SI-6690 Release reference to last dequeued element.

Avoids leaks in MutableList and its more better known child,
mutable.Queue, when the last element is dequeued or when we
take the tail of a one element collection.

Refactors copy/pasted code between the two implementations of
tail.

2 files changed, 22 insertions, 16 deletions

diff --git a/src/library/scala/collection/mutable/MutableList.scala b/src/library/scala/collection/mutable/MutableList.scala
index 183de5b727..4bbe604cd3 100644
--- a/src/library/scala/collection/mutable/MutableList.scala
+++ b/src/library/scala/collection/mutable/MutableList.scala
@@ -56,12 +56,16 @@ extends AbstractSeq[A]
   /** Returns the rest of this list
    */
   override def tail: MutableList[A] = {
-    require(nonEmpty, "tail of empty list")
     val tl = new MutableList[A]
+    tailImpl(tl)
+    tl
+  }
+
+  protected final def tailImpl(tl: MutableList[A]) {
+    require(nonEmpty, "tail of empty list")
     tl.first0 = first0.tail
-    tl.last0 = last0
     tl.len = len - 1
-    tl
+    tl.last0 = if (tl.len == 0) new LinkedList[A]() else last0
   }
 
   /** Prepends a single element to this list. This operation takes constant
diff --git a/src/library/scala/collection/mutable/Queue.scala b/src/library/scala/collection/mutable/Queue.scala
index 5ea012eb9f..8956a1105e 100644
--- a/src/library/scala/collection/mutable/Queue.scala
+++ b/src/library/scala/collection/mutable/Queue.scala
@@ -66,7 +66,7 @@ extends MutableList[A]
     else {
       val res = first0.elem
       first0 = first0.next
-      len -= 1
+      decrementLength()
       res
     }
 
@@ -82,11 +82,11 @@ extends MutableList[A]
     else if (p(first0.elem)) {
       val res: Option[A] = Some(first0.elem)
       first0 = first0.next
-      len -= 1
+      decrementLength()
       res
     } else {
       val optElem = removeFromList(p)
-      if (optElem != None) len -= 1
+      if (optElem != None) decrementLength()
       optElem
     }
 
@@ -119,7 +119,7 @@ extends MutableList[A]
       while ((first0.nonEmpty) && p(first0.elem)) {
         res += first0.elem
         first0 = first0.next
-        len -= 1
+        decrementLength()
       }
       if (first0.isEmpty) res
       else removeAllFromList(p, res)
@@ -130,10 +130,10 @@ extends MutableList[A]
     var leftlst = first0
     while (leftlst.next.nonEmpty) {
       if (p(leftlst.next.elem)) {
-	res += leftlst.next.elem
-	if (leftlst.next eq last0) last0 = leftlst
-	leftlst.next = leftlst.next.next
-	len -= 1
+        res += leftlst.next.elem
+        if (leftlst.next eq last0) last0 = leftlst
+        leftlst.next = leftlst.next.next
+        decrementLength()
       } else leftlst = leftlst.next
     }
     res
@@ -154,7 +154,7 @@ extends MutableList[A]
       else {
         val res: Option[LinkedList[A]] = Some(cell.next)
         cell.next = cell.next.next
-        len -= 1
+        decrementLength()
         res
       }
     }
@@ -170,11 +170,8 @@ extends MutableList[A]
 
   // TODO - Don't override this just for new to create appropriate type....
   override def tail: Queue[A] = {
-    require(nonEmpty, "tail of empty list")
     val tl = new Queue[A]
-    tl.first0 = first0.tail
-    tl.last0 = last0
-    tl.len = len - 1
+    tailImpl(tl)
     tl
   }
 
@@ -183,6 +180,11 @@ extends MutableList[A]
     bf ++= seq
     bf.result
   }
+
+  private[this] def decrementLength() {
+    len -= 1
+    if (len == 0) last0 = new LinkedList[A]()
+  }
 }