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

From: Junio C Hamano <gitster@pobox.com>
Date: Feb 16, 2015, 22:10 UTC
Message-ID: <xmqqtwyl4idp.fsf@gitster.dls.corp.google.com>
In-Reply-To: <54E250A6.4070605@gmail.com>

Karsten Blees <karsten.blees@gmail.com> writes:

Show 9 quoted lines

> 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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