Subject: Re: [RFC PATCH] pack-refs: fail on falsely sorted packed-refs

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Wed, 13 Feb 2019 11:08:01 +0100

On Thu, Jan 31 2019, Max Kirillov wrote:

> If packed-refs is marked as sorted but not really sorted it causes
> very hard to comprehend misbehavior of reference resolving - a reference
> is reported as not found.
>
> As the scope of the issue is not clear, make it visible by failing
> pack-refs command - the one which would not suffer performance penalty
> to verify the sortedness - when it encounters not really sorted existing
> data.
>
> Signed-off-by: Max Kirillov <[email-masked]>
> ---
> I happened to have a not really sorted packed-refs file. As you might guess,
> it was quite wtf-ing experience. It worked, mostly, but there was one branch
> which just did not resolve, regardless of existing and being presented in
> for-each-refs output.
>
> I don't know where the corruption came from. I should admit it could even be a manual
> editing but last time I did it (in that reporitory) was several years ago so it is unlikely.
>
> I am not sure what should be the proper fix. I did a minimal detection, so that
> it does not go unnoticed. Probably next step would be either fixing in `git fsck` call.
>
>  refs/packed-backend.c               | 15 +++++++++++++++
>  t/t3212-pack-refs-broken-sorting.sh | 26 ++++++++++++++++++++++++++
>  2 files changed, 41 insertions(+)
>  create mode 100755 t/t3212-pack-refs-broken-sorting.sh

This is not an area I'm very familiar with. So mostly commeting on
cosmetic issues with the patch. FWIW the "years back" issue you had
could be that an issue didn't manifest until now, i.e. in a sorted file
format you can get lucky and not see corruption for a while with a
random insert.

> diff --git a/refs/packed-backend.c b/refs/packed-backend.c
> index c01c7f5901..505f4535b5 100644
> --- a/refs/packed-backend.c
> +++ b/refs/packed-backend.c
> @@ -1088,6 +1088,7 @@ static int write_with_updates(struct packed_ref_store *refs,
>  	FILE *out;
>  	struct strbuf sb = STRBUF_INIT;
>  	char *packed_refs_path;
> +	struct strbuf prev_ref = STRBUF_INIT;
>
>  	if (!is_lock_file_locked(&refs->lock))
>  		BUG("write_with_updates() called while unlocked");
> @@ -1137,6 +1138,20 @@ static int write_with_updates(struct packed_ref_store *refs,
>  		struct ref_update *update = NULL;
>  		int cmp;
>
> +		if (iter)
> +		{
> +			if (prev_ref.len &&  strcmp(prev_ref.buf, iter->refname) > 0)

You have an extra two whitespaces after "&&" there.

> +			{
> +				strbuf_addf(err, "broken sorting in packed-refs: '%s' > '%s'",
> +					    prev_ref.buf,
> +					    iter->refname);
> +				goto error;
> +			}
> +
> +			strbuf_init(&prev_ref, 0);
> +			strbuf_addstr(&prev_ref, iter->refname);
> +		}
> +
>  		if (i >= updates->nr) {
>  			cmp = -1;
>  		} else {
> diff --git a/t/t3212-pack-refs-broken-sorting.sh b/t/t3212-pack-refs-broken-sorting.sh
> new file mode 100755
> index 0000000000..37a98a6fb1
> --- /dev/null
> +++ b/t/t3212-pack-refs-broken-sorting.sh
> @@ -0,0 +1,26 @@
> +#!/bin/sh
> +
> +test_description='tests for the falsely sorted refs'
> +. ./test-lib.sh
> +
> +test_expect_success 'setup' '
> +	git commit --allow-empty -m commit &&

Looks like just "test_commit A" would do here.

> +	for num in $(test_seq 10)
> +	do
> +		git branch b$(printf "%02d" $num) || break
> +	done &&

We can fail in these sorts of loops. There's a few ways to deal with
that. Doing it like this with "break" will still silently hide errors:

    $ for i in $(seq 1 3); do if test $i = 2; then false || break; else echo $i; fi; done && echo success
    1
    success

One way to deal with that is to e.g. before the loop say "had_fail=",
then set "had_fail=t" in that "||" case, and test for it after the loop.

But perhaps in this case we're better off e.g. running for-each-ref
after and either using test_cmp or test_line_count to see that we
created the refs successfully?

> +	git pack-refs --all &&
> +	head_object=$(git rev-parse HEAD) &&
> +	printf "$head_object refs/heads/b00\\n" >>.git/packed-refs &&

Looks like just "echo" here would be simpler since we only use printf to
add a newline.

> +	git branch b11
> +'
> +
> +test_expect_success 'off-order branch not found' '
> +	! git show-ref --verify --quiet refs/heads/b00
> +'
> +
> +test_expect_success 'subsequent pack-refs fails' '
> +	! git pack-refs --all
> +'

Instead of "! git ..." use "test_must_fail git ...". See t/README. This
will hide e.g. segfaults.

Also, perhaps:

    test_must_fail git ... 2>stderr &&
    grep "broken sorting in packed-refs" stderr

Would make this more obvious/self-documenting so we know we failed due
to that issue in particular.