Add a Task.tailRecM for stack safe loops (#846)

cache/shared/src/main/scala/coursier/util/Task.scala

@@ -29,5 +29,18 @@ object Task extends PlatformTask {
   def never[A]: Task[A] =
     Task(_ => Promise[A].future)
 
+  def tailRecM[A, B](a: A)(fn: A => Task[Either[A, B]]): Task[B] =
+    Task[B] { implicit ec =>
+      def loop(a: A): Future[B] =
+        fn(a).future().flatMap {
+          case Right(b) =>
+            Future.successful(b)
+          case Left(a) =>
+            // this is safe because recursive
+            // flatMap is safe on Future
+            loop(a)
+        }
+      loop(a)
+    }
 }
 

tests/shared/src/test/scala/coursier/util/TaskTests.scala

@@ -0,0 +1,22 @@
+package coursier.util
+
+import utest._
+
+import scala.concurrent.{Await, ExecutionContext}
+import scala.concurrent.duration.Duration
+
+object TaskTests extends TestSuite {
+
+  val tests = Tests {
+    'tailRecM {
+      import ExecutionContext.Implicits.global
+
+      def countTo(i: Int): Task[Int] =
+        Task.tailRecM(0) {
+          case x if x >= i => Task.delay(Right(i))
+          case toosmall => Task.delay(Left(toosmall + 1))
+        }
+      countTo(500000).map(_ == 500000).future().map(assert(_))
+    }
+  }
+}