Remove race condition in Scheduler.effect.delayCancellable

core/jvm/src/test/scala/fs2/SchedulerSpec.scala

@@ -37,5 +37,16 @@ class SchedulerSpec extends AsyncFs2Spec {
         assert(r == Vector(3, 6))
       }
     }
+
+    "delay cancellable: cancel after completion is no op" in {
+      val s = mkScheduler
+        .evalMap { s =>
+          s.effect.delayCancellable(IO.unit, 20.millis).flatMap(t => t._1 <* t._2)
+        }
+
+      runLogF(s).map { r =>
+        r.head shouldBe Some(())
+      }
+    }
   }
 }

core/shared/src/main/scala/fs2/Scheduler.scala

@@ -274,18 +274,20 @@ object Scheduler extends SchedulerPlatform {
     /**
       * Starts a timer for duration `d` and after completion of the timer, evaluates `fa`.
       * Returns a "gate" value which allows semantic blocking on the result of `fa` and an action which cancels the timer.
-      * If the cancellation action is invoked, the gate completes with `None`. Otherwise, the gate completes with `Some(a)`.
+      * If the cancellation action is invoked before the timer completes, the gate completes with `None`. Otherwise, the
+      * gate completes with `Some(a)`.
       */
     def delayCancellable[F[_], A](fa: F[A], d: FiniteDuration)(
         implicit F: Effect[F],
         ec: ExecutionContext): F[(F[Option[A]], F[Unit])] =
       async.promise[F, Option[A]].flatMap { gate =>
         F.delay {
+          // needs to be a val, executes the task
           val cancel = scheduler.scheduleOnce(d) {
             ec.execute(() =>
               async.unsafeRunAsync(fa.flatMap(a => gate.complete(Some(a))))(_ => IO.unit))
           }
-          gate.get -> (F.delay(cancel()) *> gate.complete(None))
+          gate.get -> (F.delay(cancel()) *> gate.complete(None).handleError(_ => ()))
         }
       }