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.

2 files changed, 16 insertions, 2 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)))
diff --git a/lisp/emacs-lisp/edebug.el b/lisp/emacs-lisp/edebug.el
index 722283b88ff..85545f9f351 100644
--- a/lisp/emacs-lisp/edebug.el
+++ b/lisp/emacs-lisp/edebug.el
@@ -57,6 +57,7 @@
 (require 'cl-lib)
 (require 'seq)
 (eval-when-compile (require 'pcase))
+(require 'debug)
 
 ;;; Options
 
@@ -3713,7 +3714,9 @@ Print result in minibuffer."
   (interactive (list (read--expression "Eval: ")))
   (princ
    (edebug-outside-excursion
-    (let ((result (edebug-eval expr)))
+    (let ((result (if debug-allow-recursive-debug
+                      (edebug-eval expr)
+                    (edebug-safe-eval expr))))
       (values--store-value result)
       (concat (edebug-safe-prin1-to-string result)
               (eval-expression-print-format result))))))