[PATCH STABLE] graft: support grafting across renames (issue4028)

[Yuya Nishihara]

On Tue, 26 Jul 2016 12:39:50 -0500, Matt Mackall wrote:
> On Wed, 2016-07-27 at 00:07 +0900, Yuya Nishihara wrote:
> > On Mon, 25 Jul 2016 13:16:32 -0500, Gábor Stefanik wrote:
> > > # HG changeset patch
> > > # User Gábor Stefanik <gabor.stefanik at nng.com>
> > > # Date 1469470573 -7200
> > > #      Mon Jul 25 20:16:13 2016 +0200
> > > # Branch stable
> > > # Node ID 395d3fa2ac89fad199c99cc137bf801502292325
> > > # Parent  9c2cc107547fd701a7604349632f2f590366f17c
> > > graft: support grafting across renames (issue4028)
> > > 
> > > Graft performs a merge with a false common ancestor, which must be taken
> > > into
> > > account when tracking renames. Compute the real common ancestor in this
> > > case,
> > > and track renames between the real and false common ancestors in reverse.
> > I don't know the detail of the copy tracing, I can't tell if this strategy
> > is correct.
> > 
> > Many tests fails with this patch.
> 
> Yes, I'm afraid it's much trickier than this.
> 
> Basically, the main copy tracing function for merges assumes all merges go
> forward in history because it was written before we had any form of rebasing or
> grafting.
> 
>   ancestor --> a - merge
>     \            /
>      --------> b 
> 
> But when we graft or rebase, we're doing a merge that looks like this:
> 
>    ancestor--<--a_parent-> a - b'
>         \                     /
>          ------------------> b
> 
> ..and twisting the graph to look like this:
> 
> a_parent -------------> a - b'
>     \                      /
>      --->-ancestor -----> b
> 
> The stretch between a_parent and ancestor is backwards in the real history but
> gets flipped for our effective merge DAG. That means a rename of x to y in that
> span effectively becomes a rename from y to x for the purposes of our operation.
> But the spans from a_parent to a and from ancestor to b remain unflipped.
> Teaching mergecopies() how to do this amounts to a complete rewrite.

Ah, thanks. I think I finally understand the basic model.

> The situation with copies is even more confusing (search the list archives for a
> discussion of "ypoc"s).

For reference,

https://www.mercurial-scm.org/pipermail/mercurial-devel/2012-December/047183.html