Followup: ensure the LSM iterators are invalidated on exceptions

[ballard26]

A follow up to the previous PR on this matter. Goes through the remainder of the LSM iterators and ensures each of them are properly invalidated when an exception is thrown. See individual commits for details on each.

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v26.1.x
• v25.3.x
• v25.2.x

Release Notes

• none

[Copilot]

Pull request overview

Follow-up exception-safety hardening for LSM iterators: ensures valid() cannot remain true after an exception during a mutating iterator operation, preventing downstream use of stale iterator state.

Changes:

• Extracted a reusable throwing_iterator test utility and wired it into existing DB iterator tests.
• Updated two_level_iterator, merging_iterator, and the block reader iterator to invalidate state at the start of mutating operations and re-establish validity only on success.
• Added targeted exception-safety tests for two_level_iterator and merging_iterator.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/v/lsm/db/tests/iter_test.cc	Switches DB iterator exception-safety tests to shared throwing_iterator utility.
src/v/lsm/db/tests/BUILD	Adds dependency on the new throwing_iterator test library.
src/v/lsm/core/internal/two_level_iterator.cc	Adds explicit invalidation/revalidation so valid() can’t remain stale-true after exceptions.
src/v/lsm/core/internal/tests/two_level_iterator_test.cc	New tests validating two_level_iterator invalidates on exceptions across seek/next/prev paths.
src/v/lsm/core/internal/tests/throwing_iterator.h	New reusable throwing iterator for exception-path testing of higher-level iterators.
src/v/lsm/core/internal/tests/merging_iterator_test.cc	Adds exception-safety tests ensuring merging iterator becomes invalid after child exceptions.
src/v/lsm/core/internal/tests/BUILD	Adds throwing_iterator test library and new two_level_iterator_test target + deps.
src/v/lsm/core/internal/merging_iterator.cc	Invalidates _current early so valid() becomes false if an awaited child op throws.
src/v/lsm/block/reader.cc	Invalidates block iterator position during mutating operations so corruption exceptions don’t leave a stale-valid position.

Comments suppressed due to low confidence (1)

src/v/lsm/db/tests/iter_test.cc:21

• std::map is used later in this test file (e.g. in DBIteratorExceptionSafetyTest::SetUp), but <map> is no longer included here. The file currently relies on throwing_iterator.h to transitively include <map>, which is brittle if that header changes. Please add an explicit #include <map> in this file.

#include "lsm/core/internal/iterator.h"
#include "lsm/core/internal/keys.h"
#include "lsm/core/internal/options.h"
#include "lsm/core/internal/tests/throwing_iterator.h"
#include "lsm/db/iter.h"
#include "lsm/db/memtable.h"

#include <gtest/gtest.h>

#include <memory>
#include <stdexcept>
#include <string_view>

[vbotbuildovich]

CI test results

test results on build#83923

test_status	test_class	test_method	test_arguments	test_kind	job_url	passed	reason	test_history
FLAKY(PASS)	TxAtomicProduceConsumeTest	test_basic_tx_consumer_transform_produce	{"with_failures": true}	integration	https://buildkite.com/redpanda/redpanda/builds/83923#019de151-4bdd-44d9-991e-ca3ca86b8feb	10/11	Test PASSES after retries.No significant increase in flaky rate(baseline=0.0019, p0=1.0000, reject_threshold=0.0100. adj_baseline=0.1000, p1=0.3487, trust_threshold=0.5000)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=TxAtomicProduceConsumeTest&test_method=test_basic_tx_consumer_transform_produce

test results on build#83966

test_status	test_class	test_method	test_arguments	test_kind	job_url	passed	reason	test_history
FLAKY(PASS)	RedpandaNodeOperationsSmokeTest	test_node_ops_smoke_test	{"cloud_storage_type": 1, "mixed_versions": false}	integration	https://buildkite.com/redpanda/redpanda/builds/83966#019de6dc-8182-4cb3-90b1-1fe7ee640a81	10/11	Test PASSES after retries.No significant increase in flaky rate(baseline=0.0000, p0=1.0000, reject_threshold=0.0100. adj_baseline=0.1000, p1=0.3487, trust_threshold=0.5000)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=RedpandaNodeOperationsSmokeTest&test_method=test_node_ops_smoke_test

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (2)

src/v/lsm/db/tests/iter_test.cc:21

• iter_test.cc still uses std::map (e.g., in DBIteratorExceptionSafetyTest::SetUp) but the direct #include <map> was removed. Consider re-adding the explicit include to avoid relying on transitive includes from throwing_iterator.h.

#include <gtest/gtest.h>

#include <memory>
#include <stdexcept>
#include <string_view>

src/v/lsm/core/internal/tests/BUILD:111

• This BUILD file ends with multiple trailing blank lines. Running buildifier will typically remove these; consider trimming to a single trailing newline to avoid formatting-only diffs from automated tooling.