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RE: [PATCH 3/3] reset.c: update files when using sparse to avoid data loss.

> -----Original Message-----
> From: git-owner@xxxxxxxxxxxxxxx [mailto:git-owner@xxxxxxxxxxxxxxx] On
> Behalf Of Duy Nguyen
> Sent: Wednesday, April 12, 2017 7:21 AM
> To: Kevin Willford <kewillf@xxxxxxxxxxxxx>
> Cc: Kevin Willford <kcwillford@xxxxxxxxx>; git@xxxxxxxxxxxxxxx;
> gitster@xxxxxxxxx; peff@xxxxxxxx
> Subject: Re: [PATCH 3/3] reset.c: update files when using sparse to avoid
> data loss.
> On Wed, Apr 12, 2017 at 5:30 AM, Kevin Willford <kewillf@xxxxxxxxxxxxx>
> wrote:
> > The loss of the skip-worktree bits is part of the problem if you are
> > talking about modified files.  The other issue that I was having is
> > when running a reset and there were files added in the commit that is
> > being reset, there will not be an entry in the index and not a file on
> > disk so the data for that file is completely lost at that point.
> > "status" also doesn't include anything about this loss of data.  On
> > modified files status will at least have the file as deleted since
> > there is still an index entry but again the previous version of the file and it's
> data is lost.
> Well, we could have "deleted" index entries, those marked with
> CE_REMOVE. They are never written down to file though, so 'status'
> won't benefit from that. Hopefully we can restore the file before the index
> file is written down and we really lose skip-worktree bits?

Because this is a reset --mixed it will never run through unpack_trees and 
The entries are never marked with CE_REMOVE.

> > To me this is totally unexpected behavior, for example if I am on a
> > commit where there are only files that where added and run a reset
> > HEAD~1 and then a status, it will show a clean working directory.
> > Regardless of skip-worktree bits the user needs to have the data in
> > the working directory after the reset or data is lost which is always bad.
> I agree we no longer have a place to save the skip-worktree bit, we have to
> restore the state back as if skip-worktree bit does not exist.
> It would be good if we could keep the logic in unpack_trees() though.
> For example, if the file is present on disk even if skip-worktree bit is on,
> unpack_trees() would abort instead of silently overwriting it.
> This is a difference between skip-worktree and assume-unchanged bits.
> If you do explicit checkout_entry() you might have to add more checks to
> keep behavior consistent.
> --
> Duy

Because this is a reset --mixed it will follow the code path calling read_from_tree
and ends up calling update_index_from_diff in the format_callback of the diff,
so unpack_trees() is never called in the --mixed case.  This code change also only applies
when the file does not exist and the skip-worktree bit is on and the updated 
index entry either will be missing (covers the added scenario) or was not missing
before (covers the modified scenario).  If there is a better way to get the previous
index entry to disk than what I am doing, I am happy to implement it correctly.