Using LSP with TRAMP starts multiple language servers instances

Description :octocat:

Spacemacs has issues using LSP via TRAMP that are not present in vanilla emacs. In particular, it opens many SSH connections, spawning a clangd (or ccls; I tested C++ only) for each source file related to the opened one. I've no idea how to debug this, I hope others will be able to reproduce this. It may be related to #11514.

Reproduction guide :beetle:

Start with a brand new .spacemacs file, only add lsp and c-c++ layers.
Prepare a test project on a remote server, with compile_commands.json file at the top level.
Create a file lsp-tramp-test.el (content quoted below); it installs lsp-mode and opens two remote source files.
Start emacs using emacs -q -l lsp-tramp-test.el. This starts vanilla emacs.
Use M-x lsp to start lsp-mode in each of the two buffers with C/C++ code.
Run lsp-describe-session.
Close emacs.
Start emacs using emacs -l lsp-tramp-test.el. This starts spacemacs.
Run lsp-describe-session.

Observed behaviour: :eyes: :broken_heart:

Vanilla emacs behaves as expected. lsp-describe-session shows only one clangd instance, both files belong to it.
Spacemacs takes a while to start, depending on how many source files are in the project because it opens many connections. lsp-describe-session shows many clangd instances, not all of them have the same source files associated with them.

I have observed many other weird issues trying to use LSP remotely, but I'm not sure which are related and which are not, so I'm just focusing on the multiple language server instances issue for now. It's the most important, I think, because even in the best case, having too many connections opened causes things to freeze at some point (where "things" means both emacs and SSH itself, depending on SSH server configuration).

Expected behaviour: :heart: :smile: Spacemacs should only start one clangd instance for the project, same as vanilla emacs.

System Info :computer:

OS: gnu/linux
Emacs: 27.1
Spacemacs: 0.300.0
Spacemacs branch: develop (rev. 59f72709b)
Graphic display: t
Distribution: spacemacs
Editing style: vim
Completion: helm

Layers:

(emacs-lisp helm lsp c-c++ multiple-cursors treemacs)

System configuration features: XPM JPEG TIFF GIF PNG RSVG CAIRO IMAGEMAGICK SOUND DBUS GLIB NOTIFY INOTIFY ACL GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF ZLIB TOOLKIT_SCROLL_BARS LUCID X11 XDBE XIM MODULES THREADS LIBSYSTEMD JSON PDUMPER LCMS2 GMP

Test file

Modify the remote file paths.

lsp-tramp-test.el

(require 'package)

(setq debug-on-error t)

(let* ((package-archives '(("melpa" . "https://melpa.org/packages/")
                           ("gnu" . "https://elpa.gnu.org/packages/")))
       (no-byte-compile t)
       (package-user-dir (expand-file-name (make-temp-name "lsp-tmp-elpa")
                                           user-emacs-directory))
       (custom-file (expand-file-name "custom.el" package-user-dir))
       (pkg-list '(lsp-mode lsp-ui helm-lsp lsp-treemacs company flycheck)))

  (package-initialize)
  (package-refresh-contents)

  (mapcar (lambda (pkg)
            (unless (package-installed-p pkg)
              (package-install pkg))
            (require pkg))
          pkg-list)

  (add-hook 'prog-mode-hook 'lsp)
  (add-hook 'kill-emacs-hook `(lambda ()
                                (delete-directory ,package-user-dir t))))

(require 'lsp)
(with-eval-after-load 'lsp-mode
 (lsp-register-client
  (make-lsp-client
   :remote? t
   :new-connection (lsp-tramp-connection "clangd")
   :major-modes '(c-mode c++-mode)
   :server-id 'clangd-remote)))

(find-file "/ssh:user@server:~/src/testproject/file1.cpp")
(find-file "/ssh:user@server:~/src/testproject/file2.cpp")

syl20bnr / spacemacs