Juri, this doesn't seem right. Eglot shouldn't be turning itself on in
hidden buffers to start with: It's totally useless there.

So you have to explain an Emacs -Q recipe that demonstrates how Eglot
reached this nonsensical state. You haven't done that (or have you and i
have missed it?).

I tried project-find-regexp with Eglot-managed files with no problems, but
I have no idea what you're doing.

Also, this problem is totally off-topic here: this is about
project-kill-buffers. Please start as new issue of you haven't already.

João

On Thu, Nov 3, 2022, 17:39 Juri Linkov <juri@linkov.net> wrote:

> > OTOH I completely support the request to make Eglot more resilient
> > to unforeseeable situations.  Currently it's so brittle, so I get a lot
> > of such errors all the time:
> >
> > Debugger entered--Lisp error: (wrong-type-argument arrayp nil)
> >   file-truename(nil)
> >   eglot--path-to-uri(nil)
> >   eglot--TextDocumentIdentifier()
> >   eglot--signal-textDocument/didClose()
> >   kill-buffer(#<buffer  *xref-temp*>)
> >   xref--convert-hits(...)
> >   xref-matches-in-files("word" ...)
> >   project--find-regexp-in-files("word" ...)
> >   apply(project--find-regexp-in-files ("word" ...))
> >   xref--show-xref-buffer(...)
> >   xref--show-xrefs(...)
> >   xref-show-xrefs(...)
> >   project-find-regexp("word")
> >   funcall-interactively(project-find-regexp "word")
> >   command-execute(project-find-regexp)
>
> Here's a patch that fixes this:
>
> diff --git a/lisp/progmodes/eglot.el b/lisp/progmodes/eglot.el
> index c5870618372..5b05f84c63c 100644
> --- a/lisp/progmodes/eglot.el
> +++ b/lisp/progmodes/eglot.el
> @@ -1792,7 +1792,9 @@ eglot--maybe-activate-editing-mode
>    (unless eglot--managed-mode
>      ;; Called when `revert-buffer-in-progress-p' is t but
>      ;; `revert-buffer-preserve-modes' is nil.
> -    (when (and buffer-file-name (eglot-current-server))
> +    (when (and buffer-file-name
> +               (not (string-match-p "\\` " (buffer-name)))
> +               (eglot-current-server))
>        (setq eglot--diagnostics nil)
>        (eglot--managed-mode)
>        (eglot--signal-textDocument/didOpen))))
>