Subject: Re: bug#58839: [Patch] Re: bug#58839: 29.0.50; project-kill-buffer fails when Eglot is running

From: Dmitry Gutov <[email-masked]>
Date: Sat, 29 Oct 2022 04:09:13 +0300

On 29.10.2022 03:15, João Távora wrote:
> Dmitry Gutov <[email-masked]> writes:
> 
>> (defun eglot-before-kill-special ()
>>    (eglot-shutdown)
>>    t)
>>
>> ;; somewhere inside that special buffer's setup:
>> (add-hook 'kill-buffer-query-functions #'eglot-before-kill-special nil t)
>>
>> Or use kill-buffer-hook, no need to watch the return value then.
> 
> Thanks, but this is simply wrong.  Besides missing a required argument,
> it doesn't respect the preference of eglot-autoshutdown: I explained
> that in the second message of this thread replying to Philip.

Sorry, I don't have Eglot checked out, or the time to test out the patch.

> I also explained in that message that Eglot doesn't "own" the process
> buffer, jsonrpc.el does.  Clients of jsonrpc.el don't even know there's
> a process buffer, they only own a handle to a jsonrpc "connection",
> which may or may not use buffers underneath.  So your "somewhere
> inside..." is a big problem.
> 
>> In either case, it will also cover the scenario of the user killing
>> the background buffer some other way.
> 
> The "background buffer" is hidden.  Users don't see it unless they go
> considerably out of their way.  Even M-x project-switch-to-buffer
> somehow doesn't list it.
> 
> Let me ask you this: can you conceive to that some buffers in Emacs's
> buffer list simply don't belong to _any_ project?

I suppose. But the current criterion depends on the value of 
default-directory, and that makes it a match.

> If you agree that
> there are such cases, then it should become clear that the buffer in
> question must be at the top of that list.

I'm not sure. Intuitively, I'd say that this buffer belongs to the 
project because it "services" the project. But if it were to work for 
several projects at the same time, I suppose I could say it doesn't 
belong to any particular one.

> There are more hints that the concept of "buffer belonging to a project"
> was not fully thought through, even in cases unrelated to this bug
> report.
> 
> * Take the *scratch* buffer.  It has a default-directory.  Does this
>    also make *scratch* belong to a project?  It doesn't make any sense to
>    me that it would.  Yet it is caught by project-buffers.

*scratch* is not that special - you can create similar buffers at will. 
So there are two ways of looking at that question. One can create a 
"scratch" for a project, and it will be part of that project.

If "~" (the usual value of default-directory in the original *scratch*) 
belongs to a project, then *scratch* also does.

> * project-buffers also catches the one-time *Completions* buffers, the
>    kind produced by hitting TAB after C-x p b.  If you type C-x p b
>    again, it quite comically offers the stale *Completions* buffer as a
>    candidate to switch to.

We could make an exception for that too.

> But back to *scratch*.  Somehow *scratch* is not killed by M-x
> project-kill-buffers.  I think it's because it doesn't have a
> buffer-file-name.  But then neither does the Eglot/Jsonrpc's "background
> buffers"!  It seems it is being targeted merely because it uses
> fundamental-mode, a most reasonable mode to use for exchanging messages
> via standard streams.
> 
> I guess this means that the hack below is enough to fix the issue, but
> it is also decidedly silly.

It's not much better than adding a function to 
kill-buffer-query-functions that returns nil. And/or behaves accordingly 
to eglot-autoshutdown.

> So please consider fixing this in project.el.  As Manuel pointed out,
> the venerable ibuffer.el's ibuffer-kill-filter-group also kills project
> buffers and handles this whole thing very well.  We should just take a
> hint from it.

I'm unable to find that message.