git-diff should not fire up $PAGER if there is no diff

git-diff should not fire up $PAGER if there is no diff

[jidanni]

git-diff should not fire up $PAGER if there is no diff output.
Just exit. The man page doesn't even mention $PAGER too.

Re: git-diff should not fire up $PAGER if there is no diff

[Jeff King]

On Tue, Dec 16, 2008 at 08:21:33AM +0800, jidanni@jidanni.org wrote:

> git-diff should not fire up $PAGER if there is no diff output.
> Just exit. The man page doesn't even mention $PAGER too.

I agree that would be nice, but it is a little difficult to implement.
The current behavior forks early and then pipes the output to the pager.
So we would have to:

  1. change that behavior to instead delay starting the pager until the
     first output. Which means intercepting every
     write/fwrite/printf/fputs/etc call.

  2. detect EOF before starting the pager. We in fact already delay
     running the pager in the forked process until we have some activity
     on the pipe, but I don't know if there is a portable way of
     detecting that that activity is EOF without performing an actual
     read() call (which is undesirable, since it eats the first byte of
     output that should go to the pager).

  3. a hacky solution to (2) above would be to make _2_ pipes, one of
     which signals to the pager sub-process either "exit now" or "proceed
     with running the pager".

The usual workaround is to ask the pager to exit immediately if the
output is small. I.e., putting "F" in your LESS variable (which git does
automatically if you don't already have LESS set).

-Peff

Re: git-diff should not fire up $PAGER if there is no diff

[Stefan Karpinski]

> On Mon, Dec 15, 2008 at 7:56 PM, Jeff King <peff@peff.net> wrote:
>  2. detect EOF before starting the pager. We in fact already delay
>     running the pager in the forked process until we have some activity
>     on the pipe, but I don't know if there is a portable way of
>     detecting that that activity is EOF without performing an actual
>     read() call (which is undesirable, since it eats the first byte of
>     output that should go to the pager).

Wouldn't ungetc work? Or is that not portable enough? (It would only
work here because the EOF has to be the first character.)

Re: git-diff should not fire up $PAGER if there is no diff

[Jeff King]

On Tue, Dec 16, 2008 at 01:35:53AM -0500, Stefan Karpinski wrote:

> > On Mon, Dec 15, 2008 at 7:56 PM, Jeff King <peff@peff.net> wrote:
> >  2. detect EOF before starting the pager. We in fact already delay
> >     running the pager in the forked process until we have some activity
> >     on the pipe, but I don't know if there is a portable way of
> >     detecting that that activity is EOF without performing an actual
> >     read() call (which is undesirable, since it eats the first byte of
> >     output that should go to the pager).
> 
> Wouldn't ungetc work? Or is that not portable enough? (It would only
> work here because the EOF has to be the first character.)

No, it won't work. ungetc works on the buffered stdio object, so it is
useful for pushing back characters onto the buffer to be read later in
the program from the same buffer. But in this case, we are going to
execv() (or on Windows, spawn) the pager, meaning it will throw away
anything that has been read() from the pipe and put in the buffer.

So we would need a system call to push a character back to the OS, so
that it was available for read() by the pager process.

-Peff

Re: git-diff should not fire up $PAGER if there is no diff

[Stefan Karpinski]

On Tue, Dec 16, 2008 at 2:44 AM, Jeff King <peff@peff.net> wrote:
> On Tue, Dec 16, 2008 at 01:35:53AM -0500, Stefan Karpinski wrote:
>
>> > On Mon, Dec 15, 2008 at 7:56 PM, Jeff King <peff@peff.net> wrote:
>> >  2. detect EOF before starting the pager. We in fact already delay
>> >     running the pager in the forked process until we have some activity
>> >     on the pipe, but I don't know if there is a portable way of
>> >     detecting that that activity is EOF without performing an actual
>> >     read() call (which is undesirable, since it eats the first byte of
>> >     output that should go to the pager).
>>
>> Wouldn't ungetc work? Or is that not portable enough? (It would only
>> work here because the EOF has to be the first character.)
>
> No, it won't work. ungetc works on the buffered stdio object, so it is
> useful for pushing back characters onto the buffer to be read later in
> the program from the same buffer. But in this case, we are going to
> execv() (or on Windows, spawn) the pager, meaning it will throw away
> anything that has been read() from the pipe and put in the buffer.
>
> So we would need a system call to push a character back to the OS, so
> that it was available for read() by the pager process.

