Re: [PATCH 3/3] checkout: allow -b/-B to work on a merge base

[Junio C Hamano <gitster@pobox.com>]

Junio C Hamano <gitster@pobox.com> writes:

> Denton Liu <liu.denton@gmail.com> writes:
>
>> -	new_branch_info->name = arg;
>> +	new_branch_info->name = strstr(arg, "...") ?
>> +		xstrdup(oid_to_hex(rev)) :
>> +		arg;
>
> Can we do better?
>
> I am not sure why we want to hardcode the knowledge of "..." syntax
> like this here.  "git checkout A...B" introduced in 2009 needed only
> a single-liner change from get_sha1() to get_sha1_mb() without making
> the ugly implementation detail seep into this layer.

I _think_ what you are trying to work around is that a syntax that
is not understood as a reference to a single revision by get_oid()
but is understood as such by get_oid_mb() can be in the .name field,
and that eventually gets passed to branch.c::create_branch() as
"start_name" in the codepath.  The function ought to know that
start_name wants to name a single revision and never a revision
range, but fails to use get_oid_mb() but just a plain get_oid(),
and fails.

If that is the case, wouldn't a better fix be more like the
attached?  This hasn't even been compile tested, but I suspect that
without taking this approach, you would introduce a new "bug",
namely, that

	$ git checkout -b newbranch master...

ought to behave exactly like

	$ git branch newbranch master...
	$ git checkout newbranch

But the first step would hit the same branch.c::create_branch()
and would not work, no?

The reason I care about hiding syntax details like "..." from users
of get_*_mb() is because I anticipate that we'll want to extend it
to things other than just "merge base between two" with syntax
different from "..." (while probably renaming _mb suffix to
something else).

The codebase is not ready to replace get_oid() with get_oid_mb()
blindly and wholesale, I think, as the former is used as an
implementation detail of parsing range syntax like A..B but places
that are end user facing *and* expect to take only a single revision
(e.g. "rebase --onto <commit>", "checkout <commit>", etc.) and never
a range that currently use get_oid() should be able to get replaced
with get_oid_mb() to learn "A...B means their merge base, not a
symmetric range" semantics for free.

 branch.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/branch.c b/branch.c
index 28b81a7e02..a84c8aaca2 100644
--- a/branch.c
+++ b/branch.c
@@ -268,7 +268,7 @@ void create_branch(struct repository *r,
 	}
 
 	real_ref = NULL;
-	if (get_oid(start_name, &oid)) {
+	if (get_oid_mb(start_name, &oid)) {
 		if (explicit_tracking) {
 			if (advice_set_upstream_failure) {
 				error(_(upstream_missing), start_name);