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

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

[Spencer Baugh]

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:
CAIRO DBUS FREETYPE GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LIBSELINUX LIBSYSTEMD LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PNG RSVG
SECCOMP SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM
XINPUT2 XPM LUCID ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: ELisp/l

Minor modes in effect:
  bug-reference-prog-mode: t
  global-so-long-mode: t
  pixel-scroll-precision-mode: t
  jane-fe-minor-mode: t
  jane-fe-jenga-minor-mode: t
  editorconfig-mode: t
  which-function-mode: t
  global-git-commit-mode: t
  magit-auto-revert-mode: t
  auto-revert-mode: t
  shell-dirtrack-mode: t
  server-mode: t
  windmove-mode: t
  savehist-mode: t
  save-place-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tab-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  context-menu-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  line-number-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Features:
(shadow emacsbug calc calc-loaddefs rect calc-macs pcmpl-unix pcmpl-gnu
mule-util reposition face-remap package-x descr-text etags fileloop
cc-mode cc-fonts cc-guess cc-menus cc-cmds cc-styles cc-align cc-engine
cc-vars cc-defs log-view bug-reference cus-start qp sort smiley
gnus-cite mail-extr textsec uni-scripts idna-mapping ucs-normalize
uni-confusable textsec-check gnus-async gnus-bcklg gnus-agent gnus-srvr
gnus-score score-mode nnvirtual nntp gnus-ml gnus-msg disp-table nndoc
gnus-cache gnus-dup mm-archive url-http-ntlm ntlm hmac-md5 hex-util md4
network-stream url-cache debbugs-gnu debbugs-compat debbugs soap-client
url-http url-gw nsm rng-xsd rng-dt rng-util xsd-regexp vc-git shortdoc
async-bytecomp async tramp-cmds tramp-sh tramp-cache time-stamp hl-line
vc vc-dispatcher sh-script treesit cl-print help-fns radix-tree misc
conf-mode dabbrev display-line-numbers goto-addr executable pulse
misearch multi-isearch dired-aux org-element org-persist org-id
org-refile avl-tree generator oc-basic ol-eww eww xdg url-queue mm-url
ol-rmail ol-mhe ol-irc ol-info ol-gnus nnselect gnus-art mm-uu mml2015
mm-view mml-smime smime gnutls dig gnus-sum shr pixel-fill kinsoku
url-file svg dom gnus-group gnus-undo gnus-start gnus-dbus dbus xml
gnus-cloud nnimap nnmail mail-source utf7 nnoo gnus-spec gnus-int
gnus-range gnus-win ol-docview doc-view jka-compr image-mode exif
ol-bibtex bibtex ol-bbdb ol-w3m ol-doi org-link-doi vc-fe find-dired
so-long jane-fe-read-feature pixel-scroll cua-base vc-hg tramp
tramp-loaddefs trampver tramp-integration tramp-compat parse-time
iso8601 ffap jane-merlin merlin-imenu let-alist merlin-xref merlin-cap
merlin jane-async-merlin jane-completion grep jane-common jane-comint
org-protocol jane-fe-project xref files-x jane-fe-menu ecaml_plugin view
gopcaml magit-bookmark bookmark image+ advice image-file image-converter
editorconfig editorconfig-core editorconfig-core-handle
editorconfig-fnmatch whitespace jane-auto-modes vba-mode markdown-mode
color jane jane-yasnippet jane-micro-features ert ewoc debug backtrace
jane-diff unified-test-mode shell-file core core-buffer core-error
jane-sexp jane-python jane-ocaml jane-tuareg-theme tuareg tuareg-compat
tuareg-opam skeleton flymake-proc flymake warnings thingatpt smie
caml-types caml-help caml-emacs find-file compile jane-cr jane-codeium
jane-align jane-deprecated jane-smerge gnu-elpa-keyring-update
jane-ocp-indent ocp-indent jane-eglot yasnippet-autoloads
swiper-autoloads htmlize-autoloads eglot-autoloads
editorconfig-autoloads codeium-autoloads jane-autoloads jane-util
ob-shell page-ext dired-x magit-extras project magit-submodule
