zio · adamgfraser · Jul 27, 2021 · Jul 27, 2021
diff --git a/core/js/src/main/scala/zio/internal/PlatformSpecific.scala b/core/js/src/main/scala/zio/internal/PlatformSpecific.scala
@@ -74,6 +74,8 @@ private[internal] trait PlatformSpecific {
     new Platform {
       val executor = executor0
 
+      val yieldOnStart = false
+
       def fatal(t: Throwable): Boolean = false
 
       def reportFatal(t: Throwable): Nothing = {

diff --git a/core/jvm/src/main/scala/zio/internal/PlatformSpecific.scala b/core/jvm/src/main/scala/zio/internal/PlatformSpecific.scala
@@ -80,6 +80,8 @@ private[internal] trait PlatformSpecific {
     new Platform {
       val executor = executor0
 
+      val yieldOnStart = true
+
       val tracing = Tracing(Tracer.globallyCached(new AkkaLineNumbersTracer), TracingConfig.enabled)
 
       def fatal(t: Throwable): Boolean =

diff --git a/core/native/src/main/scala/zio/internal/PlatformSpecific.scala b/core/native/src/main/scala/zio/internal/PlatformSpecific.scala
@@ -74,6 +74,8 @@ private[internal] trait PlatformSpecific {
     new Platform {
       val executor = executor0
 
+      val yieldOnStart = false
+
       def fatal(t: Throwable): Boolean = false
 
       def reportFatal(t: Throwable): Nothing = {

diff --git a/core/shared/src/main/scala/zio/Runtime.scala b/core/shared/src/main/scala/zio/Runtime.scala
@@ -102,7 +102,7 @@ trait Runtime[+R] {
    * This method is effectful and should only be invoked at the edges of your program.
    */
   final def unsafeRunAsyncCancelable[E, A](zio: => ZIO[R, E, A])(k: Exit[E, A] => Any): Fiber.Id => Exit[E, A] = {
-    lazy val curZio = if (Platform.isJVM) ZIO.yieldNow *> zio else zio
+    lazy val curZio = if (platform.yieldOnStart) ZIO.yieldNow *> zio else zio
     val canceler    = unsafeRunWith(curZio)(k)
     fiberId => {
       val result = internal.OneShot.make[Exit[E, A]]
@@ -168,6 +168,11 @@ trait Runtime[+R] {
    */
   def withTracing(t: Tracing): Runtime[R] = mapPlatform(_.withTracing(t))
 
+  /**
+   * Constructs a new `Runtime` with the specified yield strategy.
+   */
+  def withYieldOnStart(cond: Boolean): Runtime[R] = mapPlatform(_.withYieldOnStart(cond))
+
   /**
    * Constructs a new `Runtime` with the specified tracing configuration.
    */

diff --git a/core/shared/src/main/scala/zio/internal/Platform.scala b/core/shared/src/main/scala/zio/internal/Platform.scala
@@ -30,6 +30,11 @@ abstract class Platform { self =>
    */
   def executor: Executor
 
+  /**
+   * Specifies if ZIO should yield immediately or not.
+   */
+  def yieldOnStart: Boolean
+
   def withExecutor(e: Executor): Platform =
     new Platform.Proxy(self) {
       override def executor: Executor = e
@@ -50,6 +55,14 @@ abstract class Platform { self =>
       override val tracing: Tracing = self.tracing.copy(tracingConfig = config)
     }
 
+  /**
+   * Determines if the `Runtime` should yield right at the beginning of the evaluation.
+   */
+  def withYieldOnStart(cond: Boolean): Platform =
+    new Platform.Proxy(self) {
+      override val yieldOnStart: Boolean = cond
+    }
+
   /**
    * Determines if a throwable is fatal or not. It is important to identify
    * these as it is not recommended to catch, and try to recover from, any
@@ -97,5 +110,6 @@ object Platform extends PlatformSpecific {
     def reportFatal(t: Throwable): Nothing     = self.reportFatal(t)
     def reportFailure(cause: Cause[Any]): Unit = self.reportFailure(cause)
     def supervisor: Supervisor[Any]            = self.supervisor
+    def yieldOnStart: Boolean                  = self.yieldOnStart
   }
 }