The format is currently sitting in storage_service, but the
previous set patched all the users not to call it, instead
they use sstables_manager to get the highest supported format.
So this set finalizes this effort and places the format on
sstables_manager(s).
The set introduces the db::sstables_format_selector, that
- starts with the lowest format (ka)
- reads one on start from system tables
- subscribes on sstables-related features and bumps
up the selection if the respective feature is enabled
During its lifetime the selector holds a reference to the
sharded<database> and updates the format on it, the database,
in turn, propagates it further to sstables_managers. The
managers start with the highest known format (mc) which is
done for tests.
* https://github.com/xemul/scylla br-move-sstables-format-4:
storage_service: Get rid of one-line helpers
system_keyspace: Cleanup setup() from storage_service
format_selector: Log which format is being selected
sstables_manager: Keep format on
format_selector: Make it standalone
format_selector: Move the code into db/
format_selector: Select format locally
storage_service: Introduce format_selector
storage_service: Split feature_enabled_listener::on_enabled
storage_service: Tossing bits around
features: Introduce and use masked features
features: Get rid of per-features booleans
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