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

[Spencer Baugh]

Eli Zaretskii <eliz@gnu.org> writes:

>> 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?

My addition says:
- sometimes paging should not be disabled (for programs like "git log")
- but if you're going to disable it anyway, commands like `vc-print-log'
  become more useful (because it's an alternative to "git log")

The purpose of the addition is to communicate the second part.  (And
since I think the primary way most people will encounter this is through
"git log", I think it's worth mentioning `vc-print-log' specifically)