SI-8100 - prevent possible SOE during Stream#flatten.

This commit changes stream flatten to avoid allocating a
stack frame for every stream item. Previously, flattening
a stream whose elements are mostly empty would result in
a StackOverflowException.

This commit also adds a test demonstrating the problem.

1 files changed, 10 insertions, 8 deletions

diff --git a/src/library/scala/collection/immutable/Stream.scala b/src/library/scala/collection/immutable/Stream.scala
index 5e1de44749..e6e4d6ec18 100644
--- a/src/library/scala/collection/immutable/Stream.scala
+++ b/src/library/scala/collection/immutable/Stream.scala
@@ -960,14 +960,16 @@ self =>
    * }}}
    */
   override def flatten[B](implicit asTraversable: A => /*<:<!!!*/ GenTraversableOnce[B]): Stream[B] = {
-    def flatten1(t: Traversable[B]): Stream[B] =
-      if (!t.isEmpty)
-        cons(t.head, flatten1(t.tail))
-      else
-        tail.flatten
-
-    if (isEmpty) Stream.empty
-    else flatten1(asTraversable(head).seq.toTraversable)
+    var st: Stream[A] = this
+    while (st.nonEmpty) {
+      val h = asTraversable(st.head)
+      if (h.isEmpty) {
+        st = st.tail
+      } else {
+        return h.toStream #::: st.tail.flatten
+      }
+    }
+    Stream.empty
   }
 
   override def view = new StreamView[A, Stream[A]] {