21d9f54a9a
In partition_snapshot_row_cursor::maybe_refresh(), the !is_in_latest_version()
path calls lower_bound(_position) on the latest version's rows to find the
cursor's position in that version. When lower_bound returns null (the cursor
is positioned above all entries in the latest version in table order), the code
unconditionally sets _background_continuity = true and allows the subsequent
if(!it) block to erase the latest version's entry from the heap.
This is correct for forward traversal: null means there are no more entries
ahead, so removing the version from the heap is safe.
However, in reversed mode, null from lower_bound means the cursor is above
all entries in table order -- those entries are BELOW the cursor in query
order and will be visited LATER during reversed traversal. Erasing the heap
entry permanently loses them, causing live rows to be skipped.
The fix mirrors what prepare_heap() already does correctly: when lower_bound
returns null in reversed mode, use std::prev(rows.end()) to keep the last
entry in the heap instead of erasing it.
Add test_reversed_maybe_refresh_keeps_latest_version_entry to mvcc_test,
alongside the existing reversed cursor tests. The test creates a two-version
partition snapshot (v0 with range tombstones, v1 with a live row positioned
below all v0 entries in table order), and
traverses in reverse calling maybe_refresh() at each step -- directly
exercising the buggy code path. The test fails without the fix.
The bug was introduced by 6b7473be53 ("Handle non-evictable snapshots",
2022-11-21), which added null-iterator handling for non-evictable snapshots
(memtable snapshots lack the trailing dummy entry that evictable snapshots
have). prepare_heap() got correct reversed-mode handling at that time, but
maybe_refresh() received only forward-mode logic.
The bug is intermittent because multiple mechanisms cause iterators_valid()
to return false, forcing maybe_refresh() to take the full rebuild path via
prepare_heap() (which handles reversed mode correctly):
- Mutation cleaner merging versions in the background (changes change_mark)
- LSA segment compaction during reserve() (invalidates references)
- B-tree rebalancing on partition insertion (invalidates references)
- Debug mode's always-true need_preempt() creating many multi-version
partitions via preempted apply_monotonically()
A dtest reproducer confirmed the same root cause: with 100K overlapping range
tombstones creating a massively multi-version memtable partition (287K preemption
events), the reversed scan's latest_iterator was observed jumping discontinuously
during a version transition -- the latest version's heap entry was erased --
causing the query to walk the entire partition without finding the live row.
Fixes: SCYLLADB-1253
Closes scylladb/scylladb#29368
Scylla unit tests using C++ and the Boost test framework
The source files in this directory are Scylla unit tests written in C++ using the Boost.Test framework. These unit tests come in three flavors:
-
Some simple tests that check stand-alone C++ functions or classes use Boost's
BOOST_AUTO_TEST_CASE. -
Some tests require Seastar features, and need to be declared with Seastar's extensions to Boost.Test, namely
SEASTAR_TEST_CASE. -
Even more elaborate tests require not just a functioning Seastar environment but also a complete (or partial) Scylla environment. Those tests use the
do_with_cql_env()ordo_with_cql_env_thread()function to set up a mostly-functioning environment behaving like a single-node Scylla, in which the test can run.
While we have many tests of the third flavor, writing new tests of this type should be reserved to white box tests - tests where it is necessary to inspect or control Scylla internals that do not have user-facing APIs such as CQL. In contrast, black-box tests - tests that can be written only using user-facing APIs, should be written in one of newer test frameworks that we offer - such as test/cqlpy or test/alternator (in Python, using the CQL or DynamoDB APIs respectively) or test/cql (using textual CQL commands), or - if more than one Scylla node is needed for a test - using the test/topology* framework.
Running tests
Because these are C++ tests, they need to be compiled before running.
To compile a single test executable row_cache_test, use a command like
ninja build/dev/test/boost/row_cache_test
You can also use ninja dev-test to build all C++ tests, or use
ninja deb-build to build the C++ tests and also the full Scylla executable
(however, note that full Scylla executable isn't needed to run Boost tests).
Replace "dev" by "debug" or "release" in the examples above and below to use the "debug" build mode (which, importantly, compiles the test with ASAN and UBSAN enabling on and helps catch difficult-to-catch use-after-free bugs) or the "release" build mode (optimized for run speed).
To run an entire test file row_cache_test, including all its test
functions, use a command like:
build/dev/test/boost/row_cache_test -- -c1 -m1G
to run a single test function test_reproduce_18045() from the longer test
file, use a command like:
build/dev/test/boost/row_cache_test -t test_reproduce_18045 -- -c1 -m1G
In these command lines, the parameters before the -- are passed to
Boost.Test, while the parameters after the -- are passed to the test code,
and in particular to Seastar. In this example Seastar is asked to run on one
CPU (-c1) and use 1G of memory (-m1G) instead of hogging the entire
machine. The Boost.Test option -t test_reproduce_18045 asks it to run just
this one test function instead of all the test functions in the executable.
Unfortunately, interrupting a running test with control-C while doesn't
work. This is a known bug (#5696). Kill a test with SIGKILL (-9) if you
need to kill it while it's running.
Boost tests can also be run using test.py - which is a script that provides a uniform way to run all tests in scylladb.git - C++ tests, Python tests, etc.
Execution with pytest
To run all tests with pytest execute
pytest test/boost
To execute all tests in one file, provide the path to the source filename as a parameter
pytest test/boost/aggregate_fcts_test.cc
Since it's a normal path, autocompletion works in the terminal out of the box.
To execute only one test function, provide the path to the source file and function name
pytest --mode dev test/boost/aggregate_fcts_test.cc::test_aggregate_avg
To provide a specific mode, use the next parameter --mode dev,
if parameter isn't provided pytest tries to use ninja mode_list to find out the compiled modes.
Parallel execution is controlled by pytest-xdist and the parameter -n auto.
This command starts tests with the number of workers equal to CPU cores.
The useful command to discover the tests in the file or directory is
pytest --collect-only -q --mode dev test/boost/aggregate_fcts_test.cc
That will return all test functions in the file.
To execute only one function from the test, you can invoke the output from the previous command.
However, suffix for mode should be skipped.
For example,
output shows in the terminal something like this test/boost/aggregate_fcts_test.cc::test_aggregate_avg.dev.
So to execute this specific test function, please use the next command
pytest --mode dev test/boost/aggregate_fcts_test.cc::test_aggregate_avg
Writing tests
Because of the large build time and build size of each separate test executable, it is recommended to put test functions into relatively large source files. But not too large - to keep compilation time of a single source file (during development) at reasonable levels.
When adding new source files in test/boost, don't forget to list the new source file in configure.py and also in CMakeLists.txt. The former is needed by our CI, but the latter is preferred by some developers.