* src/eval.c (handlerlist_sentinel): New variable.

(init_eval): Use it to ensure handlerlist is non-NULL.
(unwind_to_catch): Make sure we never set handlerlist to NULL.
(Fsignal): Adjust NULLness test of handlerlist.
* src/lisp.h (PUSH_HANDLER): Assume handlerlist is non-NULL.

Fixes: debbugs:15802

1 files changed, 18 insertions, 2 deletions

diff --git a/src/eval.c b/src/eval.c
index a25dc1b5aa9..d3fcec5aef4 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -237,11 +237,22 @@ init_eval_once (void)
   Vrun_hooks = Qnil;
 }
 
+static struct handler handlerlist_sentinel;
+
 void
 init_eval (void)
 {
   specpdl_ptr = specpdl;
-  handlerlist = NULL;
+  { /* Put a dummy catcher at top-level so that handlerlist is never NULL.
+       This is important since handlerlist->nextfree holds the freelist
+       which would otherwise leak every time we unwind back to top-level.   */
+    struct handler *c;
+    handlerlist = handlerlist_sentinel.nextfree = &handlerlist_sentinel;
+    PUSH_HANDLER (c, Qunbound, CATCHER);
+    eassert (c == &handlerlist_sentinel);
+    handlerlist_sentinel.nextfree = NULL;
+    handlerlist_sentinel.next = NULL;
+  }
   Vquit_flag = Qnil;
   debug_on_next_call = 0;
   lisp_eval_depth = 0;
@@ -1129,6 +1140,8 @@ unwind_to_catch (struct handler *catch, Lisp_Object value)
 {
   bool last_time;
 
+  eassert (catch->next);
+
   /* Save the value in the tag.  */
   catch->val = value;
 
@@ -1542,7 +1555,10 @@ See also the function `condition-case'.  */)
     }
   else
     {
-      if (handlerlist != 0)
+      if (handlerlist != &handlerlist_sentinel)
+        /* FIXME: This will come right back here if there's no `top-level'
+           catcher.  A better solution would be to abort here, and instead
+           add a catch-all condition handler so we never come here.  */
         Fthrow (Qtop_level, Qt);
     }