On Thu, May 22, 2014 at 9:22 AM, David Turner <dtur...@twopensource.com> wrote:
> If I have a git repository with a clean working tree, and I delete the
> index, then I can use git reset (with no arguments) to recreate it.
> However, when I do recreate it, it doesn't come back the same.  I have
> not analyzed this in detail, but the effect is that commands like git
> status take much longer because they must read objects out of a pack
> file.  In other words, the index seems to not realize that the index (or
> at least most of it) represents the same state as HEAD.  If I do git
> reset --hard, the index is restored to the original state (it's
> byte-for-byte identical), and the pack file is no longer read.
> Before I try to dig in to why this should be so, does anyone happen to
> know off the top of their head?  Does this constitute a bug in git, or a
> bug in my understanding of git?

It's not a bug.  The index has additional stat-info it tracks about
files -- inode number, mtime, etc. -- so that it can quickly check
whether files are up to date without comparing full file contents in
the working copy to the relevant version from .git/objects.

'git reset' means make the index match the commit in HEAD.  It implies
nothing about the working copy files, which could be different.
Although you say that you have a clean working tree, git doesn't check
to verify that, so it has to treat these files as stat-dirty until the
next operation (e.g. git status) fills these in -- an operation that
involves full comparison of the files in the working copy with the
relevant version of the file from under .git/objects. (You may find
'git update-index --refresh' helpful here.)

git reset --hard means not only make the index match the commit in
HEAD, but change files in the working copy to match as well.  In such
a case, git will know that the index matches the working copy (since
it is enforcing it), so it can update all the stat-info in the index
and future git-status operations will be fast.
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