert(completed)(equalTo(0 until n))
         }
diff --git a/core/shared/src/main/scala/zio/Promise.scala b/core/shared/src/main/scala/zio/Promise.scala
index 2521939dace3..3f6e662421a9 100644
--- a/core/shared/src/main/scala/zio/Promise.scala
+++ b/core/shared/src/main/scala/zio/Promise.scala
@@ -255,12 +255,14 @@ object Promise {
     private case object Empty extends Pending[Nothing, Nothing] { self =>
       override def complete(io: IO[Nothing, Nothing]): Unit = ()
       def size                                              = 0
-      def add(waiter: IO[Nothing, Nothing] => Any): Pending[Nothing, Nothing]     = new Link[Nothing, Nothing](waiter, self) {
-        override def size = 1
-      }
-      def remove(waiter: IO[Nothing, Nothing] => Any): Pending[Nothing, Nothing]     = self
+      def add(waiter: IO[Nothing, Nothing] => Any): Pending[Nothing, Nothing] =
+        new Link[Nothing, Nothing](waiter, self) {
+          override def size = 1
+        }
+      def remove(waiter: IO[Nothing, Nothing] => Any): Pending[Nothing, Nothing] = self
     }
-    private sealed abstract class Link[E, A](val waiter: IO[E, A] => Any, val ws: Pending[E, A]) extends Pending[E, A] { self =>
+    private sealed abstract class Link[E, A](val waiter: IO[E, A] => Any, val ws: Pending[E, A]) extends Pending[E, A] {
+      self =>
       final def add(waiter: IO[E, A] => Any): Pending[E, A] = new Link(waiter, self) {
         override val size = self.size + 1
         override val ws   = self

From 73dc4130b16c133359092fe4e44e5083895d233d Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Sat, 5 Apr 2025 12:40:56 -0700
Subject: [PATCH 28/34] cleanup

---
 core/shared/src/main/scala/zio/Promise.scala | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/core/shared/src/main/scala/zio/Promise.scala b/core/shared/src/main/scala/zio/Promise.scala
index 3f6e662421a9..0dcd77996585 100644
--- a/core/shared/src/main/scala/zio/Promise.scala
+++ b/core/shared/src/main/scala/zio/Promise.scala
@@ -261,11 +261,11 @@ object Promise {
         }
       def remove(waiter: IO[Nothing, Nothing] => Any): Pending[Nothing, Nothing] = self
     }
-    private sealed abstract class Link[E, A](val waiter: IO[E, A] => Any, val ws: Pending[E, A]) extends Pending[E, A] {
+    private sealed abstract class Link[E, A](final val waiter: IO[E, A] => Any, final val ws: Pending[E, A])
+        extends Pending[E, A] {
       self =>
       final def add(waiter: IO[E, A] => Any): Pending[E, A] = new Link(waiter, self) {
         override val size = self.size + 1
-        override val ws   = self
       }
       final def complete(io: IO[E, A]): Unit =
         if (size == 1) waiter(io)

From 4ddbb3001300b99f76d896c8acd7968b3c74c817 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Sat, 5 Apr 2025 12:50:28 -0700
Subject: [PATCH 29/34] protected size

---
 core/shared/src/main/scala/zio/Promise.scala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/core/shared/src/main/scala/zio/Promise.scala b/core/shared/src/main/scala/zio/Promise.scala
index 0dcd77996585..c1f366f5123f 100644
--- a/core/shared/src/main/scala/zio/Promise.scala
+++ b/core/shared/src/main/scala/zio/Promise.scala
@@ -250,7 +250,7 @@ object Promise {
       def complete(io: IO[E, A]): Unit
       def add(waiter: IO[E, A] => Any): Pending[E, A]
       def remove(waiter: IO[E, A] => Any): Pending[E, A]
-      def size: Int
+      protected def size: Int
     }
     private case object Empty extends Pending[Nothing, Nothing] { self =>
       override def complete(io: IO[Nothing, Nothing]): Unit = ()

From 7eb086e4aaa7a61a331f54186b9c6312554060a3 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Sat, 5 Apr 2025 13:20:55 -0700
Subject: [PATCH 30/34] Revert "protected size"

This reverts commit 4ddbb3001300b99f76d896c8acd7968b3c74c817.
---
 core/shared/src/main/scala/zio/Promise.scala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/core/shared/src/main/scala/zio/Promise.scala b/core/shared/src/main/scala/zio/Promise.scala
index c1f366f5123f..0dcd77996585 100644
--- a/core/shared/src/main/scala/zio/Promise.scala
+++ b/core/shared/src/main/scala/zio/Promise.scala
@@ -250,7 +250,7 @@ object Promise {
       def complete(io: IO[E, A]): Unit
       def add(waiter: IO[E, A] => Any): Pending[E, A]
       def remove(waiter: IO[E, A] => Any): Pending[E, A]
-      protected def size: Int
+      def size: Int
     }
     private case object Empty extends Pending[Nothing, Nothing] { self =>
       override def complete(io: IO[Nothing, Nothing]): Unit = ()

