MDEV-39263 innodb_snapshot_isolation fails to prevent lost updates under contention#4936
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MDEV-39263 innodb_snapshot_isolation fails to prevent lost updates under contention#4936
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dr-m
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Apr 14, 2026
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+6349
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+6352
| if (UNIV_UNLIKELY(trx_id != 0) | ||
| && !trx_sys.is_registered(trx, trx_id)) { | ||
| err = DB_RECORD_CHANGED; | ||
| } |
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It turns out that we will need an additional condition err <= DB_SUCCESS_LOCKED_REC here. Otherwise, the mtr test in MDEV-39264 would crash a CMAKE_BUILD_TYPE=RelWithDebInfo build on shutdown:
2026-04-14 10:01:23 0 [Note] InnoDB: Buffer pool(s) dump completed at 260414 10:01:23
2026-04-14 10:01:23 0x7fc6340517c0 InnoDB: Assertion failure in file /mariadb/10.6/storage/innobase/trx/trx0trx.cc line 219
InnoDB: Failing assertion: trx->lock.wait_lock == NULL
I will revise this shortly.
…der contention lock_clust_rec_read_check_and_lock(): Recheck the read view if trx_sys.is_registered() indicates that the lock owner is active. Thanks to Vadim Tkachenko of Percona for reporting this. His test case was simplified by me and the root cause analyzed with https://rr-project.org and an additional patch that allows the race condition to be repeated under "rr record". Most likely, the patch to trx_t::commit_persist() is the essential one: diff --git a/storage/innobase/btr/btr0pcur.cc b/storage/innobase/btr/btr0pcur.cc index c7fcfd2..b549b6e12df 100644 --- a/storage/innobase/btr/btr0pcur.cc +++ b/storage/innobase/btr/btr0pcur.cc @@ -317,7 +317,7 @@ btr_pcur_t::restore_position(btr_latch_mode restore_latch_mode, mtr_t *mtr) page_cur_mode_t mode; page_cur_mode_t old_mode; mem_heap_t* heap; - + std::this_thread::yield(); ut_ad(mtr->is_active()); ut_ad(pos_state == BTR_PCUR_WAS_POSITIONED || pos_state == BTR_PCUR_IS_POSITIONED); diff --git a/storage/innobase/trx/trx0rec.cc b/storage/innobase/trx/trx0rec.cc index 5d854eb..d0c442a0f1f 100644 --- a/storage/innobase/trx/trx0rec.cc +++ b/storage/innobase/trx/trx0rec.cc @@ -1841,7 +1841,7 @@ trx_undo_report_row_operation( #ifdef UNIV_DEBUG int loop_count = 0; #endif /* UNIV_DEBUG */ - + std::this_thread::yield(); ut_a(dict_index_is_clust(index)); ut_ad(!update || rec); ut_ad(!rec || rec_offs_validate(rec, index, offsets)); diff --git a/storage/innobase/trx/trx0trx.cc b/storage/innobase/trx/trx0trx.cc index e2adc08..adc807e5943 100644 --- a/storage/innobase/trx/trx0trx.cc +++ b/storage/innobase/trx/trx0trx.cc @@ -1578,7 +1578,7 @@ void trx_t::commit_persist() noexcept { mtr_t *mtr= nullptr; mtr_t local_mtr; - + std::this_thread::yield(); if (has_logged_persistent()) { mtr= &local_mtr;
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Description
An
UPDATEcould fail to return the errorER_CHECKREADwheninnodb_snapshot_isolation=ON.Thanks to @vadimtk of Percona for reporting this.
lock_clust_rec_read_check_and_lock(): Recheck the read view iftrx_sys.is_registered()indicates that the lock owner is active.How can this PR be tested?
mysql-test/mtr --parallel=auto --repeat=1000 innodb.snapshot_isolation_race{,,,}{,,,,,}The failure of this test case, as simplified by me, was analyzed with https://rr-project.org and an additional patch that allows the race conditions to occur during the
rr recordtracing. Most likely, the patch totrx_t::commit_persist()is the essential one:Basing the PR against the correct MariaDB version
mainbranch.