Merged revisions 22897 via svnmerge from

https://lampsvn.epfl.ch/svn-repos/scala/scala/trunk

........
  r22897 | dragos | 2010-09-01 18:01:59 +0200 (Wed, 01 Sep 2010) | 1 line

  Closes #1220. Stream.tail is now thread safe. Review by odersky.
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1 files changed, 7 insertions, 3 deletions

diff --git a/src/library/scala/collection/immutable/Stream.scala b/src/library/scala/collection/immutable/Stream.scala
index 7660a1e2c8..40f71aa845 100644
--- a/src/library/scala/collection/immutable/Stream.scala
+++ b/src/library/scala/collection/immutable/Stream.scala
@@ -557,10 +557,14 @@ 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
     }
   }