Re: [PATCH v2 3/3] read_loose_refs(): treat NULL_SHA1 loose references as broken

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

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

> NULL_SHA1 is never a valid value for a reference. If a loose reference
> has that value, mark it as broken.
>
> Why check NULL_SHA1 and not the nearly 2^160 other SHA-1s that are
> also invalid in a given repository? Because (a) it is cheap to test
> for NULL_SHA1, and (b) NULL_SHA1 is often used as a "SHA-1 is invalid"
> value inside of Git client source code (not only ours!), and
> accidentally writing it to a loose reference file would be an easy
> mistake to make.
>
> Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
> ---
>  refs.c                         | 7 +++++++
>  t/t6301-for-each-ref-errors.sh | 2 +-
>  2 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/refs.c b/refs.c
> index 47e4e53..c28fde1 100644
> --- a/refs.c
> +++ b/refs.c
> @@ -1321,6 +1321,13 @@ static void read_loose_refs(const char *dirname, struct ref_dir *dir)
>  				hashclr(sha1);
>  				flag |= REF_ISBROKEN;
>  			}
> +
> +			if (!(flag & REF_ISBROKEN) && is_null_sha1(sha1)) {
> +				/* NULL_SHA1 is never a valid reference value. */
> +				hashclr(sha1);

Do you need to clear after checking with is_null_sha1()?

> +				flag |= REF_ISBROKEN;
> +			}
> +
>  			if (check_refname_format(refname.buf,
>  						 REFNAME_ALLOW_ONELEVEL)) {
>  				hashclr(sha1);
> diff --git a/t/t6301-for-each-ref-errors.sh b/t/t6301-for-each-ref-errors.sh
> index 72d2397..cdb67a0 100755
> --- a/t/t6301-for-each-ref-errors.sh
> +++ b/t/t6301-for-each-ref-errors.sh
> @@ -24,7 +24,7 @@ test_expect_success 'Broken refs are reported correctly' '
>  	test_cmp broken-err err
>  '
>  
> -test_expect_failure 'NULL_SHA1 refs are reported correctly' '
> +test_expect_success 'NULL_SHA1 refs are reported correctly' '
>  	r=refs/heads/zeros &&
>  	echo $ZEROS >.git/$r &&
>  	test_when_finished "rm -f .git/$r" &&