Re: [PATCH 02/38] rename_ref_available(): add docstring

From: Junio C Hamano <gitster@pobox.com>
Date: Jun 7, 2016, 18:10 UTC
Message-ID: <xmqq7fe0rhts.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <5eb96cc20c0f76aff39d1556803ebd4f9f422cea.1464983301.git.mhagger@alum.mit.edu>

Michael Haggerty <mhagger@alum.mit.edu> writes:

Show 23 quoted lines

> From: David Turner <dturner@twopensource.com>
>
> Signed-off-by: David Turner <dturner@twopensource.com>
> Signed-off-by: Junio C Hamano <gitster@pobox.com>
> Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
> ---
>  refs/refs-internal.h | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/refs/refs-internal.h b/refs/refs-internal.h
> index efe5847..d8a2606 100644
> --- a/refs/refs-internal.h
> +++ b/refs/refs-internal.h
> @@ -240,6 +240,11 @@ const char *find_descendant_ref(const char *dirname,
>  				const struct string_list *extras,
>  				const struct string_list *skip);
>  
> +/*
> + * Check if the new name does not conflict with any existing refs
> + * (other than possibly the old ref).  Return 0 if the ref can be
> + * renamed to the new name.
> + */
>  int rename_ref_available(const char *oldname, const char *newname);

I do not quite understand the comment. Partly because it is unclear what "conflict" means here, but I guess it means a D/F conflict that is explained near verify_refname_available()?

A new name can conflict with an existing, possibly old ref? Are you referring to this condition?

    You are trying to rename "refs/a/b" to "refs/a", which would
    conflict, but as long as there is no other ref that share the
    prefix "refs/a/", e.g. "refs/a/c", the new name "refs/a" is
    available.

I wonder if it is necessary to document that this function is not meant to protect us from others racing with us. That is, when you are renaming something to "refs/a", you call this function and it finds, by calling verify_refname_available(), that the repository has nothing that conflicts with the name and says "OK", but before you actually do the rename, somebody may push from sideways to create "refs/a/b", making the result of an earlier check with this function invalid.

Or is this to be called only under a lock that protects us from such a race?