Added tests for MacrotaskExecutor

Author: djspiewak

core/src/main/scala/org/scalajs/macrotaskexecutor/MacrotaskExecutor.scala

@@ -150,4 +150,8 @@ object MacrotaskExecutor extends ExecutionContext {
       }
     }
   }
+
+  object Implicits {
+    implicit def global: ExecutionContext = MacrotaskExecutor
+  }
 }

core/src/test/scala/org/scalajs/macrotaskexecutor/MacrotaskExecutorSuite.scala

@@ -18,6 +18,59 @@ package org.scalajs.macrotaskexecutor
 
 import munit.FunSuite
 
+import scala.concurrent.Future
+import scala.concurrent.duration._
+import scala.scalajs.js
+
 class MacrotaskExecutorSuite extends FunSuite {
-  
+  import MacrotaskExecutor.Implicits._
+
+  test("sequence a series of 10,000 recursive executions without clamping") {
+    def loop(n: Int): Future[Int] =
+      if (n <= 0)
+        Future(0)
+      else
+        Future.unit.flatMap(_ => loop(n - 1)).map(_ + 1)
+
+    val start = System.currentTimeMillis()
+    val MinimumClamp = 10000 * 2 * 4    // HTML5 specifies a 4ms clamp (https://developer.mozilla.org/en-US/docs/Web/API/WindowTimers.setTimeout#Minimum.2F_maximum_delay_and_timeout_nesting)
+
+    loop(10000) flatMap { res =>
+      Future {
+        val end = System.currentTimeMillis()
+
+        assert(res == 10000)
+        assert((end - start).toDouble / MinimumClamp < 0.25)   // we should beat the clamping by at least 4x even on slow environments
+      }
+    }
+  }
+
+  // this test fails to terminate with a Promise-based executor
+  test("preserve fairness with setTimeout") {
+    var cancel = false
+
+    def loop(): Future[Unit] =
+      Future(cancel) flatMap { canceled =>
+        if (canceled)
+          Future.unit
+        else
+          loop()
+      }
+
+    js.timers.setTimeout(100.millis) {
+      cancel = true
+    }
+
+    loop()
+  }
+
+  test("execute a bunch of stuff in 'parallel' and ensure it all runs") {
+    var i = 0
+
+    Future.sequence(List.fill(10000)(Future { i += 1 })) flatMap { _ =>
+      Future {
+        assert(i == 10000)
+      }
+    }
+  }
 }