Re: [PATCH 2/3] git-p4: Search for parent commit on branch creation

[Pete Wyckoff <pw@padd.com>]

vitor.hda@gmail.com wrote on Tue, 17 Jan 2012 00:10 +0000:
> On Mon, Jan 16, 2012 at 11:41 PM, Vitor Antunes <vitor.hda@gmail.com> wrote:
> > On Mon, Jan 16, 2012 at 6:57 PM, Pete Wyckoff <pw@padd.com> wrote:
> >> 1.  Move the tempBranch commit outside of the "for blob" loop.
> >>    It can have no parent, and the diff-tree will still tell you
> >>    if you found the same contents.  Instead of a ref for
> >>    each blob inspected for each change, you'll just have one ref
> >>    per change.  Only one checkpoint() after the tempBranch
> >>    commit should be needed.
> >
> > You're right. Completely oversaw that. Will improve the code
> > accordingly.
> 
> Apparently I did not oversee it. Assume you have added a new file to
> HEAD of parent branch, but you branched from a previous commit. When the
> new branch is committed over HEAD the new file will, incorrectly, be
> part of it and diff-tree will not work as expected.

I don't get it.  This algorithm works on the fact that a "branch"
in p4 creates a new change that looks exactly like a previous
change.

The git-p4 sync step, when it detects a branch, starts by saving
the change in a commit with parent = null, so it is its own new
branch, an orphan, with no parents.

Now the task is to find some commit that has an identical tree to
this temporary one.  You walk back all known p4 commits to try to
find one that is the same.  It doesn't matter if any of those p4
commits have other commits on top of them.

At each step in the backward walk, the comparison is against the
unchanged orphan commit.

An ascii-art picture might help me.  Or even a test case.

> I should avoid taking 6 months to submit a patch to avoid forgetting why
> I did what I did :)

Yeah, and now you have to explain it all over to me again too.  :)

		-- Pete