Re: [BUG] two-way read-tree can write null sha1s into index

[Jeff King <peff@peff.net>]

On Thu, Jan 03, 2013 at 12:34:27PM -0800, Junio C Hamano wrote:

> Jeff King <peff@peff.net> writes:
> 
> > Or are you suggesting that the three-way case should always be protected
> > by checking that there are no unmerged entries before we start it? That
> > seems sane to me, but I haven't confirmed that that is the case.
> 
> I think the normal (and hopefully only) "-m -u O A B" use case of
> threeway is the bog-standard "git merge", which requires even more:
> your index must exactly match HEAD, even though you are allowed to
> have local changes in the working tree.
> 
> That requirement is not likely to change, as cleanly merged paths
> are automatically added to the index, so "diff --cached" should show
> only the changes from cleanly merged part, while "diff" should show
> paths that still needs user's help.
> 
> If we allowed local modification to the index (let alone conflicted
> entries in it) before the merge begins, the users would not be able
> to tell which paths are in what state after a half-merge stops and
> asks for help.  Updated paths may not have anything to do with the
> merge (i.e. earlier "git add" before the merge started), conflicting
> paths may not have anything to do with the merge (i.e. leftover
> conflicts before the merge started).

Oh, I agree it's insane to try to carry through unmerged entries. I'm
just concerned that not all code paths are careful enough to check. They
probably are, though, as a null sha1 in your index would be the least of
your worries, and somebody would have noticed in the last 7 years.

-Peff