This is a refactoring patch in preparation for BTI indexes. It contains no functional changes (or at least it's not intended to).
In this patch, we modify the sstable readers to use index readers through a new virtual `abstract_index_readers` interface.
Later, we will add BTI indexes which will also implement this interface.
This interface contains the methods of `index_reader` which are needed by sstable readers, and leaves out all other methods, such as `current_clustered_cursor`.
Not all methods of this interface will be implementable by a trie-based index later. For example, a trie-based index can't provide a reliable `get_partition_key()`, because — unlike the current index — it only stores partition keys for partitions which have a row index. So the interface will have to be further restricted later. We don't do that in this patch because that will require changes to sstable reader logic, and this patch is supposed to only include cosmetic changes.
No backports needed, this is a preparation for new functionality.
Closesscylladb/scylladb#25000
* github.com:scylladb/scylladb:
sstables: add sstable::make_index_reader() and use where appropriate
sstables/mx: in readers, use abstract_index_reader instead of index_reader
sstables: in validate(), use abstract_index_reader instead of index_reader where possible
test/lib/index_reader_assertions: accept abstract_index_reader instead of index_reader
sstables/index_reader: introduce abstract_index_reader
sstables/index_reader: extract a prefetch_lower_bound() method
20693edb27