Re: What actually is a branch?

[Felipe Contreras <felipe.contreras@gmail.com>]

Sergey Organov wrote:
> Felipe Contreras <felipe.contreras@gmail.com> writes:
> 
> > Since this is not strictly related to the topic of `git switch` I
> > renamed the thread.
> >
> > Sergey Organov wrote:
> >> Felipe Contreras <felipe.contreras@gmail.com> writes:
> >> > Sergey Organov wrote:
> >
> >> >> Overall, if we aim at clear documentation, we need to define our
> >> >> documentation terms as precise as possible, and then use them
> >> >> consistently.
> >> >> 
> >> >> For example:
> >> >> 
> >> >> "branch": a chain of commits
> >> >> 
> >> >> "branch tip": the most recent commit in a branch
> >> >> 
> >> >> "branch name": specific type of symbolic reference pointing to a
> >> >> branch tip
> >> >
> >> > Completely agree on all three (although I would call it "branch head",
> >> > not "branch tip").
> >> 
> >> I see why "branch head", as you later introduce "branch tail", but a
> >> branch (of a plant) has no "head" (nor "tail"), right? BTW, how the base
> >> of a plant branch is called in English, and how one finds "branch tail"
> >> on a real tree anyway? I mean, there are probably a few of them, at
> >> every fork. In Git it's even more vague, as a branch could logically
> >> begin at any place, not necessarily at a fork point.
> >
> > We don't necessarily need a 1-to-1 mapping with common English (although
> > that would be nice). Anoher option could be "base" and "tip".
> >
> >> OTOH, "head" and "tail" are obviously taken from CS "list" concept, and,
> >> provided "chain" == "list", it does make sense.
> >
> > I took it from Mercurial, where the tip of a branch is called "head",
> > and in fact a branch can have multiple heads.
> >
> >> And then we have 'HEAD' that points to the current branch tip anyway.
> >
> > It actually points to a branch, or rather references a branch, since it
> > uses the branch name.
> 
> Yes, but it still points to the branch tip, indirectly, or even
> directly, when in "detached head" state, that, by the way, I'd vote to
> abandon, replacing it with more user-friendly "unnamed branch" or
> something like that.

Yes, but most of the time it's indirectly.

> >> Dunno, in fact I don't have any preference among "tip" and "head".
> >
> > I don't either, but from different sources (non-git-specific) I've heard
> > "head" more often.
> >
> >> As for branch tail, I do have convention of marking start of a
> >> long-standing branch with corresponding tag, where branch "foo" has
> >> corresponding "foo-bp" tag marking its "branch point". Recently I
> >> started to mark start of feature branch with yet another branch "foo-bp"
> >> rather than tag, "foo" being set to track "foo-bp", that allows to
> >> automate rebasing of "foo" against correct base.
> >
> > So foo-bp is the upstream of foo, and you do basically:
> >
> >   git rebase foo@{upstream}
> 
> Yep, but essential feature to me is that I in fact use tools that simply
> run bare
> 
>    git rebase
> 
> and that "just works" (tm).

I typed the revision explicitly, but `git rebase` would work just fine.

> > This is works if your base (or tail, or whatever) is static, but many
> > branches jump around, and that's where @{tail} comes in handy.
> 
> Yeah, I see. When I need to make a branch jump around, I do need to
> manually move my references, but that's fortunately very rare use-case
> for me. Having direct support for that is still a win.
> 
> >
> > You can do this:
> >
> >   git rebase --onto foo@{upstream} foo@{tail}
> >
> > This will always rebase the right commits (no need to look into the
> > reflog). So you can say that the branch is foo@{tail}..foo.
> 
> I see where and when it's useful, but for a feature branch 99% of times
> I don't want to rebase it onto some true upstream. I rather want to just
> fiddle with the branch in place, and I prefer to setup things the way
> that ensures that bare "git rebase" does "the right thing".

But that's precisely the point: when you do `git rebase` you don't have
to type the base or --onto anymore. It's done automatically.

Not just for your long-standing branches, but for *any* branch.

> Probably that could be solved by a branch-local configuration that makes
> "git rebase" become "git rebase @{tail}" for the branch instead of "git
> rebase @{upstream}"

No. @{upstream} is where you want to rebase *to*, @{tail} is where you
want to rebase *from*.

When you do:

  git rebase foo@{upstream}

This is basically the same as:

  git checkout foo@{upstream}^0
  git cherry-pick --right-only foo@{upstream}...foo

git is smart enough to figure out what commits are already part of
foo@{upstream}, and those are skipped, but at no point was any "base"
calculated (at least not from `git rebase`).

Most of the time `git rebase` works fine, because there aren't too many
commits to figure out where they should go, but it's definitely not
efficient, and there's many corner-cases (see a Linux kernel maintaner
baffled by what the hell `git rebase` is doing [1]).

> > Another advantage of having this notion is that `git rebase`
> > automatically updates the tail (in this case to foo@{upstream}).
> 
> Yep, looks useful. Is it all local to given repo, or else?

I implented it as 'refs/tails' (as opposed to 'refs/heads'), so it's
local to a given repo, but could easily be exported.

[1] https://lore.kernel.org/git/60b272ff6bfa4_265861208d6@natae.notmuch/

-- 
Felipe Contreras