Yeah, I realized that after I sent the message. Late night sending bad!

git-diff should not fire up $PAGER, period!

[jidanni]

Gentlemen, I have found the solution to your problem.

Unbundle git-diff and $PAGER.

Ask yourself, does diff(1) call $PAGER?

No. That's because the Unix designers were smart enough not to glue
everything together.

Now's your chance to repent, as you haven't even yet mentioned $PAGER
on the git-diff man page. Yes, do mention it: "EXAMPLES: git-diff|less"
I.e., the user can page the output if he feels inclined, just like any
other output. I mean one already has a wallet. The bank need not give
the user one every time they make a withdraw.

I mean here I am in emacs, and

-*- mode: compilation; default-directory: "...coreutils/" -*-
Compilation started at Thu Dec 18 03:15:14
git-diff
WARNING: terminal is not fully functional^M
^M-  (press RETURN)

"It's all emacs' fault for emulating a tty too well"... no, it's all
your fault for gumming things together. No I don't want my cookies
with obligatory milk. I'll using git-diff|cat for now instead of
complaining that emacs is all wrong. Even using git-diff|cat|less is
better than messing with the LESS=F bug. Repent, whippersnappers!

OK, doing test x$EMACS = xt && PAGER=cat in .bashrc. That will help
for emacs' shell buffers, but not compilation mode buffers... "then
just make a hook"... 13 hooks to combat one poor design choice.
And one notices git-show is gummed up too.

Hmm, looking in changelogs, we see

 * Error messages used to be sent to stderr, only to get hidden,
   when $PAGER was in use.  They now are sent to stdout along
   with the command output to be shown in the $PAGER.

Well, if you had left paging to the user, no one would have blamed you
for making error messages disappear, and you could have left stderr as
the elders intended.

Wait,
$ git-config --global core.pager ""
Cool. Bye.

Re: git-diff should not fire up $PAGER, period!

[Junio C Hamano]

jidanni@jidanni.org writes:

> I mean here I am in emacs, and
>
> -*- mode: compilation; default-directory: "...coreutils/" -*-
> Compilation started at Thu Dec 18 03:15:14
> git-diff
> WARNING: terminal is not fully functional^M
> ^M-  (press RETURN)

Any semi-good emacs users (let alone hackers) export PAGER=cat to be used
in compilation mode (and possibly shell mode), so this is not a problem in
practice.

I have something like this in my .emacs:

    (setenv "PAGER" "cat")

I suspect (I am just a user not a hacker) this will have bad interaction
with emacs terminal emulation mode, but I do not use the mode, so it is
enough for me.

Re: git-diff should not fire up $PAGER, period!

[Linus Torvalds]

On Thu, 18 Dec 2008, jidanni@jidanni.org wrote:
>
> Gentlemen, I have found the solution to your problem.

Umm. YOUR problem.

Do that whole

	[core]
		pager = 

and you can get the behaviour you want.

Or just get rid of emacs. Your problem has nothing to do with git, and 
everything to do with emacs. And then you have the _gall_ to talk about 
"unix design" and not gumming programs together, when you yourself use the 
most gummed-up piece of absolute sh*t there is!

			Linus

Re: git-diff should not fire up $PAGER, period!

[Miles Bader]

Linus Torvalds <torvalds@linux-foundation.org> writes:
> And then you have the _gall_ to talk about "unix design"...

Another beer?

