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

[sbaugh@catern.com]

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.