magit-obsolete magit-blame magit-stash magit-reflog magit-bisect
magit-push magit-pull magit-fetch magit-clone magit-remote magit-commit
magit-sequence magit-notes magit-worktree magit-tag magit-merge
magit-branch magit-reset magit-files magit-refs magit-status magit
magit-repos magit-apply magit-wip magit-log which-func imenu magit-diff
smerge-mode diff diff-mode git-commit log-edit message sendmail
yank-media puny dired dired-loaddefs rfc822 mml mml-sec epa derived epg
rfc6068 epg-config mm-decode mm-bodies mm-encode mail-parse rfc2231
rfc2047 rfc2045 ietf-drums mailabbrev gmm-utils mailheader pcvs-util
add-log magit-core magit-autorevert autorevert filenotify magit-margin
magit-transient magit-process with-editor shell server magit-mode
transient edmacro kmacro magit-git magit-section magit-utils crm dash
gnus nnheader gnus-util text-property-search mail-utils range mm-util
mail-prsvr cl-extra help-mode windmove org ob ob-tangle ob-ref ob-lob
ob-table ob-exp org-macro org-src ob-comint org-pcomplete pcomplete
org-list org-footnote org-faces org-entities time-date noutline outline
ob-emacs-lisp ob-core ob-eval org-cycle org-table ol rx org-fold
org-fold-core org-keys oc org-loaddefs find-func cal-menu calendar
cal-loaddefs org-version org-compat org-macs format-spec gdb-mi bindat
gud easy-mmode comint ansi-osc ansi-color ring vundo modus-vivendi-theme
modus-themes pcase savehist saveplace cus-edit pp cus-load icons
wid-edit adaptive-wrap-autoloads csv-mode-autoloads
cyberpunk-theme-autoloads evil-autoloads exwm-autoloads helm-autoloads
helm-core-autoloads async-autoloads ivy-autoloads magit-autoloads
git-commit-autoloads finder-inf magit-section-autoloads dash-autoloads
popup-autoloads url-http-ntlm-autoloads url-auth vc-hgcmd-autoloads
vundo-autoloads info with-editor-autoloads xelb-autoloads package
browse-url url url-proxy url-privacy url-expand url-methods url-history
url-cookie generate-lisp-file url-domsuf url-util mailcap url-handlers
url-parse auth-source cl-seq eieio eieio-core cl-macs password-cache
json subr-x map byte-opt gv bytecomp byte-compile url-vars cl-loaddefs
cl-lib rmc iso-transl tooltip cconv eldoc paren electric uniquify
ediff-hook vc-hooks lisp-float-type elisp-mode mwheel term/x-win x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode lisp-mode prog-mode register
page tab-bar menu-bar rfn-eshadow isearch easymenu timer select
scroll-bar mouse jit-lock font-lock syntax font-core term/tty-colors
frame minibuffer nadvice seq simple cl-generic indonesian philippine
cham georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao
korean japanese eucjp-ms cp51932 hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese composite emoji-zwj charscript
charprop case-table epa-hook jka-cmpr-hook help abbrev obarray oclosure
cl-preloaded button loaddefs theme-loaddefs faces cus-face macroexp
files window text-properties overlay sha1 md5 base64 format env
code-pages mule custom widget keymap hashtable-print-readable backquote
threads dbusbind inotify dynamic-setting system-font-setting
font-render-setting cairo x-toolkit xinput2 x multi-tty
make-network-process emacs)

Memory information:
((conses 16 1290183 152090)
 (symbols 48 57370 1)
 (strings 32 222722 19140)
 (string-bytes 1 8502817)
 (vectors 16 116575)
 (vector-slots 8 2371281 184939)
 (floats 8 770 630)
 (intervals 56 99161 3193)
 (buffers 976 171))

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

[Eli Zaretskii]

> From: Spencer Baugh <sbaugh@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?

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

[Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors]

>> 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