GNU bug report logs - #65897
29.1.50; Debugging errors surfaced through emacsclient is hard

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Spencer Baugh <sbaugh <at> janestreet.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 29.1.50; Debugging errors surfaced through emacsclient is hard
Date: Tue, 12 Sep 2023 15:42:37 -0400

Errors which happen through emacsclient never trigger the debugger and
only print a simple error message, which makes it very difficult to
debug them.  If there's an error which, for some reason, only happens
when running emacsclient, then life becomes very difficult.  I just
encountered this with one of my users - I still have no idea why the
error only affected emacsclient, or how I could have reproduced it
without emacsclient, I eventually fixed it by bisecting their .emacs.

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.


In GNU Emacs 29.1.50 (build 4, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.15.12, Xaw scroll bars) of 2023-09-11 built on

Repository revision: f9bc92d0b36bc631d11c194e4b580f43b7b8dcba
Repository branch: emacs-29
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Rocky Linux 8.8 (Green Obsidian)

Configured using:
 'configure --config-cache --with-x-toolkit=lucid
 --with-gif=ifavailable'

Configured features:
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Message #8 received at 65897 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Spencer Baugh <sbaugh <at> janestreet.com>,
 Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: 65897 <at> debbugs.gnu.org
Subject: Re: bug#65897: 29.1.50;
 Debugging errors surfaced through emacsclient is hard
Date: Thu, 14 Sep 2023 17:54:28 +0300

> From: Spencer Baugh <sbaugh <at> janestreet.com>
> Date: Tue, 12 Sep 2023 15:42:37 -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.

Stefan, any comments?

Message #11 received at 65897 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: Spencer Baugh <sbaugh <at> janestreet.com>, 65897 <at> debbugs.gnu.org
Subject: Re: bug#65897: 29.1.50; Debugging errors surfaced through
 emacsclient is hard
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

Message #14 received at 65897 <at> debbugs.gnu.org (full text, mbox):

From: Spencer Baugh <sbaugh <at> janestreet.com>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: Eli Zaretskii <eliz <at> gnu.org>, 65897 <at> debbugs.gnu.org
Subject: Re: bug#65897: 29.1.50; Debugging errors surfaced through
 emacsclient is hard
Date: Fri, 29 Aug 2025 17:02:09 -0400

[Message part 1 (text/plain, inline)]

Stefan Monnier <monnier <at> iro.umontreal.ca> writes:
>>> 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

I tested this and it works for me.  I suggest we should install this.
Here's a modernized version as a patch which applies to trunk.

[0001-Allow-entering-the-debugger-on-error-in-server-conne.patch (text/x-patch, inline)]

From b9aa445c6eb92481ac5b02f69eb99f404aca5cdf Mon Sep 17 00:00:00 2001
From: Spencer Baugh <sbaugh <at> janestreet.com>
Date: Fri, 29 Aug 2025 17:00:47 -0400
Subject: [PATCH] Allow entering the debugger on error in server connection

* lisp/server.el (server--process-filter-1): Use
condition-case-unless-debug. (bug#65897)
---
 lisp/server.el | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lisp/server.el b/lisp/server.el
index 4415c45971e..70299d52f18 100644
--- a/lisp/server.el
+++ b/lisp/server.el
@@ -1232,7 +1232,7 @@ server--process-filter-1
     (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
-- 
2.43.7

Message #17 received at 65897 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Spencer Baugh <sbaugh <at> janestreet.com>
Cc: Eli Zaretskii <eliz <at> gnu.org>, 65897 <at> debbugs.gnu.org
Subject: Re: bug#65897: 29.1.50; Debugging errors surfaced through
 emacsclient is hard
Date: Sun, 31 Aug 2025 00:33:06 -0400

> I tested this and it works for me.  I suggest we should install this.
> Here's a modernized version as a patch which applies to trunk.

LGTM,


        Stefan

Message #22 received at 65897-done <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: sbaugh <at> janestreet.com, 65897-done <at> debbugs.gnu.org
Subject: Re: bug#65897: 29.1.50; Debugging errors surfaced through
 emacsclient is hard
Date: Sun, 31 Aug 2025 09:55:43 +0300

> From: Stefan Monnier <monnier <at> iro.umontreal.ca>
> Cc: Eli Zaretskii <eliz <at> gnu.org>,  65897 <at> debbugs.gnu.org
> Date: Sun, 31 Aug 2025 00:33:06 -0400
> 
> > I tested this and it works for me.  I suggest we should install this.
> > Here's a modernized version as a patch which applies to trunk.
> 
> LGTM,

Thanks, installed on master (with the missing NEWS entry), and
closing the bug.

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