Subject: Re: bug#65897: 29.1.50; Debugging errors surfaced through emacsclient is hard

From: Stefan Monnier <[email-masked]>
Date: Mon, 18 Sep 2023 10:00:42 -0400

>> Toy example:
>> 1. emacs -Q
>> 2. (server-start)
>> 3. (add-hook 'before-make-frame-hook (lambda () (error "frame hook error")))
>> 4. emacsclient -c
>> 
>> The output of emacsclient is:
>> 
>> Waiting for Emacs...
>> *ERROR*: frame hook error
>> 
>> That's all, there is no way to get more information about the error that
>> happened.
>> 
>> Ideally, when emacsclient encounters an error and debug-on-error is
>> non-nil, it should cause Emacs to enter the debugger.
>
> I'm not sure we can allow entering the debugger from server-execute.
> That function is documented as being called from timers and process
> filters, so entering the debugger might be unsafe.

AFAIK it is perfectly safe to drop into the debugger when running timers
and process-filters.  I believe the reason why this doesn't happen here is
because of the `condition-case` used to grab the error and send it to
Emacsclient's output.  See patch below.

We could also offer a way to grab the backtrace and send it over to
Emacsclient's output.

        Stefan

diff --git a/lisp/server.el b/lisp/server.el
index 10f15598221..2536a9c6591 100644
--- a/lisp/server.el
+++ b/lisp/server.el
@@ -1175,7 +1175,7 @@ server-process-filter
     (when prev
       (setq string (concat prev string))
       (process-put proc 'previous-string nil)))
-  (condition-case err
+  (condition-case-unless-debug err
       (progn
 	(server-add-client proc)
 	;; Send our pid