Fix #780: improve fixed rate scheduling on JS

monix-execution/js/src/test/scala/monix/execution/internal/AsyncSchedulerSuite.scala

@@ -20,7 +20,8 @@ package monix.execution.internal
 import minitest.TestSuite
 import monix.execution.atomic.Atomic
 import monix.execution.schedulers.{AsyncScheduler, StandardContext}
-import monix.execution.{ExecutionModel, Scheduler, UncaughtExceptionReporter}
+import monix.execution.{Cancelable, ExecutionModel, Scheduler, UncaughtExceptionReporter}
+
 import scala.concurrent.Promise
 import scala.concurrent.duration._
 
@@ -88,6 +89,26 @@ object AsyncSchedulerSuite extends TestSuite[Scheduler] {
     }
   }
 
+  testAsync("scheduleWithFixedRate should compensate for scheduling inaccuracy") { implicit s =>
+    import concurrent.duration._
+    val p = Promise[Unit]()
+    val startAt = s.clockMonotonic(MILLISECONDS)
+    var count = 0
+    lazy val c: Cancelable = s.scheduleAtFixedRate(Duration.Zero, 20.millis) {
+      count += 1
+      if (count == 500) {
+        c.cancel()
+        p.success(())
+      }
+    }
+    c // trigger evaluation
+
+    for (_ <- p.future) yield {
+      val duration = s.clockMonotonic(MILLISECONDS) - startAt
+      assert(Math.abs(duration - 10000) <= 20, "Error <= 20ms")
+    }
+  }
+
   test("clockRealTime") { s =>
     val t1 = System.currentTimeMillis()
     val t2 = s.clockRealTime(MILLISECONDS)

monix-execution/shared/src/main/scala/monix/execution/schedulers/ReferenceScheduler.scala

@@ -57,12 +57,13 @@ trait ReferenceScheduler extends Scheduler {
   override def scheduleAtFixedRate(initialDelay: Long, period: Long, unit: TimeUnit, r: Runnable): Cancelable = {
     val sub = OrderedCancelable()
 
-    def loop(initialDelayMs: Long, periodMs: Long): Unit =
+    def loop(initialDelayMs: Long, periodMs: Long): Unit = {
+      // Measuring the duration of the task + possible scheduler lag
+      val startedAtMillis = clockMonotonic(MILLISECONDS) + initialDelayMs
+
       if (!sub.isCanceled) {
         sub := scheduleOnce(initialDelayMs, MILLISECONDS, new Runnable {
           def run(): Unit = {
-            // Measuring the duration of the task
-            val startedAtMillis = clockMonotonic(MILLISECONDS)
             r.run()
 
             val delay = {
@@ -76,6 +77,7 @@ trait ReferenceScheduler extends Scheduler {
           }
         })
       }
+    }
 
     val initialMs = MILLISECONDS.convert(initialDelay, unit)
     val periodMs = MILLISECONDS.convert(period, unit)

monix-execution/shared/src/main/scala/monix/execution/schedulers/TestScheduler.scala

@@ -392,7 +392,6 @@ object TestScheduler {
 
     val newID = state.lastID + 1
     SingleAssignCancelable()
-
     val task = Task(newID, r, state.clock + delay)
     val cancelable = new Cancelable {
       def cancel(): Unit = cancelTask(task)

monix-execution/shared/src/test/scala/monix/execution/schedulers/ReferenceSchedulerSuite.scala

@@ -39,6 +39,11 @@ object ReferenceSchedulerSuite extends SimpleTestSuite {
       underlying.scheduleOnce(initialDelay, unit, r)
   }
 
+  class DummyTimeScheduler extends DummyScheduler() {
+    override def clockRealTime(unit: TimeUnit): Long = underlying.clockRealTime(unit)
+    override def clockMonotonic(unit: TimeUnit): Long = underlying.clockMonotonic(unit)
+  }
+
   test("clockRealTime") {
     val s = new DummyScheduler
     val ws = s.withExecutionModel(SynchronousExecution)
@@ -73,7 +78,7 @@ object ReferenceSchedulerSuite extends SimpleTestSuite {
   }
 
   test("schedule at fixed rate") {
-    val s = new DummyScheduler
+    val s = new DummyTimeScheduler
     val ws = s.withExecutionModel(SynchronousExecution)
 
     var effect = 0