bug#62958: [PATCH] Set PAGER=cat in comint.el

[Eli Zaretskii <eliz@gnu.org>]

> From: sbaugh@catern.com
> Date: Tue, 09 May 2023 14:55:31 +0000 (UTC)
> Cc: Spencer Baugh <sbaugh@janestreet.com>, 62958@debbugs.gnu.org
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> From: Spencer Baugh <sbaugh@janestreet.com>
> >> Cc: 62958@debbugs.gnu.org
> >> Date: Mon, 08 May 2023 15:38:12 -0400
> >> 
> >> >   Pagination is not needed, and gets in the way, when the output of
> >> >   the program is directed to an Emacs buffer, so in those cases
> >> >   pagination should be disabled.  To disable pagination, this
> >> >   variable's value should be a string that names a program, such
> >> >   as \"cat\", which passes through all of the output without any
> >> >   filtering or delays.  Comint will then set the PAGER variable
> >> >   to name that program, when it invokes external programs."
> >> 
> >> I think it's worth noting the fact that there are programs which depend
> >> on pagination for their normal functionality.  How about this short
> >> additional paragraph added after your text:
> >> 
> >> "Disabling pagination means programs will produce their entire output
> >> immediately.  If only part of the output is useful, this can be
> >> wasteful.  For some programs, this can be avoided by using commands (for
> >> example, `vc-print-log') which run the program, limit the output it
> >> produces, and let the user interactively browse the output inside
> >> Emacs."
> >
> > Sorry, I don't understand: why would it matter that a program produces
> > its output immediately when that output is redirected to an Emacs
> > buffer?  Whatever the amount of output, the user can always page
> > through it interactively and conveniently using the normal movement
> > commands, no?  What am I missing?
> 
> If the command produces a lot of output it can take a long time to run
> and slow down Emacs.
> 
> For example, if a user disables pagination, runs M-x shell, and then
> runs "git log" in the Emacs repo, it takes ~3.5 minutes to run and
> produces ~1.5 million lines.  Even if all they want is to see the 10
> most recent commits.  There are several negative effects of this, in
> decreasing order of importance:
> 
> - The shell buffer now has way more output, which slows down navigation
> and operations on that buffer forever after, including font lock.
> - The user's shell is blocked during that whole time, preventing further
> shell commands.
> - git log producing unnecessary output wastes CPU and IO.
> 
> If "git log" is run with a pager, it can complete as soon as the user
> exits the pager, and it only produces a few lines.

But this is impossible in general, AFAIU: most comint clients aren't
prepared for interactive paging anyway.  If someone wants paging, they
should use "M-x term" or "M-x ansi-term", the terminal emulators that
Emacs provides.