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

[Eli Zaretskii]

> From: Spencer Baugh <sbaugh@janestreet.com>
> Cc: sbaugh@catern.com,  62958@debbugs.gnu.org
> Date: Tue, 09 May 2023 12:30:42 -0400
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> 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.
> 
> Yes.  I of course know that, that's why I filed this bug :)
> 
> Nevertheless, today by default many programs run in comint clients will
> run a pager, and even though it is a hassle and gets in the way inside
> Emacs, it does prevent some programs from producing lots of output and
> yielding the negative effects I mentioned previously:
> 
> >> - 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.
> 
> Because of these negative effects which will start happening for some
> programs when the user disables pagination, I think it's important for
> users to know that there's alternatives for such programs: namely, using
> native Emacs interfaces like vc-print-log.

OK, but what's the purpose of the addition you were proposing?  It
basically says that sometimes paging should not be disabled, but since
this is a defcustom, and the default is not to disable paging, that
already speaks volumes about the same issue, right?