patch 9.0.2123: Problem with initializing the length of range() lists

Problem:  Problem with initializing the length of range() lists
Solution: Set length explicitly when it shouldn't contain any items

range() may cause a wrong calculation of list length, which may later
then cause a segfault in list_find().  This is usually not a problem,
because range_list_materialize() calculates the length, when it
materializes the list.

In addition, in list_find() when the length of the range was wrongly
initialized, it may seem to be valid, so the check for list index
out-of-bounds will not be true, because it is called before the list is
actually materialized. And so we may eventually try to access a null
pointer, causing a segfault.

So this patch does 3 things:

- In f_range(), when we know that the list should be empty, explicitly
  set the list->lv_len value to zero. This should happen, when
  start is larger than end (in case the stride is positive) or
  end is larger than start when the stride is negative.
  This should fix the underlying issue properly. However,

- as a safety measure, let's check that the requested index is not
  out of range one more time, after the list has been materialized
  and return NULL in case it suddenly is.

- add a few more tests to verify the behaviour.

fixes: #13557
closes: #13563

Co-authored-by: Tim Pope <tpope@github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/evalfunc.c

@@ -8646,7 +8646,10 @@ f_range(typval_T *argvars, typval_T *rettv)
     list->lv_u.nonmat.lv_start = start;
     list->lv_u.nonmat.lv_end = end;
     list->lv_u.nonmat.lv_stride = stride;
-    list->lv_len = (end - start) / stride + 1;
+    if (stride > 0 ? end < start : end > start)
+	list->lv_len = 0;
+    else
+	list->lv_len = (end - start) / stride + 1;
 }
 
 /*

src/list.c

@@ -415,6 +415,10 @@ list_find(list_T *l, long n)
 
     CHECK_LIST_MATERIALIZE(l);
 
+    // range_list_materialize may reset l->lv_len
+    if (n >= l->lv_len)
+	return NULL;
+
     // When there is a cached index may start search from there.
     if (l->lv_u.mat.lv_idx_item != NULL)
     {

src/testdir/test_functions.vim

@@ -3052,6 +3052,9 @@ func Test_range()
   " get()
   call assert_equal(4, get(range(1, 10), 3))
   call assert_equal(-1, get(range(1, 10), 42, -1))
+  call assert_equal(0, get(range(1, 0, 2), 0))
+  call assert_equal(0, get(range(0, -1, 2), 0))
+  call assert_equal(0, get(range(-2, -1, -2), 0))
 
   " index()
   call assert_equal(1, index(range(1, 5), 2))

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2123,
 /**/
     2122,
 /**/