Re: [PATCH 14/26] shallow.c: implement a generic shallow boundary finder based on rev-list

> Instead of a custom commit walker like get_shallow_commits(), this new
> function uses rev-list to mark NOT_SHALLOW to all reachable commits,
> except borders. The definition of reachable is to be defined by the
> protocol later. This makes it more flexible to define shallow boundary.
>
> The way we find find border is paint all reachable commits NOT_SHALLOW.

> Any of them that "touches" commits without NOT_SHALLOW flag are
> considered shallow (e.g. zero parents via grafting mechanism). Shallow
> commits and their true parents are all marked SHALLOW. Then NOT_SHALLOW
> is removed from shallow commits at the end.
>
> There is an interesting observation. With a generic walker, we can
> produce all kinds of shallow cutting. In the following graph, every
> commit but "x" is reachable. "b" is a parent of "a".
>
>            x -- a -- o
>           /    /
>     x -- c -- b -- o
>
> After this function is run, "a" and "c" are both considered shallow
> commits. After grafting occurs at the client side, what we see is
>
>                 a -- o
>                     /
>          c -- b -- o
>
> Notice that because of grafting, "a" has zero parents, so "b" is no
> longer a parent of "a".
>
> This is unfortunate and may be solved in two ways. The first is change
> the way shallow grafting works and keep "a -- b" connection if "b"
> exists and always ends at shallow commits (iow, no loose ends). This is
> hard to detect, or at least not cheap to do.
>
> The second way is mark one "x" as shallow commit instead of "a" and
> produce this graph at client side:
>
>            x -- a -- o
>                /    /
>          c -- b -- o
>
> More commits, but simpler grafting rules.
>
> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>