Re: [Monotone-devel] Re: 3-way merge considered harmful

[Florian Weimer]

* Richard Levitte:

> In message <address@hidden> on Sun, 1 May 2005 00:29:48 -0700, Nathaniel 
> Smith <address@hidden> said:
>
> njs> Here's another pathological case for 3-way merge:
> njs>    A
> njs>    |
> njs>    B
> njs>   / \
> njs>  C   D
> njs> Suppose that a file was added on the A->B edge, and then removed again
> njs> in B->C, while it was left alone on the B->D edge.  We want to merge
> njs> C and D.  Let's assume that for some reason we decide to use A as a
> njs> common ancestor.  What will happen?  Our file does not exist in either
> njs> A or C, so when comparing A and C 3-way merge will think that nothing
> njs> has happened.  Our file _does_ exist in D, though, so when comparing A
> njs> and D, 3-way merge will decide that a file has been added.  Therefore,
> njs> says 3-way merge, this file should exist in the new merged node.
> njs> But, this is obviously wrong; the file was deleted between B and C,
> njs> and this delete should have caused a delete of D's copy as well.
>
> I disagree with that conclusion.  It's quite possible the removal of
> the file in the B->C edge is a mistake, and that the person leaving it
> along in the B->D edge had it right.  Especially in a fairly loose
> network of cooperating programmers, disagreements of that kind are
> bound to happen (and have happened).  Others might call them "oopses"
> rather than disagreements...

I think Nathaniel wanted to say that there is an implied conflict
which 3-way merge cannot detect, not that one choice is better than
the other.  The argument that there is a hidden conflict which should
be flagged as such is quite convincing (and, as a result, the "zombie
changeset" term is misleading).