Fix triggeredBy with :=

Fixes #1444

sbt/src/sbt-test/actions/triggeredby/build.sbt

@@ -0,0 +1,21 @@
+val taskA = taskKey[File]("")
+val taskB = taskKey[File]("")
+val taskC = taskKey[File]("")
+val taskD = taskKey[File]("")
+
+taskA := touch(target.value / "a")
+taskB := touch(target.value / "b")
+taskC := touch(target.value / "c")
+taskD := touch(target.value / "d")
+
+//   a <<= a triggeredBy b
+// means "a" will be triggered by "b"
+// said differently, invoking "b" will run "b" and then run "a"
+
+taskA <<= taskA triggeredBy taskB
+taskC := (taskC triggeredBy taskD).value
+
+
+
+// test utils
+def touch(f: File): File = { IO touch f; f }

sbt/src/sbt-test/actions/triggeredby/test

@@ -0,0 +1,7 @@
+> taskB
+$ exists target/b
+$ exists target/a
+
+> taskD
+$ exists target/d
+$ exists target/c

tasks-standard/src/main/scala/sbt/std/TaskExtra.scala

@@ -120,8 +120,14 @@ trait TaskExtra {
     def failure: Task[Incomplete] = mapFailure(idFun)
     def result: Task[Result[S]] = mapR(idFun)
 
+    private val triggeredByKey = AttributeKey[Seq[Task[_]]]("triggered-by")
+    private def newInfo[A]: Info[A] = {
+      val i = Info[A]()
+      (in.info get triggeredByKey).fold(i)(i.set(triggeredByKey, _))
+    }
+
     def flatMapR[T](f: Result[S] => Task[T]): Task[T] = Task(Info(), new FlatMapped[T, K](in, f, ml))
-    def mapR[T](f: Result[S] => T): Task[T] = Task(Info(), new Mapped[T, K](in, f, ml))
+    def mapR[T](f: Result[S] => T): Task[T] = Task(newInfo, new Mapped[T, K](in, f, ml))
     def dependsOn(tasks: Task[_]*): Task[S] = Task(Info(), new DependsOn(in, tasks))
 
     def flatMap[T](f: S => Task[T]): Task[T] = flatMapR(f compose successM)