From: Stefan Sperling <stsp@stsp.name>
Subject: Re: [patch] simplify ancestry checks
To: James Cook <falsifian@falsifian.org>
Cc: gameoftrees@openbsd.org
Date: Wed, 24 May 2023 17:58:20 +0200

On Sun, May 21, 2023 at 03:09:33AM +0000, James Cook wrote:
> As far as I can tell, the below patch doesn't change any behaviour
> except maybe to save a bit of time.
>
> I added a comment to got_commit_graph_find_youngest_ancestor mostly to
> make it easier to verify that yca_id can't be NULL when the new line
> 11313 is reached.
>
> The calls to check_same_branch that I replaced were equivalent to just
> checking whether one commit was the youngest common ancestor of both. It
> is complicated a bit by whether non-linear ancestors were considered,
> i.e. the first_parent_traversal argument to
> got_commit_graph_find_youngest_common_ancestor, but I believe it all
> works out:
>
> - The check in cmd_rebase only considers linear history, before and
>   after this change.
>
> - The check in cmd_merge considers full history, both before and after
>   this change. (The reason it considered full history before this change
>   is that one of the first things check_same_branch did was to compare
>   commit_id to yca_id, and yca_id was based on full history.)
>
> I was working on making "got merge" fast-forward when possible as
> discussed in the other thread when I noticed this apparently unneeded
> complexity. If this change is correct, I think it makes sense to do it
> first as a separate change, because it's already tricky enough to reason
> about on its own.

Indeed. After reading over this diff for a while with plenty of context lines
added, I have to agree with this simplification. Thanks for pointing it out.

Just one question about terminology, not a technical question:

> +/*
> + * Sets *yca_id to the last commit that can be reached from both commit_id and
> + * commit_id2. Returns got_error(GOT_ERR_ANCESTRY) if there are none.

The above wording is a bit misleading.

The term "last commit" is not defined in Got's or Git's vocabulary.
It is unclear which commit this refers to. I would suggest to use the phrase
"youngest common ancestor commit" (even though it is already in the name of
the function). This phrase seems self-explanatory to me.
Do you agree or do you have another suggestion?

And should it not say "if there is none" rather than "if there are none"?
I cannot think of a case where multiple YCA commits would exist. Unless
perhaps if merge-commits were excluded from the set of possible YCAs which
would leave their parent commits as a set of YCAs... ?
Maybe I am missing something?

> + *
> + * If first_parent_traversal is nonzero, only linear history is considered.
> + */
>  const struct got_error *
>  got_commit_graph_find_youngest_common_ancestor(struct got_object_id **yca_id,
>      struct got_object_id *commit_id, struct got_object_id *commit_id2,
>
>