Re: [PATCH] rebase: set REF_HEAD_DETACH in checkout_up_to_date()

From: Junio C Hamano <gitster@pobox.com>
Date: Mar 11, 2022, 05:55 UTC
Message-ID: <xmqq5yolav8l.fsf@gitster.g>
In-Reply-To: <pull.1226.git.git.1646975144178.gitgitgadget@gmail.com>

"John Cai via GitGitGadget" <gitgitgadget@gmail.com> writes:

Show 10 quoted lines

> This is happening because on a fast foward with an oid as a <branch>,
> update_refs() will only call update_ref() with REF_NO_DEREF if
> RESET_HEAD_DETACH is set. This change was made in 176f5d96 (built-in rebase
> --autostash: leave the current branch alone if possible,
> 2018-11-07). In rebase, we are not setting the RESET_HEAD_DETACH flag,
> which means that the update_ref() call ends up dereferencing
> HEAD and updating it to the oid used as <branch>.
>
> The correct behavior is that git rebase should update HEAD to $(git
> rev-parse topic) without dereferencing it.

It is unintuitive that unconditionally setting the RESET_HEAD_DETACH bit is the right solution.

If the command weren't "rebase master side^0" but "rebase master side", i.e. "please rebase the side branch itself, not an unnamed branch created out of the side branch, on master", according to <reset.h>, we ought to end up being on a detached HEAD, as reset_head() with the bit

    /* Request a detached checkout */
    #define RESET_HEAD_DETACH (1<<0)

requests a detached checkout. But that apparently is not what would happen with your patch applied.

Puzzled. The solution to the puzzle probably deserves to be in the proposed log message.

Thanks.