Re: Teaching emacsclient to act as a pager, and more

[dieter@duenenhof-wilhelm.de (H. Dieter Wilhelm)]

Spencer Baugh <sbaugh@catern.com> writes:

> Hi emacs-devel,
>
> I've added the ability for emacsclient to act as a pager, such that
> you can pipe data to emacsclient and page through it in the emacs
> frame of your choice. The attached patches are on top of commit

+1  thanks


> 549470fdf234acb4da7941e3bb9b28ed63a51876
>
> Here is a video demo: https://catern.com/files/emacspager.webm
>
> To do this, I've used the file descriptor passing feature of Unix
> sockets, which allows a process to transmit a duplicate of any of its
> open file descriptors over a Unix socket to another process. The other
> process can then make full use of that file descriptor, including, of
> course, reading and writing from it.
>
> In the attached patches, I taught emacsclient to (when a new option
> --pipeline/-l is passed) send its stdin/stdout/stderr to the emacs
> server, and I taught the emacs server to accept those file descriptors
> and make an Elisp process out of them. Then the process machinery does
> the rest of the work of reading data from the Elisp process (which is
> actually data coming in on emacsclient's stdin) and putting it in a
> buffer.
>
> This functionality is exposed to Elisp by simply directly passing
> received file descriptor numbers to the process filter function (with
> a new argument, to processes that have opted in with a new keyword
> argument :ancillary). Those file descriptor numbers can be passed to a
> new function make-fd-process.
>
> I've written a function in Elisp, server-pager, which should be run
> with emacsclient -l --eval (server-pager). I added a new dynamic
> variable server-emacsclient-proc which is non-nil if we are currently
> evaluating Lisp for an emacsclient, and holds the Elisp process
> corresponding to that Emacsclient. server-pager uses this variable to
> retrieve the file descriptors for the current emacs-client, invoke
> make-fd-process, and pop-to-buffer the output buffer. server-pager
> stores the Elisp process it creates in the plist of the corresponding
> emacsclient, so when the Elisp process is killed, emacsclient is told
> to exit (if it didn't also open frames). Likewise if emacsclient is
> killed, the server-pager Elisp process is killed.
>
> The primary issue here, it seems to me, is that this leads to huge
> amounts of file descriptor leakage - if any passed-in file descriptor
> is unused, it is eternally left open. I think a good way to fix this
> is to add a native Elisp file descriptor type, so the file descriptor
> can be closed on garbage collect. But perhaps there's a better
> solution? In any case I would need guidance on how to create such a
> new Elisp type - this is my first attempt to hack on the Emacs C
> codebase.
>
> I am sure that these patches are terrible style, not in keeping with
> the Emacs coding conventions, and totally unportable (I wrote this on
> GNU/Linux) - I just wanted to get out a quick proof of concept.
>
>
>

-- 
Best wishes
H. Dieter Wilhelm
Kelkheim, Germany