From c818f4c6080501be8e8dbd72d6fb70d042a5b0c0 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Sat, 26 Apr 2025 07:04:24 -0700
Subject: [PATCH 31/34] fix merge

---
 project/MimaSettings.scala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/project/MimaSettings.scala b/project/MimaSettings.scala
index 01c8bbd1ee93..69fd86d52bc3 100644
--- a/project/MimaSettings.scala
+++ b/project/MimaSettings.scala
@@ -36,7 +36,7 @@ object MimaSettings {
         exclude[NewMixinForwarderProblem]("zio.Exit.mapErrorCause"),
         exclude[NewMixinForwarderProblem]("zio.Exit.unit"),
         exclude[Problem]("zio.Promise#internal*"),
-        exclude[Problem]("zio.Promise$internal*")
+        exclude[Problem]("zio.Promise$internal*"),
         exclude[Problem]("zio.Queue#Strategy*.shutdown")
       ),
       mimaFailOnProblem := failOnProblem

From 9b906aa0f8da669126af6accd24c2ae7269b8d32 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Sat, 10 May 2025 05:48:05 -0700
Subject: [PATCH 32/34] fix merge

---
 core-tests/shared/src/test/scala/zio/FiberRuntimeSpec.scala | 1 +
 1 file changed, 1 insertion(+)

diff --git a/core-tests/shared/src/test/scala/zio/FiberRuntimeSpec.scala b/core-tests/shared/src/test/scala/zio/FiberRuntimeSpec.scala
index 7769cd472549..ebc2ffc1e235 100644
--- a/core-tests/shared/src/test/scala/zio/FiberRuntimeSpec.scala
+++ b/core-tests/shared/src/test/scala/zio/FiberRuntimeSpec.scala
@@ -99,6 +99,7 @@ object FiberRuntimeSpec extends ZIOBaseSpec {
           )
         }
       } @@ TestAspect.nonFlaky(10)
+    ),
     suite("runtime metrics")(
       test("Failures are counted once for the fiber that caused them and exits are not") {
         val nullErrors = ZIO.foreachParDiscard(1 to 2)(_ => ZIO.attempt(throw new NullPointerException))

From 207bd2740469fd708f25a691dabf006ead5dd5f6 Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Wed, 14 May 2025 12:04:51 -0600
Subject: [PATCH 33/34] address in person review

---
 .../src/test/scala/zio/PromiseSpec.scala      | 13 ++--
 core/shared/src/main/scala/zio/Promise.scala  | 61 +++++++++++--------
 2 files changed, 43 insertions(+), 31 deletions(-)

diff --git a/core-tests/shared/src/test/scala/zio/PromiseSpec.scala b/core-tests/shared/src/test/scala/zio/PromiseSpec.scala
index e62a093563e0..747a9024e6e6 100644
--- a/core-tests/shared/src/test/scala/zio/PromiseSpec.scala
+++ b/core-tests/shared/src/test/scala/zio/PromiseSpec.scala
@@ -10,6 +10,8 @@ object PromiseSpec extends ZIOBaseSpec {
   private def empty[E, A]: Promise.internal.Pending[E, A] =
     Promise.internal.State.empty[E, A].asInstanceOf[Promise.internal.Pending[E, A]]
 
+  val n = 10000
+
   def spec: Spec[Any, TestFailure[Any]] = suite("PromiseSpec")(
     test("complete a promise using succeed") {
       for {
@@ -127,7 +129,7 @@ object PromiseSpec extends ZIOBaseSpec {
     test("waiter stack safety") {
       for {
         p      <- Promise.make[Nothing, Unit]
-        fibers <- ZIO.foreach(1 to 100000)(_ => p.await.forkDaemon)
+        fibers <- ZIO.foreach(1 to n)(_ => p.await.forkDaemon)
         _      <- p.complete(Exit.unit)
         _      <- ZIO.foreach(fibers)(_.await)
       } yield assertCompletes
@@ -147,7 +149,6 @@ object PromiseSpec extends ZIOBaseSpec {
           assert(increment)(equalTo(1))
         },
         test("multiple") {
-          val n         = 10
           var increment = 0
           val state     = (0 until n).foldLeft(empty[Nothing, Unit])((acc, _) => acc.add(_ => increment += 1))
           state.complete(ZIO.unit)
@@ -165,7 +166,6 @@ object PromiseSpec extends ZIOBaseSpec {
           assert(increment)(equalTo(0))
         },
         test("multiple") {
-          val n        = 10
           var fired    = 0
           val cb       = (_: IO[Nothing, Unit]) => ()
           val toRemove = (_: IO[Nothing, Unit]) => fired += 1
@@ -185,11 +185,10 @@ object PromiseSpec extends ZIOBaseSpec {
           assert(completed)(equalTo(1))
         },
         test("multiple") {
-          val n         = 10
-          var completed = Seq.empty[Int]
-          val state     = (0 until n).foldLeft(empty[Nothing, Unit])((acc, i) => acc.add(_ => completed = completed :+ i))
+          var completed = List.empty[Int]
+          val state     = (0 until n).foldLeft(empty[Nothing, Unit])((acc, i) => acc.add(_ => completed = i :: completed))
           state.complete(ZIO.unit)
-          assert(completed)(equalTo(0 until n))
+          assert(completed)(equalTo(List.range(0, n)))
         }
       )
     )
diff --git a/core/shared/src/main/scala/zio/Promise.scala b/core/shared/src/main/scala/zio/Promise.scala
index 0dcd77996585..7a922d86120c 100644
--- a/core/shared/src/main/scala/zio/Promise.scala
+++ b/core/shared/src/main/scala/zio/Promise.scala
@@ -269,41 +269,54 @@ object Promise {
       }
       final def complete(io: IO[E, A]): Unit =
         if (size == 1) waiter(io)
-        else Link.materialize(self, size).foreach(_(io))
+        else {
+          var current: Pending[E, A] = self
+          while (current ne Empty) {
+            current match {
+              case link: Link[?, ?] =>
+                link.waiter(io)
+                current = link.ws
+              case _ => // Empty
+                current = Empty.asInstanceOf[Pending[E, A]]
+            }
+          }
+        }
 
