rwlock was added to protect iterations against concurrent updates to the map.
the updates can happen when allocating a new tablet replica or removing an old one (tablet cleanup).
the rwlock is very problematic because it can result in topology changes blocked, as updating
token metadata takes the exclusive lock, which is serialized with table wide ops like
split / major / explicit flush (and those can take a long time).
to get rid of the lock, we can copy the storage group map and guard individual groups with a gate
(not a problem since map is expected to have a maximum of ~100 elements).
so cleanup can close that gate (carefully closed after stopping individual groups such that
migrations aren't blocked by long-running ops like major), and ongoing iterations (e.g. triggered
by nodetool flush) can skip a group that was closed, as such a group is being migrated out.
Check documentation added to compaction_group.hh to understand how
concurrent iterations and updates to the map work without the rwlock.
Yielding variants that iterate over groups are no longer returning group
id since id stability can no longer be guaranteed without serializing split
finalization and iteration.
Fixes#18821.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
(cherry picked from commit c539b7c861)
31451ec2a0