Re: [PATCH 1/4] vinyl: fix EQ check in run iterator

[Konstantin Osipov <kostja@tarantool.org>]

* Vladimir Davydov <vdavydov.dev@gmail.com> [18/05/15 17:10]:

I am pushing this as is now, please a couple of comments below.

> vy_run_iterator_seek() is supposed to check that the resulting statement
> matches the search key in case of ITER_EQ, but if the search key lies at
> the beginning of the slice, it doesn't. As a result, vy_point_lookup()
> may fail to find an existing tuple as demonstrated below.
> 
> Suppose we are looking for key {10} in the primary index which consists
> of an empty mem and two runs:
> 
>     run 1: DELETE{15}
>     run 2: INSERT{10}
> 
> vy_run_iterator_next() returns DELETE{15} for run 1 because of the
> missing EQ check and vy_point_lookup() stops at run 1 (since the
> terminal statement is found) and mistakenly returns NULL.
> 
> The issue manifests itself as crash in vinyl_iterator_secondary_next(),
> when we fail to find the tuple in the primary index corresponding to a
> statement found in a secondary index.

I believe this explanation belongs to the code, not only to the
changeset comment.

Ideally, this special case should be covered in a unit test, not
just in the code or in CS comment.

> Part of #3393
> ---
>  src/box/vy_run.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/src/box/vy_run.c b/src/box/vy_run.c
> index 587cb002..8c922895 100644
> --- a/src/box/vy_run.c
> +++ b/src/box/vy_run.c
> @@ -1316,6 +1316,7 @@ vy_run_iterator_seek(struct vy_run_iterator *itr,
>  {
>  	const struct key_def *cmp_def = itr->cmp_def;
>  	struct vy_slice *slice = itr->slice;
> +	const struct tuple *check_eq_key = NULL;
>  	int cmp;
>  
>  	if (slice->begin != NULL &&
> @@ -1340,6 +1341,8 @@ vy_run_iterator_seek(struct vy_run_iterator *itr,
>  			return 0;
>  		}
>  		if (cmp < 0 || (cmp == 0 && iterator_type != ITER_GT)) {
> +			if (iterator_type == ITER_EQ)
> +				check_eq_key = key;
>  			iterator_type = ITER_GE;
>  			key = slice->begin;
>  		}
> @@ -1365,7 +1368,15 @@ vy_run_iterator_seek(struct vy_run_iterator *itr,
>  		}
>  	}
>  
> -	return vy_run_iterator_do_seek(itr, iterator_type, key, ret);
> +	if (vy_run_iterator_do_seek(itr, iterator_type, key, ret) != 0)
> +		return -1;
> +
> +	if (check_eq_key != NULL && *ret != NULL &&
> +	    vy_stmt_compare(check_eq_key, *ret, cmp_def) != 0) {
> +		vy_run_iterator_stop(itr);
> +		*ret = NULL;
> +	}

As far as I understand the code flow, this adds an extra check for
cases when key != slice->begin. This is 99.9% of cases. Can we
avoid an extra check if it is not needed?

> +	return 0;
>  }

-- 
Konstantin Osipov, Moscow, Russia, +7 903 626 22 32
http://tarantool.io - www.twitter.com/kostja_osipov