Part #1 of cancel-task-hooks - Hooks EvaluateTask.

* Create a new EvaluateTaskConfig which gives us a bit more freedom
  over changign config options to EvaluateTask in the future.
* Create adapted from old EvaluateTask to new EvaluateTask
* Add hooks into signals class to register/remote  a signal listener
  directly, rather than in an "arm" block.
* Create TaskEvaluationCancelHandler to control the strategy of
  who/whom can cancel (sbt-server vs. sbt-terminal).
* Create a null-object for the "can't cancel" scenario so the
  code path is exactly the same.

This commit does not wire settings into the build yet, nor does it
fix the config extractio methods.

main/src/main/scala/sbt/EvaluateTask.scala

@@ -14,7 +14,73 @@ package sbt
 	import std.Transform.{DummyTaskMap,TaskAndValue}
 	import TaskName._
 
+@deprecated("Use EvaluateTaskConfig instead.", "0.13.2")
 final case class EvaluateConfig(cancelable: Boolean, restrictions: Seq[Tags.Rule], checkCycles: Boolean = false, progress: ExecuteProgress[Task] = EvaluateTask.defaultProgress)
+
+
+
+/** Represents something that can be cancelled. */
+trait TaskCancel {
+	/** cancels whatever this points at. */
+	def cancel(): Unit
+}
+/** A handler for registering/remmoving listeners that allow you to cancel tasks. */
+trait TaskEvaluationCancelHandler {
+  /* Evaluation is starting, here's a mechanism to cancel things. */
+  def start(canceller: TaskCancel): Unit
+  /* Task Evaluation is complete, whether success or failure. */
+  def finish(): Unit
+}
+object TaskEvaluationCancelHandler {
+   /** An empty handler that does nothing. */
+   object Null extends TaskEvaluationCancelHandler {
+   	  def start(canceller: TaskCancel): Unit = ()
+      def finish(): Unit = ()
+   }
+   /** Cancel handler which registers for SIGINT and cancels tasks when it is received. */
+   object Signal extends TaskEvaluationCancelHandler {
+   	 private var registration: Option[Signals.Registration] = None
+   	 def start(canceller: TaskCancel): Unit = {
+   	 	registration = Some(Signals.register(() => canceller.cancel()))
+   	 }
+   	 def finish(): Unit =
+   	   registration match {
+   	   	 case Some(value) => 
+   	   	   value.remove()
+   	   	   registration = None
+         case None =>
+   	   }
+   }
+}
+
+
+/** The new API for running tasks.  
+ *
+ * This represents all the hooks possible when running the task engine.
+ * We expose this trait so that we can, in a binary compatible way, modify what is used
+ * inside this configuration and how to construct it.
+ */
+sealed trait EvaluateTaskConfig {
+	def restrictions: Seq[Tags.Rule]
+	def checkCycles: Boolean
+	def progressReporter: ExecuteProgress[Task]
+	def cancelHandler: TaskEvaluationCancelHandler
+}
+final object EvaluateTaskConfig {
+	/** Pulls in the old configuration format. */
+	def apply(old: EvaluateConfig): EvaluateTaskConfig = {
+	  object AdaptedTaskConfig extends EvaluateTaskConfig {
+        def restrictions: Seq[Tags.Rule] = old.restrictions
+        def checkCycles: Boolean = old.checkCycles
+        def progressReporter: ExecuteProgress[Task] = old.progress
+        def cancelHandler: TaskEvaluationCancelHandler =
+          if(old.cancelable) TaskEvaluationCancelHandler.Signal
+          else TaskEvaluationCancelHandler.Null
+	  }
+	  AdaptedTaskConfig
+	}
+}
+
 final case class PluginData(dependencyClasspath: Seq[Attributed[File]], definitionClasspath: Seq[Attributed[File]], resolvers: Option[Seq[Resolver]], report: Option[UpdateReport], scalacOptions: Seq[String])
 {
 	val classpath: Seq[Attributed[File]] = definitionClasspath ++ dependencyClasspath
@@ -176,12 +242,18 @@ object EvaluateTask
 	def nodeView[HL <: HList](state: State, streams: Streams, roots: Seq[ScopedKey[_]], dummies: DummyTaskMap = DummyTaskMap(Nil)): NodeView[Task] =
 		Transform((dummyRoots, roots) :: (dummyStreamsManager, streams) :: (dummyState, state) :: dummies )
 
