diff --git a/main/src/main/scala/sbt/Defaults.scala b/main/src/main/scala/sbt/Defaults.scala
index 0498650b8..8e379b6b5 100644
--- a/main/src/main/scala/sbt/Defaults.scala
+++ b/main/src/main/scala/sbt/Defaults.scala
@@ -2039,6 +2039,7 @@ object Defaults extends BuildCommon {
     } catch {
       case e: Throwable if !promise.isCompleted =>
         promise.failure(e)
+        ConcurrentRestrictions.cancelAllSentinels()
         throw e
     } finally {
       i.setup.reporter match {
diff --git a/main/src/main/scala/sbt/Project.scala b/main/src/main/scala/sbt/Project.scala
index a86dd155a..232410149 100755
--- a/main/src/main/scala/sbt/Project.scala
+++ b/main/src/main/scala/sbt/Project.scala
@@ -53,6 +53,7 @@ import sbt.util.{ Show, Level }
 import sjsonnew.JsonFormat
 
 import language.experimental.macros
+import scala.concurrent.TimeoutException
 import scala.concurrent.duration.FiniteDuration
 
 sealed trait ProjectDefinition[PR <: ProjectReference] {
@@ -887,8 +888,20 @@ object Project extends ProjectExtra {
       (Def
         .task {
           val p = i.value
-          val result: A = Await.result(p.underlying.future, atMost)
-          result
+          var result: Option[A] = None
+          if (atMost == Duration.Inf) {
+            while (result.isEmpty) {
+              try {
+                result = Some(Await.result(p.underlying.future, Duration("1s")))
+                Thread.sleep(10)
+              } catch {
+                case _: TimeoutException => ()
+              }
+            }
+          } else {
+            result = Some(Await.result(p.underlying.future, atMost))
+          }
+          result.get
         })
         .tag(Tags.Sentinel)
   }
diff --git a/tasks/src/main/scala/sbt/CompletionService.scala b/tasks/src/main/scala/sbt/CompletionService.scala
index 347c514a6..f9586a5da 100644
--- a/tasks/src/main/scala/sbt/CompletionService.scala
+++ b/tasks/src/main/scala/sbt/CompletionService.scala
@@ -29,6 +29,7 @@ import java.util.concurrent.{
   Executor,
   ExecutorCompletionService,
   Executors,
+  Future => JFuture,
   RejectedExecutionException,
   CompletionService => JCompletionService
 }
@@ -52,8 +53,12 @@ object CompletionService {
       def take() = completion.take().get()
     }
   def submit[T](work: () => T, completion: JCompletionService[T]): () => T = {
+    val future = submitFuture[T](work, completion)
+    () => future.get
+  }
+  private[sbt] def submitFuture[A](work: () => A, completion: JCompletionService[A]): JFuture[A] = {
     val future = try completion.submit {
-      new Callable[T] {
+      new Callable[A] {
         def call =
           try {
             work()
@@ -66,7 +71,7 @@ object CompletionService {
       case _: RejectedExecutionException =>
         throw Incomplete(None, message = Some("cancelled"))
     }
-    () => future.get()
+    future
   }
   def manage[A, T](
       service: CompletionService[A, T]
diff --git a/tasks/src/main/scala/sbt/ConcurrentRestrictions.scala b/tasks/src/main/scala/sbt/ConcurrentRestrictions.scala
index e3ed01c60..b5172ed57 100644
--- a/tasks/src/main/scala/sbt/ConcurrentRestrictions.scala
+++ b/tasks/src/main/scala/sbt/ConcurrentRestrictions.scala
@@ -11,7 +11,8 @@ import java.util.concurrent.atomic.AtomicInteger
 
 import sbt.internal.util.AttributeKey
 import java.util.concurrent.atomic.AtomicBoolean
-import java.util.concurrent.RejectedExecutionException
+import java.util.concurrent.{ Future => JFuture, RejectedExecutionException }
+import scala.collection.mutable
 
 /**
  * Describes restrictions on concurrent execution for a set of tasks.
@@ -45,6 +46,10 @@ trait ConcurrentRestrictions[A] {
   def valid(g: G): Boolean
 }
 
+private[sbt] sealed trait CancelSentiels {
+  def cancelSentinels(): Unit
+}
+
 import java.util.{ LinkedList, Queue }
 import java.util.concurrent.{ Executor, Executors, ExecutorCompletionService }
 import annotation.tailrec
@@ -57,6 +62,11 @@ object ConcurrentRestrictions {
     case _                =>
   }
 
+  private[sbt] def cancelAllSentinels() = completionServices.keySet.asScala.toVector.foreach {
+    case a: CancelSentiels => a.cancelSentinels()
+    case _                 =>
+  }
+
   /**
    * A ConcurrentRestrictions instance that places no restrictions on concurrently executing tasks.
    * @param zero the constant placeholder used for t
@@ -181,12 +191,12 @@ object ConcurrentRestrictions {
       tags: ConcurrentRestrictions[A],
       warn: String => Unit,
       isSentinel: A => Boolean,
-  ): CompletionService[A, R] with AutoCloseable = {
+  ): CompletionService[A, R] with CancelSentiels with AutoCloseable = {
 
     // Represents submitted work for a task.
     final class Enqueue(val node: A, val work: () => R)
 
-    new CompletionService[A, R] with AutoCloseable {
+    new CompletionService[A, R] with CancelSentiels with AutoCloseable {
       completionServices.put(this, true)
       private[this] val closed = new AtomicBoolean(false)
       override def close(): Unit = if (closed.compareAndSet(false, true)) {
@@ -206,11 +216,20 @@ object ConcurrentRestrictions {
       /** Tasks that cannot be run yet because they cannot execute concurrently with the currently running tasks.*/
       private[this] val pending = new LinkedList[Enqueue]
 
+      private[this] val sentinels: mutable.ListBuffer[JFuture[_]] = mutable.ListBuffer.empty
+
+      def cancelSentinels(): Unit = {
+        sentinels.toList foreach { s =>
+          s.cancel(true)
+        }
+        sentinels.clear
+      }
+
       def submit(node: A, work: () => R): Unit = synchronized {
         if (closed.get) throw new RejectedExecutionException
         else if (isSentinel(node)) {
           // skip all checks for sentinels
-          CompletionService.submit(work, jservice)
+          sentinels += CompletionService.submitFuture(work, jservice)
         } else {
           val newState = tags.add(tagState, node)
           // if the new task is allowed to run concurrently with the currently running tasks,
@@ -232,7 +251,7 @@ object ConcurrentRestrictions {
         val wrappedWork = () =>
           try work()
           finally cleanup(node)
-        CompletionService.submit(wrappedWork, jservice)
+        CompletionService.submitFuture(wrappedWork, jservice)
         ()
       }
       private[this] def cleanup(node: A): Unit = synchronized {