Closes #1220.

3 files changed, 46 insertions, 3 deletions

diff --git a/src/library/scala/collection/immutable/Stream.scala b/src/library/scala/collection/immutable/Stream.scala
index 5475c59809..cf9a48983a 100644
--- a/src/library/scala/collection/immutable/Stream.scala
+++ b/src/library/scala/collection/immutable/Stream.scala
@@ -565,10 +565,14 @@ object Stream extends SeqFactory[Stream] {
   final class Cons[+A](hd: A, tl: => Stream[A]) extends Stream[A] {
     override def isEmpty = false
     override def head = hd
-    private[this] var tlVal: Stream[A] = _
-    def tailDefined = tlVal ne null
+    @volatile private[this] var tlVal: Stream[A] = _
+    def tailDefined: Boolean = tlVal ne null
     override def tail: Stream[A] = {
-      if (!tailDefined) { tlVal = tl }
+      if (!tailDefined)
+        synchronized {
+          if (!tailDefined) tlVal = tl
+        }
+
       tlVal
     }
   }
diff --git a/test/files/run/concurrent-stream.check b/test/files/run/concurrent-stream.check
new file mode 100644
index 0000000000..d4adf84490
--- /dev/null
+++ b/test/files/run/concurrent-stream.check
@@ -0,0 +1,3 @@
+Testing standard cons.
+Evaluation 0: List(0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
+Evaluation 1: List(0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
diff --git a/test/files/run/concurrent-stream.scala b/test/files/run/concurrent-stream.scala
new file mode 100644
index 0000000000..42c695964e
--- /dev/null
+++ b/test/files/run/concurrent-stream.scala
@@ -0,0 +1,36 @@
+// test concurrent calls to Stream.tail
+object Test  {
+
+def slowRange(from: Int, until: Int, cons: (Int, => Stream[Int]) => Stream[Int]): Stream[Int] = {
+  var current = from
+  def next: Stream[Int] = {
+    Thread.sleep(100)
+    if (current >= until) Stream.empty
+    else {
+      val stream = cons(current, next)
+      current += 1
+      stream
+    }
+  }
+  next
+}
+
+def testCons(cons: (Int, => Stream[Int]) => Stream[Int]): Unit = {
+  import scala.actors.Actor._
+
+  val stream = slowRange(0, 10, cons)
+  val main = self
+  actor { main ! stream.toList }
+  actor { main ! stream.toList }
+  val eval0 = receive { case list: List[Int] => list }
+  val eval1 = receive { case list: List[Int] => list }
+  println("Evaluation 0: " + eval0)
+  println("Evaluation 1: " + eval1)
+}
+
+  def main(args: Array[String]) {
+    println("Testing standard cons.")
+    testCons(Stream.cons.apply(_, _))
+  }
+}
+