From: Dennis Kaarsemaker <dennis@kaarsemaker.net>
To: Junio C Hamano <gitster@pobox.com>
Cc: git <git@vger.kernel.org>
Subject: Re: Reset sometimes updates mtime
Date: Fri, 10 Jul 2015 09:30:57 +0200 [thread overview]
Message-ID: <1436513457.9537.28.camel@kaarsemaker.net> (raw)
In-Reply-To: <xmqqa8v5w6xi.fsf@gitster.dls.corp.google.com>
On do, 2015-07-09 at 10:56 -0700, Junio C Hamano wrote:
> Dennis Kaarsemaker <dennis@kaarsemaker.net> writes:
>
> > I'm seeing some behaviour with git reset that I find odd. Basically if I
> > do
> >
> > git fetch && \
> > git reset --hard simple-tag-that-points-to-the-current-commit
> >
> > sometimes the reset will update the mtime of all files and directories
> > in the repo and sometimes it will leave them alone. Changing it to
> >
> > git fetch && \
> > git status && \
> > git reset --hard simple-tag-that-points-to-the-current-commit
> >
> > Cause the mtime update to reliably not happen.
>
> If my theory on what is happening is correct, I do not think there
> is any bug in what "reset --hard" is doing.
>
> My theory is that something is causing the stat info that is cached
> in your index and the lstat(2) return you get from your working tree
> files go out of sync. Even though you are not actively touching any
> working tree files (otherwise, you wouldn't be complaining about
> mtime changing in the first place), perhaps your build of Git
> records timestamps in NS but your filesystem and the operating
> system does not preserve nanosecond resolution of timestamps when it
> evicts inode data from the core, or something like that? If that is
> what is happening, I think that "fetch" is a red herring, but any
> operation that takes some time and/or hits filesystem reasonably
> hard would trigger it.
>
> And the reason why I say there is no bug in what "reset --hard" is
> doing here, if the above theory is correct, is because:
>
> - The user asked "reset --hard" to "make sure that my working tree
> files are identical to those of HEAD";
>
> - "reset --hard" looks at lstat(2) return and the cached stat info
> in the index and find them not to match. It can do one of two
> things:
>
> (1) see if the user did something stupid, like "touch file", that
> modifies only lstat(2) info without actually changing its
> contents, by reading from the working tree, reading HEAD:file
> from the object database, and comparing them, and overwrite
> the working tree file only when they do not match.
>
> or
>
> (2) the contents might happen to be the same, but the end result
> user desires to have is that the contents of the working tree
> file is the same as that from the HEAD, so overwrite it
> without wasting time reading two and compare before doing so.
>
> and it is perfectly reasonable to do the latter. After all, the
> whole point of having its cached lstat(2) data in the index is to
> so that we do not have to always compare the contents before
> deciding something has changed in the working tree.
>
> Running "git update-index --refresh" immediately before "reset" may
> alleviate the issue. "git status" has the same effect, only because
> it does "update-index --refresh" at the beginning of its processing,
> but it wastes a lot more time and resource doing other things.
>
> But unless/until you know _why_ the cached stat info in your index
> goes stale relative to what lstat(2) tells you, it would not "solve"
> it, because that magical thing (and my theory is cached data in your
> operating system that keeps a file timestamp with more precision
> than your underlying filesystem can represent is being flushed, and
> reading the file timestamp back from the disk has to truncate the
> nanoseconds part) can happen at any time between the "--refresh" and
> your "reset".
Thanks Junio!
If I understand you correctly, reset should not touch files if it thinks
they are up-to-date, so at least that assumption is safe to make. I'll
test your theory about why reset thinks all the files are outdated.
I did notice 'fetch' updates the index (well, mtime of .git/index
changes, I didn't look at index content yet), so maybe fetch isn't quite
a red herring. I'll try to eliminate this variable as well.
--
Dennis Kaarsemaker
www.kaarsemaker.net
prev parent reply other threads:[~2015-07-10 7:31 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-09 14:02 Reset sometimes updates mtime Dennis Kaarsemaker
2015-07-09 17:56 ` Junio C Hamano
2015-07-10 7:30 ` Dennis Kaarsemaker [this message]
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