bug#31919: 26.1.50; Lisp Debugger doesn't work when at stack limit

[Gemini Lasswell <gazally@runbox.com>]

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Eli Zaretskii <eliz@gnu.org> writes:

> Yes, but please add a comment there describing the reason.  How much
> more depth out of 40 is actually needed to allow cl-print calls, and
> how much is safety margin?

I determined by experiment that 77 needs to be added to
max-lisp-eval-depth to permit the debugger to print
((1 (2 (3 (4 (5 (6 (7 (8))))))))).  So I changed the increment to 100.
But I really have no idea what is reasonable for a safety margin.
Here's a new patch with comments.


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From ab293d12ef045042b62df7670cf9fe05f175ce19 Mon Sep 17 00:00:00 2001
From: Gemini Lasswell <gazally@runbox.com>
Date: Wed, 20 Jun 2018 13:58:33 -0700
Subject: [PATCH] Increase max-lisp-eval-depth adjustment while in debugger

* src/eval.c (call_debugger): Increase the amount of extra Lisp
evaluation depth given to the debugger to allow it to call cl-print.
* lisp/emacs-lisp/debug.el (debugger-setup-buffer): Add a comment
to suggest updating call_debugger when changing print-level.
---
 lisp/emacs-lisp/debug.el | 1 +
 src/eval.c               | 8 ++++++--
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/lisp/emacs-lisp/debug.el b/lisp/emacs-lisp/debug.el
index 593fab9727..821d674882 100644
--- a/lisp/emacs-lisp/debug.el
+++ b/lisp/emacs-lisp/debug.el
@@ -322,6 +322,7 @@ debugger-setup-buffer
                  (backtrace-frames 'debug)))
         (print-escape-newlines t)
         (print-escape-control-characters t)
+        ;; If you increase print-level, add more depth in call_debugger.
         (print-level 8)
         (print-length 50)
         (pos (point)))
diff --git a/src/eval.c b/src/eval.c
index ca1eb84ff3..40cba3bb1c 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -282,8 +282,12 @@ call_debugger (Lisp_Object arg)
   /* Do not allow max_specpdl_size less than actual depth (Bug#16603).  */
   EMACS_INT old_max = max (max_specpdl_size, count);
 
-  if (lisp_eval_depth + 40 > max_lisp_eval_depth)
-    max_lisp_eval_depth = lisp_eval_depth + 40;
+  /* The previous value of 40 is too small now that the debugger
+     prints using cl-prin1 instead of prin1.  Printing lists nested 8
+     deep (which is the value of print-level used in the debugger)
+     currently requires 77 additional frames.  See bug#31919.  */
+  if (lisp_eval_depth + 100 > max_lisp_eval_depth)
+    max_lisp_eval_depth = lisp_eval_depth + 100;
 
   /* While debugging Bug#16603, previous value of 100 was found
      too small to avoid specpdl overflow in the debugger itself.  */
-- 
2.16.4