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git reset for index restoration?

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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?

posted May 22, 2014 by Garima Jain

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Are you sure it's reading a packfile? I would expect that it is rather reading the files in the working tree. One of the functions of the index is to maintain a cache of the sha1 of the working tree file content and the stat information. Once that is populated, a subsequent "git status" can then just lstat() the files to see that its sha1 cache is up to date.
commented May 22, 2014 by Seema Siddique

1 Answer

+1 vote

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.

answer May 22, 2014 by anonymous
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