This adaptor adapts a mutation reader pausable consumer to the frozen
mutation visitor interface. The pausable consumer protocol allows the
consumer to skip the remaining parts of the partition and resume the
consumption with the next one. To do this, the consumer just has to
return stop_iteration::yes from one of the consume() overloads for
clustering elements, then return stop_iteration::no from
consume_end_of_partition(). Due to a bug in the adaptor, this sequence
leads to terminating the consumption completely -- so any remaining
partitions are also skipped.
This protocol implementation bug has user-visible effects, when the
only user of the adaptor -- read repair -- happens during a query which
has limitations on the amount of content in each partition.
There are two such queries: select distinct ... and select ... with
partition limit. When converting the repaired mutation to to query
result, these queries will trigger the skip sequence in the consumer and
due to the above described bug, will skip the remaining partitions in
the results, omitting these from the final query result.
This patch fixes the protocol bug, the return value of the underlying
consumer's consume_end_of_partition() is now respected.
A unit test is also added which reproduces the problem both with select
distinct ... and select ... per partition limit.
Follow-up work:
* frozen_mutation_consumer_adaptor::on_end_of_partition() calls the
underlying consumer's on_end_of_stream(), so when consuming multiple
frozen mutations, the underlying's on_end_of_stream() is called for
each partition. This is incorrect but benign.
* Improve documentation of mutation_reader::consume_pausable().
Fixes: #20084Closesscylladb/scylladb#23657
(cherry picked from commit d67202972a)
Closesscylladb/scylladb#23694
251db77fcb