"
Row level repair, when using a local reader, is prone to deadlocking on
the streaming reader concurrency semaphore. This has been observed to
happen with at least two participating nodes, running more concurrent
repairs than the maximum allowed amount of reads by the concurrency
semaphore. In this situation, it is possible that two repair instances,
competing for the last available permits on both nodes, get a permit on
one of the nodes and get queued on the other one respectively. As
neither will let go of the permit it already acquired, nor give up
waiting on the failed-to-acquired permit, a deadlock happens.
To prevent this, we make the local repair reader evictable. For this we
reuse the already existing evictable reader mechanism of the multishard
combining reader. This patchset refactors this evictable reader
mechanism into a standalone flat mutation reader, then exposes it to the
outside world.
The repair reader is paused after the repair buffer is filled, which is
currently 32MB, so the cost of a possible reader recreation is amortized
over 32MB read.
The repair reader is said to be local, when it can use the shard-local
partitioner. This is the case if the participating nodes are homogenous
(their shard configuration is identical), that is the repair instance
has to read just from one shard. A non-local reader uses the multishard
reader, which already makes its shard readers evictable and hence is not
prone to the deadlock described here.
Fixes: #6272
Tests: unit(dev, release, debug)
"
* 'repair-row-level-evictable-local-reader/v3' of https://github.com/denesb/scylla:
repair: row_level: destroy reader on EOS or error
repair: row_level: use evictable_reader for local reads
mutation_reader: expose evictable_reader
mutation_reader: evictable_reader: add auto_pause flag
mutation_reader: make evictable_reader a flat_mutation_reader
mutation_reader: s/inactive_shard_read/inactive_evictable_reader/
mutation_reader: move inactive_shard_reader code up
mutation_reader: fix indentation
mutation_reader: shard_reader: extract remote_reader as evictable_reader
mutation_reader: reader_lifecycle_policy: make semaphore() available early
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