bug#62907: Eglot does not start managing LaTeX buffer despite reporting successful connection

[Michael Eliachevitch]

User-agent: mu4e 1.11.2; emacs 30.0.50

When I run `M-x eglot' in a LaTeX-buffer and select "texlab" as a 
language-server, I get an echo-area message indicating that the connection was successful:

 [eglot] Connected! Server `TexLab' now managing `(tex-mode context-mode 
texinfo-mode bibtex-mode)' buffers in project `phd-thesis'.

However, I don't get any mode-line indication and the variable 
`eglot--managed-mode' is nil, so it seems it didn't properly activate.

I initally noticed the error in that when I tried auto-completing with 
`eglot-completion-at-point', I got the error message

 (jsonrpc-error "No current JSON-RPC connection" (jsonrpc-error-code . 32603) 
(jsonrpc-error-message . "No current JSON-RPC connection"))

I get the same message when e.g. running `M-x eglot-stderr-buffer', so I cannot 
attach that, but I attached the contents of eglot-events-buffer. With 
debug-on-error enabled I get the backtrace

 Debugger entered--Lisp error: (jsonrpc-error "No current JSON-RPC connection" 
(jsonrpc-error-code . 32603) (jsonrpc-error-message . "No current JSON-RPC connection"))
   jsonrpc-error("No current JSON-RPC connection")
   eglot--current-server-or-lose()
   byte-code("\300 C\207" [eglot--current-server-or-lose] 1)
   command-execute(eglot-stderr-buffer record)
   execute-extended-command(nil "eglot-stderr-buffer" nil)
   funcall-interactively(execute-extended-command nil "eglot-stderr-buffer" nil)
   command-execute(execute-extended-command)

But the initial bug is that Eglot didn't start managing the buffer when I run 
the command and that doesn't raise an error initially.

I can reproduce the error with "emacs -Q". It seemed to work in the past 
(couple of weeks ago) and now it doesn't anymore. I changed a lot in my Emacs config but 
that it works with emacs -Q suggests something else changed rather than just my emacs 
init file.

I tried two texlab versions, 5.4.1 and 5.5.0 and also other latex 
language-servers such as digestif and ltex-ls and got the same issue. Also it 
doesn't matter whether I use AucTeX or plain latex.el (which I had in emacs -Q 
anyway). I also disabled my dir-local variables when testing. So it doesn't 
seem to depend on the latex language server version.

However, I don't have problems running Eglot in other major modes, e.g. it 
works in Python with the pylsp  server.

For reproducing, any latex file seemed to work for me, but I attached a very 
minimal latex-project for testing anyway. texlab can be downloaded as a static 
binary from https://github.com/latex-lsp/texlab/releases/tag/v5.5.0. Then, to 
reproduce, just open the latex file in the attached project, run `M-x eglot', 
select texlab, and check if it starts managing the buffer. Then, 
`eglot--managed-mode' should be `t' and I expect auto-completion to work.

I'm still not 100% sure this is a bug or something messed up in my local 
configuration, but I just couldn't find the issue, so I thought I might as well 
report it just in case it's not known. If you cannot reproduce it with the 
above instructions, feel free to close this bug. Below is the additional system 
and config information as provided by report-emacs-bug.

Cheers, Michael

---

In GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
3.24.37, cairo version 1.17.8) of 2023-04-17 built on e490
Repository revision: a46201f57eb114b8107435caf3588edbb666829e
Repository branch: master
System Description: Arch Linux

Configured using:
'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
--localstatedir=/var --mandir=/usr/share/man --with-gameuser=:games
--with-modules --without-libotf --without-m17n-flt --without-gconf
--enable-link-time-optimization --with-native-compilation=yes
--with-xinput2 --with-pgtk --without-xaw3d --with-sound=alsa
--with-xwidgets --with-tree-sitter --without-gpm
--without-compress-install
'--program-transform-name=s/\([ec]tags\)/\1.emacs/'
'CFLAGS=-march=native -mtune=generic -O3 -pipe -fno-plt -fexceptions
-Wp,-D_FORTIFY_SOURCE=2 -Wformat -Werror=format-security
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LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

Configured features:
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TREE_SITTER WEBP XIM XWIDGETS GTK3 ZLIB

Important settings:
 value of $LC_ALL: C.UTF-8
 value of $LC_COLLATE: C.UTF-8
 value of $LC_CTYPE: en_GB.UTF-8
 value of $LC_MESSAGES: en_US.UTF-8
 value of $LC_MONETARY: de_DE.UTF-8
 value of $LC_NUMERIC: en_US.UTF-8
 value of $LC_TIME: de_DE.UTF-8
 value of $LANG: C
 value of $XMODIFIERS: @im=fcitx
 locale-coding-system: utf-8-unix

Major mode: LaTeX

Minor modes in effect:
 shell-dirtrack-mode: t
 tooltip-mode: t
 global-eldoc-mode: t
 show-paren-mode: t
 electric-indent-mode: t
 mouse-wheel-mode: t
 tool-bar-mode: t
 menu-bar-mode: t
 file-name-shadow-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

Load-path shadows:
None found.

Features:
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Memory information:
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eglot-events-buffer.eld
Description: Text document

latex-mwe.tar.gz
Description: application/gzip