-	def runTask[T](root: Task[T], state: State, streams: Streams, triggers: Triggers[Task], config: EvaluateConfig)(implicit taskToNode: NodeView[Task]): (State, Result[T]) =
+	@deprecated("Use new EvalauteTaskConfig option to runTask", "0.13.2")
+    def runTask[T](root: Task[T], state: State, streams: Streams, triggers: Triggers[Task], config: EvaluateConfig)(implicit taskToNode: NodeView[Task]): (State, Result[T]) =
+    {
+    	val newConfig = EvaluateTaskConfig(config)
+    	runTask(root, state, streams, triggers, config)(taskToNode)
+    }
+	def runTask[T](root: Task[T], state: State, streams: Streams, triggers: Triggers[Task], config: EvaluateTaskConfig)(implicit taskToNode: NodeView[Task]): (State, Result[T]) =
 	{
-			import ConcurrentRestrictions.{completionService, TagMap, Tag, tagged, tagsKey}
+		import ConcurrentRestrictions.{completionService, TagMap, Tag, tagged, tagsKey}
 
 		val log = state.log
-		log.debug("Running task... Cancelable: " + config.cancelable + ", check cycles: " + config.checkCycles)
+		log.debug("Running task... Cancel: " + config.cancelHandler + ", check cycles: " + config.checkCycles)
 		val tags = tagged[Task[_]](_.info get tagsKey getOrElse Map.empty, Tags.predicate(config.restrictions))
 		val (service, shutdown) = completionService[Task[_], Completed](tags, (s: String) => log.warn(s))
 
@@ -191,7 +263,7 @@ object EvaluateTask
 			case _ => true
 		}
 		def run() = {
-			val x = new Execute[Task]( Execute.config(config.checkCycles, overwriteNode), triggers, config.progress)(taskToNode)
+			val x = new Execute[Task]( Execute.config(config.checkCycles, overwriteNode), triggers, config.progressReporter)(taskToNode)
 			val (newState, result) =
 				try {
 					val results = x.runKeep(root)(service)
@@ -203,15 +275,17 @@ object EvaluateTask
 			logIncResult(replaced, state, streams)
 			(newState, replaced)
 		}
-		val cancel = () => {
-			println("")
-			log.warn("Canceling execution...")
-			shutdown()
+		object taskCancel extends TaskCancel { 
+			def cancel(): Unit = {
+			  println("")
+			  log.warn("Canceling execution...")
+			  shutdown()
+			}
 		}
-		if(config.cancelable)
-			Signals.withHandler(cancel) { run }
-		else
-			run()
+		// Register with our cancel handler we're about to start.
+		config.cancelHandler.start(taskCancel)
+		try run()
+		finally config.cancelHandler.finish()
 	}
 
 	private[this] def storeValuesForPrevious(results: RMap[Task, Result], state: State, streams: Streams): Unit =

util/collection/src/main/scala/sbt/Signal.scala

@@ -19,6 +19,38 @@ object Signals
 			case Right(v) => v
 		}
 	}
+
+	/** Helper interface so we can expose internals of signal-isms to others. */
+	sealed trait Registration {
+		def remove(): Unit
+	}
+    /** Register a signal handler that can be removed later. 
+     * NOTE: Does not stack with other signal handlers!!!!
+     */
+    def register(handler: () => Unit, signal: String = INT): Registration =
+    	// TODO - Maybe we can just ignore things if not is-supported.
+    	if(supported(signal)) {
+	    	import sun.misc.{Signal,SignalHandler}
+	    	val intSignal = new Signal(signal)
+	    	val newHandler = new SignalHandler {
+				def handle(sig: Signal) { handler() }
+			}
+			val oldHandler = Signal.handle(intSignal, newHandler)
+			object unregisterNewHandler extends Registration {
+				override def remove(): Unit = {
+					Signal.handle(intSignal, oldHandler)
+				}
+			}
+			unregisterNewHandler
+		} else {
+			// TODO - Maybe we should just throw an exception if we don't support signals...
+			object NullUnregisterNewHandler extends Registration {
+				override def remove(): Unit = ()
+			}
+			NullUnregisterNewHandler
+		}
+
+
 	def supported(signal: String): Boolean =
 		try
 		{