This one-line patch fixes a failure in the dtest
lwt_schema_modification_test.py::TestLWTSchemaModification
::test_table_alter_delete
Where an update sometimes failed due to an internal server error, and the
log had the mysterious warning message:
"std::logic_error (Empty materialized view updated)"
We've also seen this log-message in the past in another user's log, and
never understood what it meant.
It turns out that the error message was generated (and warning printed)
while building view updates for a base-table mutation, and noticing that
the base mutation contains an *empty* row - a row with no cells or
tombstone or anything whatsoever. This case was deemed (8 years ago,
in d5a61a8c48) unexpected and nonsensical,
and we threw an exception. But this case actually *can* happen - here is
how it happened in test_table_alter_delete - which is a test involving
a strange combination of materialized views, LWT and schema changes:
1. A table has a materialized view, and also a regular column "int_col".
2. A background thread repeatedly drops and re-creates this column
int_col.
3. Another thread deletes rows with LWT ("IF EXISTS").
4. These LWT operations each reads the existing row, and because of
repeated drop-and-recreate of the "int_col" column, sometimes this
read notices that one node has a value for int_col and the other
doesn't, and creates a read-repair mutation setting int_col (the
difference between the two reads includes just in this column).
5. The node missing "int_col" receives this mutation which sets only
int_col. It upgrade()s this mutation to its most recent schema,
which doesn't have int_col, so it removes this column from the
mutation row - and is left with a completely empty mutation row.
This completely empty row is not useful, but upgrade() doesn't
remove it.
6. The view-update generation code sees this empty base-mutation row
and fails it with this std::logic_error.
7. The node which sent the read-repair mutation sees that the read
repair failed, so it fails the read and therefore fails the LWT
delete operation.
It is this LWT operation which failed in the test, and caused
the whole test to fail.
The fix is trivial: an empty base-table row mutation should simply be
*ignored* when generating view updates - it shouldn't cause any error.
Before this patch, test_table_alter_delete used to fail in roughly
20% of the runs on my laptop. After this patch, I ran it 100 times
without a single failure.
Fixes#15228Fixes#17549
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Closesscylladb/scylladb#17607
19bcea6216