From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Tue, 15 May 2018 22:23:59 +0300 From: Vladimir Davydov Subject: Re: [PATCH 1/4] vinyl: fix EQ check in run iterator Message-ID: <20180515192359.zvevt7wbswkgvat6@esperanza> References: <6028746ee469d1ba463c1e9740e27a3516ffc7f7.1526392563.git.vdavydov.dev@gmail.com> <20180515190512.GA24119@atlas> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180515190512.GA24119@atlas> To: Konstantin Osipov Cc: tarantool-patches@freelists.org List-ID: On Tue, May 15, 2018 at 10:05:12PM +0300, Konstantin Osipov wrote: > * Vladimir Davydov [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. Well, all those ITER_EQ checks are scattered throughout iterator code and none of them has a comment as they all are pretty self-explaining. Adding a comment to just one of them looks pointless IMO. Actually, I was thinking about extracting all the EQ checks out of source iterators and moving them to vy_read_iterator as this would allow to reduce the number of EQ comparisons, but I didn't get my hands on it as there were some problems with the cache iterator. May be, later. > > Ideally, this special case should be covered in a unit test, not > just in the code or in CS comment. After the last patch in the series is applied, vinyl/select_consistency functional test triggers this bug in 100% cases. Anyway, I'll think about a unit test. > > > 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? We do this extra tuple comparison only if cmp == 0 and the original iterator_type is ITER_EQ. The (cmp < 0) case is excluded, because we stop the iteration in this case for ITER_EQ - see several lines above: > cmp = vy_stmt_compare_with_key(key, slice->begin, cmp_def); > if (cmp < 0 && iterator_type == ITER_EQ) { > vy_run_iterator_stop(itr); > return 0; > } > if (cmp < 0 || (cmp == 0 && iterator_type != ITER_GT)) { > if (iterator_type == ITER_EQ) > check_eq_key = key; ^^^^^^^^^^^^^^^^^^ cmp can't be < 0 if this assignment takes place > iterator_type = ITER_GE; > key = slice->begin; > } Looks rather messy, but then again I think we should try to clean up this mess by extracting ITER_EQ check out of source iterators. Thanks.