scylladb

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Tomasz Grabiec f97268d8f2 row_cache: Fix violation of the "oldest version are evicted first" when evicting last dummy

Consider the following MVCC state of a partition:

   v2: ==== <7> [entry2] ==== <9> ===== <last dummy>
   v1: ================================ <last dummy> [entry1]

Where === means a continuous range and --- means a discontinuous range.

After two LRU items are evicted (entry1 and entry2), we will end up with:

   v2: ---------------------- <9> ===== <last dummy>
   v1: ================================ <last dummy> [entry1]

This will cause readers to incorrectly think there are no rows before
entry <9>, because the range is continuous in v1, and continuity of a
snapshot is a union of continuous intervals in all versions. The
cursor will see the interval before <9> as continuous and the reader
will produce no rows.

This is only temporary, because current MVCC merging rules are such
that the flag on the latest entry wins, so we'll end up with this once
v1 is no longer needed:

   v2: ---------------------- <9> ===== <last dummy>

...and the reader will go to sstables to fetch the evicted rows before
entry <9>, as expected.

The bug is in rows_entry::on_evicted(), which treats the last dummy
entry in a special way, and doesn't evict it, and doesn't clear the
continuity by omission.

The situation is not easy to trigger because it requires certain
eviction pattern concurrent with multiple reads of the same partition
in different versions, so across memtable flushes.

Closes #12452

2023-01-09 16:10:52 +02:00