[PATCH] backout: handle file moves correctly (issue1932)

[Matt Mackall]

On Thu, 2015-01-08 at 13:39 -0800, Mateusz Kwapich wrote:
> # HG changeset patch
> # User Mateusz Kwapich <mitrandir at fb.com>
> # Date 1420752870 28800
> #      Thu Jan 08 13:34:30 2015 -0800
> # Node ID b140af105987b9db7a296d24584a557bdc67fc6e
> # Parent  7ad155e13f0f51df8e986a0ec4e58ac9a0ccedbb
> backout: handle file moves correctly (issue1932)

> The merge logic (ab)used by backout

It's actually not abuse. It's very important core functionality.

>  wasn't detecting file moves when the
> parameter which should be common ancestor of merge heads was in fact child of
> one of them.

In the abstract, merge is simply the operation that given three state
vectors A, X, Y, creates a new state M = A + (X - A) + (Y - A) (where +
is "patch" and - is "diff"). It doesn't matter what the relationships
between the three inputs are. Don't get too distracted that we call 'A'
the 'ancestor' in the code, it's simply more convenient to reason about
in terms of a normal merge.

However, various parts of the merge and copy code were indeed originally
written with the assumption that a would be an ancestor of x and y and
made various assumptions about revlog ordering, which
has implications for every single thing that calls merge in a
non-vanilla fashion: backout, rebase, graft, evolve, histedit, etc..

Most of that is fixed. The primary offender at this point is
mergecopies(). I started down the path to replacing that by writing
pathcopies() a few years back, which doesn't care about direction in
history and deals with most of our non-merge copy-detection needs.

But finishing the job requires rewriting mergecopies() based on
pathcopies() or otherwise making it direction-agnostic... which is what
I've told everyone who's asked about merge+copy bugs for the past few
years.

Since you only mention "backout" in your subject and only hit a very
specific special case, I suspect you're missing most of the related
problems here and are just making things that much more confusing for
whoever comes along to do the larger fix.

-- 
Mathematics is the supreme nostalgia of our time.