multishard_mutation_reader starts read-aheads on the
shards-to-be-read-soon. When doing this it didn't check whether the
respective shards had an ongoing read-ahead already. This lead to a
single shard executing multiple concurrent read-aheads. This is damaging
for multiple reasons:
* Can lead to concurrent access of the remote reader's data members.
* The `shard_reader` was designed around a single read-ahead and
thus will synchronise foreground reads with only the last one.
The practical implications of this seen so far was that queries reading
a large number of rows (large enough to reliably trigger the
bug) would stop the read early, due the `combined_mutation_reader`'s
internal accounting being messed up by concurrent access.
Also add a unit test. Instead of coming up with a very specific, and
very contrived unit test, use the test-case that detected this bug in
the first place: count(*) on a table with lots of rows (>1000). This
unit-test should serve well for detecting any similar bugs in the
future.
Signed-off-by: Botond Dénes <bdenes@scylladb.com>
Message-Id: <ff1c49be64e2fb443f9aa8c5c8d235e682442248.1536746388.git.bdenes@scylladb.com>
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