Re: [RFC/PATCH] config: add core.trustmtime

From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date: Nov 25, 2015, 09:00 UTC
Message-ID: <CACBZZX4YSvfA=BRayk8CC8ipk_nDdvJfznxjyb-upwP9PKRBSA@mail.gmail.com>
In-Reply-To: <1448433323-21037-1-git-send-email-chriscool@tuxfamily.org>

On Wed, Nov 25, 2015 at 7:35 AM, Christian Couder <christian.couder@gmail.com> wrote:

> At Booking.com we know that mtime works everywhere and we don't
> want the untracked cache to stop working when a kernel is upgraded
> or when the repo is copied to a machine with a different kernel.
> I will add tests later if people are ok with this.

I bit more info: I rolled Git out internally with this patch: https://github.com/avar/git/commit/c63f7c12c2664631961add7cf3da901b0b6aa2f2

The --untracked-cache feature hardcodes the equivalent of:

    pwd; uname --kernel-name --kernel-release --kernel-version

Into the index. If any of those change it prints out the "cache is disabled" warning.

This patch will make it stop being so afraid of itself to the point of disabling itself on minor kernel upgrades :)

A few other issues with this feature I've noticed:

 * There's no way to just enable it globally via the config. Makes it
a bit of a hassle to use it. I wanted to have a config option to
enable it via the config, how about "index.untracked_cache = true" for
the config variable name?

 * Doing "cd /tmp: git --git-dir=/git/somewhere/else/.git update-index
--untracked-cache" doesn't work how I'd expect. It hardcodes "/tmp" as
the directory that "works" into the index, so if you use the working
tree you'll never use the untracked cache. I spotted this because I
carry out a bunch of git maintenance commands with --git-dir instead
of cd-ing to the relevant directories. This works for most other
things in git, is it a bug that it doesn't work here?

 * If you "ctrl+c" git update-index --untracked-cache at an
inopportune time you'll end up with a mtime-test-XXXXXX directory in
your working tree. Perhaps this tempdir should be created in the .git
directory instead?

 * Maybe we should have a --test-untracked-cache option, so you can
run the tests without enabling it.

Aside from the slight hassle of enabling this and keeping it enabled this feature is great. It's sped up "git status" across the board by about 40%. Slightly less than that on faster spinning disks, slightly more than that on slower ones.