fixes #17696 (#18276)

lib/pure/collections/sharedlist.nim

@@ -35,8 +35,10 @@ template withLock(t, x: untyped) =
   release(t.lock)
 
 proc iterAndMutate*[A](x: var SharedList[A]; action: proc(x: A): bool) =
-  ## iterates over the list. If 'action' returns true, the
+  ## Iterates over the list. If 'action' returns true, the
   ## current item is removed from the list.
+  ##
+  ## .. warning:: It may not preserve the element order after some modifications.
   withLock(x):
     var n = x.head
     while n != nil:
@@ -47,8 +49,9 @@ proc iterAndMutate*[A](x: var SharedList[A]; action: proc(x: A): bool) =
         if action(n.d[i]):
           acquire(x.lock)
           let t = x.tail
+          dec t.dataLen # TODO considering t.dataLen == 0,
+                        # probably the module should be refactored using doubly linked lists
           n.d[i] = t.d[t.dataLen]
-          dec t.dataLen
         else:
           acquire(x.lock)
           inc i