-Miles

-- 
Suburbia: where they tear out the trees and then name streets after them.

Re: git-diff should not fire up $PAGER, period!

[Jeff King]

On Thu, Dec 18, 2008 at 05:45:35AM +0800, jidanni@jidanni.org wrote:

> Gentlemen, I have found the solution to your problem.
> 
> Unbundle git-diff and $PAGER.

If you are going to argue this, please at least go back and read the
numerous times it has been brought up in the past on the list archive.

The last discussion ended up showing that some people really like the
automatic pager for some commands, and some people really detest it.
So I implemented 4e10738 (Allow per-command pager config, 2008-07-03),
and now you can do:

  git config pager.diff false

and be happy (or, as you obviously disocvered, simply unsetting
core.pager will disable all).

-Peff

Re: git-diff should not fire up $PAGER, period!

[Miles Bader]

Just (setenv "PAGER" "cat") in .emacs.

[I actually have it set to /bin/cat, not sure if that's meaningful or not.]

-Miles

-- 
.Numeric stability is probably not all that important when you're guessing.

Re: git-diff should not fire up $PAGER, period!

[Johannes Sixt]

Miles Bader schrieb:
> Just (setenv "PAGER" "cat") in .emacs.
> 
> [I actually have it set to /bin/cat, not sure if that's meaningful or not.]

No, really, you should set it to plain "cat": As a special case git
recognizes this token and does not run any pager. If you set it to
"/bin/cat" it does run a pager, namely /bin/cat.

-- Hannes

Re: git-diff should not fire up $PAGER, period!

[Junio C Hamano]

Johannes Sixt <j.sixt@viscovery.net> writes:

> Miles Bader schrieb:
>> Just (setenv "PAGER" "cat") in .emacs.
>> 
>> [I actually have it set to /bin/cat, not sure if that's meaningful or not.]
>
> No, really, you should set it to plain "cat": As a special case git
> recognizes this token and does not run any pager. If you set it to
> "/bin/cat" it does run a pager, namely /bin/cat.

But that would not hurt ;-).

On the other hand, using PAGER=cat or PAGER=/bin/cat is the right thing to
do in compilation and shell modes in Emacs, regardless of your use of git.

Re: git-diff should not fire up $PAGER if there is no diff

[jidanni]

>>>>> "JS" == Johannes Schindelin <Johannes.Schindelin@gmx.de> writes:

JS> Maybe you are not aware just how annoying these "this should do that, that
JS> should do this" without a patch are?

Yes but I must first learn the thing I am about to make a patch for.

In the mean time I could like most beginners, just feel dumb.

Or I could let you have my valuable insights on how things look to a
beginner. You would else have no way of knowing. Sorry I always come
across as Don Rickles the pushy customer... it's all a charade... now
let's get back to business,

>>>>> "JK" == Jeff King <peff@peff.net> writes:

JK> The usual workaround is to ask the pager to exit immediately if the
JK> output is small. I.e., putting "F" in your LESS variable (which git does
JK> automatically if you don't already have LESS set).

Ah, but who doesn't have LESS set these days? OK, the man page should
mention that so people will put the F on their LESS variable, like I
recall another man page already does.

Re: git-diff should not fire up $PAGER if there is no diff

[Jeff King]

On Tue, Dec 16, 2008 at 09:16:29AM +0800, jidanni@jidanni.org wrote:

> JK> The usual workaround is to ask the pager to exit immediately if the
> JK> output is small. I.e., putting "F" in your LESS variable (which git does
> JK> automatically if you don't already have LESS set).
> 
> Ah, but who doesn't have LESS set these days? OK, the man page should
> mention that so people will put the F on their LESS variable, like I
> recall another man page already does.

Not unreasonable. Care to make a patch to the documentation?

-Peff

Re: git-diff should not fire up $PAGER if there is no diff

[jidanni]

Oh barf, having LESS=F in one's environment messes up less +G:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=508919