Re: [PATCH v3 5/8] rebase: rename merge_base to branch_base

[Phillip Wood <phillip.wood123@gmail.com>]

On 17/10/2022 12:27, Ævar Arnfjörð Bjarmason wrote:
> 
> On Mon, Oct 17 2022, Phillip Wood wrote:
> 
>> On 13/10/2022 20:16, Ævar Arnfjörð Bjarmason wrote:
>>> On Thu, Oct 13 2022, Phillip Wood via GitGitGadget wrote:
>>>
>>>> From: Phillip Wood <phillip.wood@dunelm.org.uk>
>>>>
>>>> merge_base is not a very descriptive name, the variable always holds
>>>> the merge-base of 'branch' and 'onto' which is commit at the base of
>>>> the branch being rebased so rename it to branch_base.
>>> To me "branch" means or has heavier implications of "named branch"
>>> than
>>> just a merge base, and this command is perfectly happy to work on
>>> commits disconnected from any named branch.
>>>> But more to the point, the rebase docs for --onto discuss a "merge
>>> base", so you'd read those, and then encounter this code talking about a
>>> "branch base", and wonder what the difference was...
>>
>> Aren't the docs saying the merge base is the base of the commits
>> (i.e. branch) being rebased? I don't think merge_base is a
>> particularly helpful name as it doesn't tell us what it is the merge
>> base of and branch_base was the best I could come up with. I see what
>> you mean in the detached HEAD case, but as the command also works with
>> named branches I hope it is fairly obvious what "branch_base" is in
>> the detached HEAD case.
> 
> It *optionally* works with a <branch>, but doesn't require one. E.g. try
> this on git.git:

Maybe I wasn't clear, I was referring to the fact that if HEAD isn't 
detached then it rebases the current branch not about the optional 
<branch> argument. I also think that the docs are for users, they are 
not a guide to the code. With this change if you search for merge_base 
in builtin/rebase.c you still find the part where we calculate the merge 
base. This commit was added in response to a review comment from Junio 
on V1, as far as I know he is happy with it and at this stage I'm 
disinclined to change it.

Best Wishes

Phillip

> 	git checkout origin/next
> 	touch f && git add f && git commit -m"file"
> 	git rebase --onto origin/master^{} HEAD~
> 
> Here we transplant a commit on top of "next" to "master", without either
> of those *names* being involved, or their branches, just the
> corresponding OIDs/tips.
> 
> That will go through e.g. can_fast_forward() which you're modifying
> here, and now populate a "branch_base" variable, instead of a
> "merge_base".
> 
> I know that we conflate the meaning of "branch" somewhat, even in our
> own docs. E.g. we sometimes use "branch" and "named branch", but usually
> by "branch" we mean "named branch", and otherwise talk about a detached
> HEAD, <commit> or "tip".
> 
> But in this case it's especially confusing in the post-image, because
> "git rebase --onto" explicitly uses an optional "<branch>" to
> distinguish the "named branch" case from the case where we're operating
> on detached a HEAD, or otherwise don't care about the "<branch>" (except
> as generic "restore us to where we were" behavior).
> 
> So, if anything I'd think that we'd want something like this in various
> places in git-rebase.txt to make the distinction clearer:
> 	
> 	diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt
> 	index 9cb8931c7ac..e4700a6e777 100644
> 	--- a/Documentation/git-rebase.txt
> 	+++ b/Documentation/git-rebase.txt
> 	@@ -18,7 +18,7 @@ DESCRIPTION
> 	 -----------
> 	 If `<branch>` is specified, `git rebase` will perform an automatic
> 	 `git switch <branch>` before doing anything else.  Otherwise
> 	-it remains on the current branch.
> 	+it remains on the current tip or named branch.
> 	
> 	 If `<upstream>` is not specified, the upstream configured in
> 	 `branch.<name>.remote` and `branch.<name>.merge` options will be used (see
> 
> But your post-image seems to be to make this sort of thing explicitly
> more confusing, and e.g. these parts:
> 
> 	@@ -206,8 +206,8 @@ OPTIONS
> 	 --onto <newbase>::
> 	 	Starting point at which to create the new commits. If the
> 	 	`--onto` option is not specified, the starting point is
> 	-	`<upstream>`.  May be any valid commit, and not just an
> 	-	existing branch name.
> 	+	`<upstream>`.  May be any valid commit, and not just an <-- this
> 	+	existing branch name. <--- this
> 	 +
> 	 As a special case, you may use "A\...B" as a shortcut for the
> 	 merge base of A and B if there is exactly one merge base. You can
> 
> To sum up why I find this confusing: Reading this from the docs onwards
> I'd think (as is the case) that "<branch>" is optional. Then when I read
> the code I'd think a "branch_base" is something that *only* had to do
> with the "<branch>" case.
> 
> But that's not the case, it's just a generic "merge base" in the same
> sense that "git merge-base" accepts all of these
> 
> 	$ git merge-base origin/master origin/next
> 	d420dda0576340909c3faff364cfbd1485f70376
> 
> (These two are equivalent, just demo'ing that we don't need the peel
> syntax):
> 
> 	$ git merge-base $(git rev-parse origin/master) $(git rev-parse origin/next)
> 	d420dda0576340909c3faff364cfbd1485f70376
> 	$ git merge-base origin/master^{} origin/next^{}
> 	d420dda0576340909c3faff364cfbd1485f70376
> 
> What *would* make things much clearer is e.g. calling a variable
> "branch_merge_base" *if* there is a case where that's a merge base only
> for named branches, but I don't know (and didn't look carefully enough)
> if you've got such a case or cases here. It just seems like a generic
> "merge-base".
> 
>