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, 23 insertions, 3 deletions

diff --git a/doc/lispref/debugging.texi b/doc/lispref/debugging.texi
index c258a9adc0e..058c9319544 100644
--- a/doc/lispref/debugging.texi
+++ b/doc/lispref/debugging.texi
@@ -196,6 +196,17 @@ echo area.  For example, this can be useful when trying to find the
 cause of a particular message.
 @end defvar
 
+@defvar debug-allow-recursive-debug
+You can evaluate forms in the current stack frame in the
+@samp{*Backtrace*} buffer with the @key{e} command, and while
+edebugging you can use the @key{e} and @key{C-x C-e} commands to do
+something similar.  By default, the debugger is inhibited by these
+commands (because (re-)entering the debugger at this point will
+usually take you out of the debugging context you're in).  Set
+@code{debug-allow-recursive-debug} to a non-@code{nil} value to allow
+these commands to enter the debugger recursively.
+@end defvar
+
   To debug an error that happens during loading of the init
 file, use the option @samp{--debug-init}.  This binds
 @code{debug-on-error} to @code{t} while loading the init file, and
@@ -520,6 +531,7 @@ Flag the current frame like @kbd{b}.  Then continue execution like
 @kbd{c}, but temporarily disable break-on-entry for all functions that
 are set up to do so by @code{debug-on-entry}.
 
+@vindex debug-allow-recursive-debug
 @item e
 Read a Lisp expression in the minibuffer, evaluate it (with the
 relevant lexical environment, if applicable), and print the
@@ -528,7 +540,11 @@ variables, and the current buffer, as part of its operation; @kbd{e}
 temporarily restores their values from outside the debugger, so you can
 examine and change them.  This makes the debugger more transparent.  By
 contrast, @kbd{M-:} does nothing special in the debugger; it shows you
-the variable values within the debugger.
+the variable values within the debugger.  By default, this command
+suppresses the debugger during evaluation, so that an error in the
+evaluated expression won't add a new error on top of the existing one.
+Set the @code{debug-allow-recursive-debug} user option to a
+non-@code{nil} value to override this.
 
 @item R
 Like @kbd{e}, but also save the result of evaluation in the
diff --git a/doc/lispref/edebug.texi b/doc/lispref/edebug.texi
index eff9621628e..0fc5271d5ad 100644
--- a/doc/lispref/edebug.texi
+++ b/doc/lispref/edebug.texi
@@ -700,8 +700,12 @@ on this process.
 @table @kbd
 @item e @var{exp} @key{RET}
 Evaluate expression @var{exp} in the context outside of Edebug
-(@code{edebug-eval-expression}).  That is, Edebug tries to minimize its
-interference with the evaluation.
+(@code{edebug-eval-expression}).  That is, Edebug tries to minimize
+its interference with the evaluation.  By default, this command
+suppresses the debugger during evaluation, so that an error in the
+evaluated expression won't add a new error on top of the existing one.
+Set the @code{debug-allow-recursive-debug} user option to a
+non-@code{nil} value to override this.
 
 @item M-: @var{exp} @key{RET}
 Evaluate expression @var{exp} in the context of Edebug itself