Fix up length_internal with degenerate length inputs

* src/fns.c (length_internal): Protect against edge conditions.

2 files changed, 16 insertions, 2 deletions

diff --git a/src/fns.c b/src/fns.c
index 0fded92aeb2..6aad119d1f9 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -159,14 +159,14 @@ EMACS_INT length_internal (Lisp_Object sequence, int len)
   if (len < 0xffff)
     while (CONSP (sequence))
       {
-        if (--len == 0)
+        if (--len <= 0)
           return -1;
         sequence = XCDR (sequence);
       }
   /* Signal an error on circular lists. */
   else
     FOR_EACH_TAIL (sequence)
-      if (--len == 0)
+      if (--len <= 0)
         return -1;
   return len;
 }
@@ -210,6 +210,9 @@ counted.  */)
   CHECK_FIXNUM (length);
   EMACS_INT len = XFIXNUM (length);
 
+  if (len < 0)
+    return Qnil;
+
   if (CONSP (sequence))
     return length_internal (sequence, len + 1) == 1? Qt: Qnil;
   else
diff --git a/test/src/fns-tests.el b/test/src/fns-tests.el
index 3486c745bf3..e66dad44a1a 100644
--- a/test/src/fns-tests.el
+++ b/test/src/fns-tests.el
@@ -1025,6 +1025,17 @@
   (should (length= "abc" 3))
   (should-not (length= "abc" 4))
 
+  (should-not (length< (list 1 2 3) -1))
+  (should-not (length< (list 1 2 3) 0))
+  (should-not (length< (list 1 2 3) -10))
+
+  (should (length> (list 1 2 3) -1))
+  (should (length> (list 1 2 3) 0))
+
+  (should-not (length= (list 1 2 3) -1))
+  (should-not (length= (list 1 2 3) 0))
+  (should-not (length= (list 1 2 3) 1))
+
   (should-error
    (let ((list (list 1)))
      (setcdr list list)