Subject: Re: bug#61350: Eglot over Tramp freezes with large project

From: Michael Albinus <[email-masked]>
Date: Thu, 02 Mar 2023 12:01:32 +0100

João Távora <[email-masked]> writes:

Hi João,

> But JUST-THIS-ONE _is_ relevant when there is more than one process.
> Here, there is.  There's one process, the jsonrpc.el process, henceforth
> 'jprocess', and the Tramp process, henceforth 'tprocess'.  jprocess
> receives only JSONRPC data from the LSP server.  It "thinks" it is
> talking directly to a JSONRPC server, but in Tramp scenarios it is being
> fed data from tprocess, which is the process connected to the remote
> host.  In tprocess, other things, such as shell interactions are going
> on.
>
> Michael can probably confirm, correct or deny this.

More or less correct. But I still can't say which process gets output
when, because I cannot debug accept-process-output (it's a C
function). And running Emacs under gdb changes timings, which is
important I believe.

> When one (accept-process-output tprocess nil nil 'JUST-THIS-ONE=t) one
> must be absolutely sure that tprocess is going to send _something_
> "soon".  If it doesn't, we'll hang indefinitely (until the process dies
> or the user quits)

Yes. But Tramp calls accept-process-output only, if it has send a
command to the remote shell, and it expects something to be returned. At
least the shell prompt.

During my tests I have also changed this to

--8<---------------cut here---------------start------------->8---
(while (accept-process-output tprocess 0 nil 'JUST-THIS-ONE))
--8<---------------cut here---------------end--------------->8---

but it didn't help either.

> That's what has been confirmed through a backtrace.  It's a particular
> accept-process-output call in tramp-wait-for-regexp that hangs,
> understandibly so.

Yes.

> Now, 'tramp-check-for-regexp' uses a somewhat non-standard technique of
> searching for messages: it searches them from the back, from the end of
> the tprocess's buffer.  I don't know what motivated this, but I find it
> odd.  I find one of its callees, tramp-search-regexp, particularly
> suspicious:
>
>    (defun tramp-search-regexp (regexp)
>      "Search for REGEXP backwards, starting at point-max.""
>      (goto-char (point-max))
>      ;; We restrict ourselves to the last 256 characters.  There were
>      ;; reports of a shell command "git ls-files -zco --exclude-standard"
>      ;; with 85k files involved, which has blocked Tramp forever.
>      (re-search-backward regexp (max (point-min) (- (point) 256)) 'noerror))
>
> See the comment there?  Only 256 characters back are inspected.

Yes. But the regexp it searches for is the final shell prompt. Something
like "///4b3b7d4fa561141e84c94a1cf25e8575#$", which is shorter than 256
bytes for sure.

> So, finally, here's my conjecture:
>
> 1. Tramp goes into 'tramp-wait-for-regexp'.  tprocess's buffer already
>    the message that 'found' is supposed to return, but it also has a lot
>    more stuff, say a lot of JSONRPC data from the LSP server that also
>    came into that tprocess buffer and is awaiting to be delivered to
>    jprocess.
>
> 2. This data is for piping into jprocess, where the JSONRPC message will
>    be decoded, but it will probably never arrive at its destination.
>
> 3. 'found' will be nil in tramp-wait-for-regexp, because of the
>    tramp-search-regexp limitation.
>
> 4. tramp-wait-for-regexp will issue the "risky" accept-process-output
>    call.
>
> 5. there is no more data that accept-process-output wants to put in the
>    buffer,  because the LSP server is fine for the moment.
>
> 6. Emacs hang
>
> Just a conjecture.

Yes, this is more or less the scenario. But I still don't understand why
not all data are delivered through the socket ssh is using. Could it be
there is a limitation, how much data could be buffered by ssh?

>> I have a vague feeling, that Tramp could be improved with a work queue
>> such that requests to tramp from notification or timer threads get
>> blocked while another tramp command is still waiting for a
>> reply.
>
> There are no (usable) threads in Emacs.

There are. I made Tramp using threads, and it worked fine, when no
interactive dialogue inside a thread happened.

> Timers are events, and so are runs of each processe's process filter.
> Those two are what creates asynchronicity and the emulation of
> simultaneity in Emacs.  When jprocess's filter sees a whole JSONRPC
> message, it calls the message handler.

Timers and process filters are the cause of the "Forbidden reentrant
call in Tramp" errors. Wwe must do anything, solving this.

> João

Best regards, Michael.