Is there an extension with a revset for 'all ancestors along the p1 line'?

[Sietse Brouwer]

(Apologies, Gregory, I forgot to copy my reply to the list.)

On 04/06/2016, Gregory Szorc <gregory.szorc at gmail.com> wrote:

> Such a revset does not exist. Even if you built one, it wouldn't do what
> you think. The reason is that Mercurial sorts the parent nodes during a
> merge commit and stores the node with the smaller bit-wise value in p1.

Are you certain that that is what Mercurial does? For tag 3.8.2, This
is how I read merge.py, def update:

* line 1439 sets p1 to parent 1 of the current commit -- this is
before this function sets the current commit's parents.
* line 1465 aborts if the current commit already has more than 1
parent -- in other words, when line 1439 sets p1 successfully, it sets
it to the 'current' parent.
* line 1453 sets `p2 = repo[node]`; `node` is the command-line
argument passed to `hg merge` (you can check this by following
commands.py def merge > hg.py def merge > merge.py def update), so
that is always the incoming commit.

As a sanity check, I tried creating two heads and merging them both
ways: once merging 1 into 2, and once merging 2 into 1. When I then
used `hg log` to print p1 and p2 of each commit, I found the two
merges had different p1/p2-ordering: in each case, p1 was the
'receiving' parent, and p2 the incoming one.

    hg --version
    Mercurial Distributed SCM (version 3.8.2+20160517)

    hg log -T '{rev}\np1: {p1rev}\np2: {p2rev}'
    @    4  merge 2 into 1
    |\   p1: 1
    | |  p2: 2
    +---o  3 merge 1 into 2
    | |/   p1: 2
    | |    p2: 1
    | o  2
    | |  p1: 0
    | |  p2: -1
    o |  1
    |/   p1: 0
    |    p2: -1
    o  0
       p1: -1
       p2: -1

If this looks alright to you, too, that would mean the p1/p2 distinction is
indeed meaningful, which would be good news.

Cheers,
Sietse