From: Matthieu Moy <matthieu.moy@imag.fr>
Date: Fri, 03 Aug 2007 23:01:23 GMT
Subject: Re: [PATCH] Add --show-touched option to show "diff --git" line when contents are unchanged
Message-ID: <vpqir7wi5oc.fsf@bauges.imag.fr>
In-Reply-To: <7v1wekmgo8.fsf@assigned-by-dhcp.cox.net>

Junio C Hamano <gitster@pobox.com> writes:

> Matthieu Moy <Matthieu.Moy@imag.fr> writes:
>
>> Also, is there any particular reason not to update the index stat
>> information when files are found to be identical?
>
> Very much --- diff is a read-only operation.
>
> "git-status $args" on the other hand is a preview of "what would
> happen if I say 'git-commit $args'", and in order to compute
> that, you would fundamentally need to be able to write into the
> object store.  In a special case of giving empty $args it can be
> read-only.

Can you give an example where it _could_ not be read-only?

> The commit Dscho quoted earlier was to hack that around so that
> "git-status" can pretend to be a read-only operation in a repository
> you do not have write permission to.

I really, really, really, don't understand your argument.

There are two different concepts:

* Should git-diff be "read-only for the user"?
  (i.e. external specification)

* Should git-diff be actually read-only for the filesystem?
  (i.e. implementation)


"read-only for the user" is a user-interface thing. It just means that
running

$ git-diff >& /dev/null; whatever

will give the same result as

$ whatever

In another context, "cat", for example, is a read-only operation for
the user. I can run "cat whatever-file" without influencing the
behavior of subsequent operations. Now, somewhere in the kernel of my
OS, I do hope that reading this file will have side-effects (putting
the file in the cache, and why not decide to physically move the file
on disk).


Here, obviously, git-diff is a read-only operation for the user. I
don't expect git-diff to modify the behavior of subsequent commands,
but I appreciate if git-diff can improve the speed of subsequent
commands.

Now, both of us agree that git-status should not be read-only for the
filesystem, both of us agree that git-diff should be read-only for the
user, but we disagree on the two other cases.

In the same way, I expect git-status to be read-only for the user. You
say "what _would_ happen _if_ I say commit $args". But you don't
commit, the sentence is conditionnal. I don't expect any tool to have
visible side-effects when I say "what would happen if ...".

-- 
Matthieu