Subject: Re: [PATCH 4/3] branch: fix "copy" to never touch HEAD

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Tue, 26 Sep 2017 23:39:40 +0200

On Fri, Sep 22 2017, Junio C. Hamano jotted:

> When creating a new branch B by copying the branch A that happens to
> be the current branch, it also updates HEAD to point at the new
> branch.  It probably was made this way because "git branch -c A B"
> piggybacked its implementation on "git branch -m A B",
>
> This does not match the usual expectation.  If I were sitting on a
> blue chair, and somebody comes and repaints it to red, I would
> accept ending up sitting on a chair that is now red (I am also OK to
> stand, instead, as there no longer is my favourite blue chair).  But
> if somebody creates a new red chair, modelling it after the blue
> chair I am sitting on, I do not expect to be booted off of the blue
> chair and ending up on sitting on the new red one.
>
> Let's fix this before it hits 'next'.  Those who want to create a
> new branch and switch to it can do "git checkout B" after doing a
> "git branch -c B", and if that operation is so useful and deserves a
> short-hand way to do so, perhaps extend "git checkout -b B" to copy
> configurations while creating the new branch B.
>
> Signed-off-by: Junio C Hamano <[email-masked]>
> ---
>
>  * Let's send an updated one as a follow-up to the discussion thread
>    that it is a follow-up to.  The patch in this message is the same
>    as the one I previously sent; the proposed log message has been
>    updated somewhat.
>
>  builtin/branch.c  |  9 +++------
>  t/t3200-branch.sh | 10 +++++-----
>  2 files changed, 8 insertions(+), 11 deletions(-)

This mostly looks good to me.

I've already spent several screenfuls here on-list arguing for not
applying the equivalent of this patch, which I won't repeat here.

But just a brief comment: I think this doing this is good, and it's
something I agree with given that the consensus is going this way. I
thought "do what -m does" was less confusing, but since everyone (you,
Brad, even Sahil) seems to disagree I'm happy to be shown to be wrong &
to have this go in so thoroughly reviewed & thought about.

I do think however that we also need to update the docs, the relevant
origin/master...gitster/sd/branch-copy diff is currently:

    +The `-c` and `-C` options have the exact same semantics as `-m` and
    +`-M`, except instead of the branch being renamed it along with its
    +config and reflog will be copied to a new name.

Suggestions welcome, but I think that should probably be changed to
something like the following as part of this patch:

    The `-c` and `-C` options have the exact same semantics as `-m` and
    `-M`, except instead of the branch being renamed it along with its
    config and reflog will be copied to a new name. Furthermore, unlike
    its `-m` and `-M` cousins the `-c` and `-C` options will never move
    the HEAD, whereas the move option will do so if the source branch is
    the currently checked-out branch.

> diff --git a/builtin/branch.c b/builtin/branch.c
> index 89f64f4123..e2e3692838 100644
> --- a/builtin/branch.c
> +++ b/builtin/branch.c
> @@ -506,12 +506,9 @@ static void copy_or_rename_branch(const char *oldname, const char *newname, int
>  				oldref.buf + 11);
>  	}
>
> -	if (replace_each_worktree_head_symref(oldref.buf, newref.buf, logmsg.buf)) {
> -		if (copy)
> -			die(_("Branch copied to %s, but HEAD is not updated!"), newname);
> -		else
> -			die(_("Branch renamed to %s, but HEAD is not updated!"), newname);
> -	}
> +	if (!copy &&
> +	    replace_each_worktree_head_symref(oldref.buf, newref.buf, logmsg.buf))
> +		die(_("Branch renamed to %s, but HEAD is not updated!"), newname);
>
>  	strbuf_release(&logmsg);
>
> diff --git a/t/t3200-branch.sh b/t/t3200-branch.sh
> index 5d03ad16f6..be9b3784c6 100755
> --- a/t/t3200-branch.sh
> +++ b/t/t3200-branch.sh
> @@ -422,7 +422,7 @@ test_expect_success 'git branch --copy is a synonym for -c' '
>  	test_cmp expect actual
>  '
>
> -test_expect_success 'git branch -c ee ef should copy and checkout branch ef' '
> +test_expect_success 'git branch -c ee ef should copy to create branch ef' '
>  	git checkout -b ee &&
>  	git reflog exists refs/heads/ee &&
>  	git config branch.ee.dummy Hello &&
> @@ -431,7 +431,7 @@ test_expect_success 'git branch -c ee ef should copy and checkout branch ef' '
>  	git reflog exists refs/heads/ef &&
>  	test $(git config branch.ee.dummy) = Hello &&
>  	test $(git config branch.ef.dummy) = Hello &&
> -	test $(git rev-parse --abbrev-ref HEAD) = ef
> +	test $(git rev-parse --abbrev-ref HEAD) = ee
>  '
>
>  test_expect_success 'git branch -c f/f g/g should work' '
> @@ -494,12 +494,12 @@ test_expect_success 'git branch -C c1 c2 should succeed when c1 is checked out'
>  	git checkout -b c1 &&
>  	git branch c2 &&
>  	git branch -C c1 c2 &&
> -	test $(git rev-parse --abbrev-ref HEAD) = c2
> +	test $(git rev-parse --abbrev-ref HEAD) = c1
>  '
>
> -test_expect_success 'git branch -C c1 c2 should add entries to .git/logs/HEAD' '
> +test_expect_success 'git branch -C c1 c2 should never touch HEAD' '
>  	msg="Branch: copied refs/heads/c1 to refs/heads/c2" &&
> -	grep "$msg$" .git/logs/HEAD
> +	! grep "$msg$" .git/logs/HEAD
>  '
>
>  test_expect_success 'git branch -C master should work when master is checked out' '