Fix recent 'unpack_trees()'-related changes breaking 'git stash'

Fix recent 'unpack_trees()'-related changes breaking 'git stash'

On Sat, 15 Mar 2008, SZEDER G?bor wrote:
>
> The testcase usually fails during the first 25 run, but sometimes it
> runs more than 100 times before failing.

Damn, this series has had more subtle issues than I ever expected.

'git stash' creates its saved working tree object with:

        # state of the working tree
        w_tree=$( (
                rm -f "$TMP-index" &&
                cp -p ${GIT_INDEX_FILE-"$GIT_DIR/index"} "$TMP-index" &&
                GIT_INDEX_FILE="$TMP-index" &&
                export GIT_INDEX_FILE &&
                git read-tree -m $i_tree &&
                git add -u &&
                git write-tree &&
                rm -f "$TMP-index"
        ) ) ||
                die "Cannot save the current worktree state"

which creates a new index file with the updates, and writes the tree from
that.

We have this logic where we compare the timestamp of the index with the
timestamp of the files and we then write them out "smudged" if they are
the same, and it basically depends on the fact that the date on the index
file is compared with the date encoded in the stat information itself.

And what is going on is:

 - we create a new index file with that "cp". We are careful to preserve
   the timestamps by using "-p", so this one should be all ok.

 - then we *update* that index by resetting it to the tree with git
   read-tree, but now we do *not* preserve the timestamp on this new copy
   any more, even though we copy over all the timestamps on the files that
   are indexed from the stat information!

Now, we always had that problem when re-writing the index, but we had this
clever workaround in the writing part: if the source had racily clean
entries, then when we wrote those out (and thus can't depend on the index
fiel timestamp showing that they are racily clean any more!), we would
smudge them when writing.

IOW, we handle this issue by having write_index() do this:

	for (i = 0; i < entries; i++) {
		...
		if (is_racy_timestamp(istate, ce))
			ce_smudge_racily_clean_entry(ce);
		..

when writing out entries. And that all took care of it, because now when
we wrote the new index, we'd change the timestamp on the index, yes, but
we'd smudge the entries we wrote out, so now the resulting index would
still show that file as not-up-to-date any more.

But with commit 34110cd4e394e3f92c01a4709689b384c34645d8 ("Make
'unpack_trees()' have a separate source and destination index"), this
logic no longer triggers, because we now write out the "result" index, and
that one never got its timestamp updated from the source index, so it had
lost all that "is_racy_timestamp()" information!

This trivial patch fixes it. It looks trivial, and it's a simple fix, but
boy did it take me way too much thinking and explaining to myself to
explain why there was a problem in the first place!

The trivial fix is to just copy the index timestamp from the source index
into the result index. But we only do this if we *have* a source index, of
course, and if we will even bother to use the result.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>