Subject: Re: [PATCH 0/3] Win32: nanosecond-precision file times

From: Junio C Hamano <[email-masked]>
Date: Mon, 16 Feb 2015 14:10:58 -0800

Karsten Blees <[email-masked]> writes:

> However, the Makefile has this to say on the subject:
>
> # Define USE_NSEC below if you want git to care about sub-second file mtimes
> # and ctimes. Note that you need recent glibc (at least 2.2.4) for this, and
> # it will BREAK YOUR LOCAL DIFFS! show-diff and anything using it will likely
> # randomly break unless your underlying filesystem supports those sub-second
> # times (my ext3 doesn't).
>
> Am I missing something?

I think "it would break" is about show-diff which wanted to use the
cached stat information for freshness.

	>foo
	git update-index --add foo
        sleep 2
        >foo
        git diff-files ;# modern counterpart of show-diff

would say that "foo" is *different*, because the plumbing commands
like diff-files expect you to refresh the index before you call
them.

And if you did "git update-index --refresh" after touching "foo" the
last time before running "git diff-files" in the above sequence, you
should expect that it does not say "foo" is different, no matter how
much time passes between the time you run that "refresh" and
"diff-files" (or between the time you last touched "foo" and you run
"refresh", for that matter), as long as you do not touch "foo" in
the meantime.  The following should say "foo" is *not* different,
that is:

	>foo
	git update-index --add foo
        sleep 2
        >foo
        sleep arbitrary
        git update-index --refresh
        sleep arbitrary
        git diff-files ;# modern counterpart of show-diff

If you use NSEC, however, and "refresh" grabbed a subsecond time and
then later "diff-files" learned a truncated/rounded time because the
filesystem later purged the cached inodes and re-read it from the
underlying filesystem with no subsecond time resolution, the times
would not match so you will again see "diff-files" report that "foo"
is now different.

That is what the comment you cited is about.

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