       final def remove(waiter: IO[E, A] => Any): Pending[E, A] =
-        if (size == 1 && (waiter eq self.waiter)) ws
+        if (size == 1) if (waiter eq self.waiter) ws else self
         else {
           val arr = Link.materialize(self, size)
+          var i = size - 1
+          var acc: Pending[E, A] = Empty.asInstanceOf[Pending[E, A]]
 
-          @annotation.tailrec
-          def tabulate(i: Int, acc: Pending[E, A]): Pending[E, A] =
-            if (i >= 0) {
-              if (arr(i) ne waiter)
-                tabulate(i - 1, acc.add(arr(i)))
-              else
-                tabulate(i - 1, acc)
-            } else acc
-
-          tabulate(size - 1, Empty.asInstanceOf[Pending[E, A]])
+          while (i >= 0) {
+            if (arr(i) ne waiter) {
+              acc = acc.add(arr(i))
+            }
+            i -= 1
+          }
+          acc
         }
     }
 
     private object Link {
+      /**
+        * Materializes the pending state into an array of waiters in reverse order.
+        */
       def materialize[E, A](pending: Pending[E, A], size: Int): Array[IO[E, A] => Any] = {
         val array = new Array[IO[E, A] => Any](size)
-
-        @annotation.tailrec
-        def fill(pending: Pending[E, A], i: Int): Unit =
-          if (i >= 0)
-            pending match {
-              case link: Link[?, ?] =>
-                array(i) = link.waiter
-                fill(link.ws, i - 1)
-              case _ => () // Empty
-            }
-
-        fill(pending, size - 1)
+        var current = pending
+        var i = size - 1
+
+        while (i >= 0) {
+          current match {
+            case link: Link[?, ?] =>
+              array(i) = link.waiter
+              current = link.ws
+            case _ => () // Empty
+          }
+          i -= 1
+        }
         array
       }
     }

From dd855c31eaf04293eca0defd4219009488d2716a Mon Sep 17 00:00:00 2001
From: Adam Hearn <22334119+hearnadam@users.noreply.github.com>
Date: Wed, 14 May 2025 12:05:34 -0600
Subject: [PATCH 34/34] format

---
 core/shared/src/main/scala/zio/Promise.scala | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/core/shared/src/main/scala/zio/Promise.scala b/core/shared/src/main/scala/zio/Promise.scala
index 7a922d86120c..9e54ef47bf85 100644
--- a/core/shared/src/main/scala/zio/Promise.scala
+++ b/core/shared/src/main/scala/zio/Promise.scala
@@ -285,8 +285,8 @@ object Promise {
       final def remove(waiter: IO[E, A] => Any): Pending[E, A] =
         if (size == 1) if (waiter eq self.waiter) ws else self
         else {
-          val arr = Link.materialize(self, size)
-          var i = size - 1
+          val arr                = Link.materialize(self, size)
+          var i                  = size - 1
           var acc: Pending[E, A] = Empty.asInstanceOf[Pending[E, A]]
 
           while (i >= 0) {
@@ -300,13 +300,15 @@ object Promise {
     }
 
     private object Link {
+
       /**
-        * Materializes the pending state into an array of waiters in reverse order.
-        */
+       * Materializes the pending state into an array of waiters in reverse
+       * order.
+       */
       def materialize[E, A](pending: Pending[E, A], size: Int): Array[IO[E, A] => Any] = {
-        val array = new Array[IO[E, A] => Any](size)
+        val array   = new Array[IO[E, A] => Any](size)
         var current = pending
-        var i = size - 1
+        var i       = size - 1
 
         while (i >= 0) {
           current match {