The test dtest materialized_views_test.py::TestMaterializedViews::
test_mv_populating_from_existing_data_during_truncate reproduces an
assertion failure, and crash, while doing a CREATE MATERIALIZED VIEW
during a TRUNCATE operation.
This patch fixes the crash by removing the assert() call for a view
(replacing it by a warning message) - we'll explain below why this is fine.
Also for base tables change we change the assertion to an on_internal_error
(Refs #7871).
This makes the test stop crashing Scylla, but it still fails due to
issue #17635.
Let's explain the crash, and the fix:
The test starts TRUNCATE on table that doesn't yet have a view.
truncate_table_on_all_shards() begins by disabling compaction on
the table and all its views (of which there are none, at this
point). At this point, the test creates a new view is on this table.
The new view has, by default, compaction enabled. Later, TRUNCATE
calls discard_sstables() on this new view, asserts that it has
compaction disabled - and this assertion fails.
The fix in this patch is to not do the assert() for views. In other words,
we acknowledge that in this use case, the view *will* have compactions
enabled while being truncated. I claim that this is "good enough", if we
remember *why* we disable compaction in the first place: It's important
to disable compaction while truncating because truncating during compaction
can lead us to data resurection when the old sstable is deleted during
truncation but the result of the compaction is written back. True,
this can now happen in a new view (a view created *DURING* the
truncation). But I claim that worse things can happen for this
new view: Notably, we may truncate a view and then the ongoing
view building (which happens in a new view) might copy data from
the base to the view and only then truncate the base - ending up
with an empty base and non-empty view. This problem - issue #17635 -
is more likely, and more serious, than the compaction problem, so
will need to be solved in a separate patch.
Fixes#17543.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Closesscylladb/scylladb#17634
8df2ea3f95