From: Junio C Hamano <gitster@pobox.com>
Date: Thu, 03 Jan 2013 15:34:53 GMT
Subject: Re: [BUG] two-way read-tree can write null sha1s into index
Message-ID: <7vehi2xote.fsf@alter.siamese.dyndns.org>
In-Reply-To: <20130103083712.GC32377@sigill.intra.peff.net>

Jeff King <peff@peff.net> writes:

> On Tue, Jan 01, 2013 at 02:24:46PM -0800, Junio C Hamano wrote:
>
>> Jeff King <peff@peff.net> writes:
>> 
>> > So I think we need to update twoway_merge to recognize unmerged entries,
>> > which gives us two options:
>> >
>> >   1. Reject the merge.
>> >
>> >   2. Throw away the current unmerged entry in favor of the "new" entry
>> >      (when old and new are the same, of course; otherwise we would
>> >      reject).
>> >
>> > I think (2) is the right thing.
>> 
>> As "--reset" in "read-tree --reset -u A B" is a way to say "we know
>> we are based on A and we want to go to B no matter what", I agree we
>> should not reject the merge.
>> 
>> With -m instead of --reset, I am not sure what the right semantics
>> should be, though.
>
> Good point; I was just thinking about the --reset case.
>
> With "-m", though, we could in theory carry over the unmerged entries
> (again, assuming that "old" and "new" are the same; otherwise it is an
> obvious reject). But those entries would be confused with any new
> unmerged entries we create. It seems we already protect against this,
> though: "read-tree -m" will not run at all if you have unmerged entries.
>
> Likewise, "checkout" seems to have similar protections.
>
> So I think it may be a non-issue.

Yeah.  Also earlier in the thread you mentioned three-way case, but
I do not think we ever would want --reset with three trees, so I
think that too is a non-issue for the same reason.

I would still feel safer if we expressed the expectation of
the callee in the code, perhaps like this in the two-way case:

	if (current->ce_flags & CE_CONFLICTED) {
        	if (!o->reset) {
                	... either die or fail ...
		} else {
                	... your fix ...
		}
	}