"When moving whole partition entries from memtable to cache, we move
snapshots as well. It is incorrect to evict from such snapshots
though, because associated readers would miss data.
Solution is to record evictability of partition version references (snapshots)
and avoiding eviction from non-evictable snapshots.
Could affect scanning reads, if the reader uses partition entry from
memtable, and the partition is too large to fit in reader's buffer,
and that entry gets moved to cache (was absent in cache), and then
gets evicted (memory pressure). The reader will not see the remainder
of that entry. Found during code review.
Introduced in ca8e3c4, so affects 2.1+
Fixes#3186.
Tests: unit (release)"
* 'tgrabiec/do-not-evict-memtable-snapshots' of github.com:tgrabiec/scylla:
tests: mvcc: Add test for eviction with non-evictable snapshots
mutation_partition: Define + operator on tombstones
tests: mvcc: Check that partition is fully discontinuous after eviction
tests: row_cache: Add test for memtable readers surviving flush and eviction
memtable: Make printable
mvcc: Take partition_entry by const ref in operator<<()
mvcc: Do not evict from non-evictable snapshots
mvcc: Drop unnecessary assignment to partition_snapshot::_version
tests: Use partition_entry::make_evictable() where appropriate
mvcc: Encapsulate construction of evictable entries
6ccd317c38