Don't enter the debugger from *Backtrace* or edebug on eval errors

* doc/lispref/debugging.texi (Error Debugging): Document it.

* doc/lispref/edebug.texi (Edebug Eval): Mention it.
* lisp/emacs-lisp/debug.el (debug-allow-recursive-debug): New user
option (bug#36145).
(debugger-eval-expression): Use it.

* lisp/emacs-lisp/edebug.el (edebug-eval-expression): Ditto.

This patch is based on a patch by Noam Postavsky.

1 files changed, 12 insertions, 1 deletions

diff --git a/lisp/emacs-lisp/debug.el b/lisp/emacs-lisp/debug.el
index 46b0306d64f..91e9b0716d0 100644
--- a/lisp/emacs-lisp/debug.el
+++ b/lisp/emacs-lisp/debug.el
@@ -90,6 +90,11 @@ The value used here is passed to `quit-restore-window'."
   :group 'debugger
   :version "24.3")
 
+(defcustom debug-allow-recursive-debug nil
+  "If non-nil, erroring in debug and edebug won't recursively debug."
+  :type 'boolean
+  :version "29.1")
+
 (defvar debugger-step-after-exit nil
   "Non-nil means \"single-step\" after the debugger exits.")
 
@@ -534,7 +539,13 @@ The environment used is the one when entering the activation frame at point."
                       (error 0)))) ;; If on first line.
 	(base (debugger--backtrace-base)))
     (debugger-env-macro
-      (let ((val (backtrace-eval exp nframe base)))
+      (let ((val (if debug-allow-recursive-debug
+                     (backtrace-eval exp nframe base)
+                   (condition-case err
+                       (backtrace-eval exp nframe base)
+                     (error (format "%s: %s"
+                                    (get (car err) 'error-message)
+			            (car (cdr err))))))))
         (prog1
             (debugger--print val t)
           (let ((str (eval-expression-print-format val)))