* eval.c (call_debugger): Grow specpdl if the debugger was

entered due to specpdl overflow (Bug#16603) and allow more
specpdl space for the debugger itself.

1 files changed, 19 insertions, 12 deletions

diff --git a/src/eval.c b/src/eval.c
index 91b8d17769d..da68a3014dd 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -273,6 +273,8 @@ restore_stack_limits (Lisp_Object data)
   max_lisp_eval_depth = XINT (XCDR (data));
 }
 
+static void grow_specpdl (void);
+
 /* Call the Lisp debugger, giving it argument ARG.  */
 
 Lisp_Object
@@ -281,22 +283,27 @@ call_debugger (Lisp_Object arg)
   bool debug_while_redisplaying;
   ptrdiff_t count = SPECPDL_INDEX ();
   Lisp_Object val;
-  EMACS_INT old_max = max_specpdl_size;
-
-  /* Temporarily bump up the stack limits,
-     so the debugger won't run out of stack.  */
-
-  max_specpdl_size += 1;
-  record_unwind_protect (restore_stack_limits,
-                         Fcons (make_number (old_max),
-                                make_number (max_lisp_eval_depth)));
-  max_specpdl_size = old_max;
+  EMACS_INT old_max = max_specpdl_size, old_depth = max_lisp_eval_depth;
 
   if (lisp_eval_depth + 40 > max_lisp_eval_depth)
     max_lisp_eval_depth = lisp_eval_depth + 40;
 
-  if (max_specpdl_size - 100 < SPECPDL_INDEX ())
-    max_specpdl_size = SPECPDL_INDEX () + 100;
+  /* While debugging Bug#16603, previous value of 100 was found
+     too small to avoid specpdl overflow in the debugger itself.  */
+  if (max_specpdl_size - 200 < count)
+    max_specpdl_size = count + 200;
+
+  if (old_max == count)
+    {
+      /* We can enter the debugger due to specpdl overflow (Bug#16603).  */
+      specpdl_ptr--;
+      grow_specpdl ();
+    }
+
+  /* Restore limits after leaving the debugger.  */
+  record_unwind_protect (restore_stack_limits,
+                         Fcons (make_number (old_max),
+                                make_number (old_depth)));
 
 #ifdef HAVE_WINDOW_SYSTEM
   if (display_hourglass_p)