Preparing for #10459, this series defines sstables::generation_type::int_t
as `int64_t` at the moment and use that instead of naked `int64_t` variables
so it can be changed in the future to hold e.g. a `std::variant<int64_t, sstables::generation_id>`.
sstables::new_generation was defined to generation new, unique generations.
Currently it is based on incrementing a counter, but it can be extended in the future
to manufacture UUIDs.
The unit tests are cleaned up in this series to minimize their dependency on numeric generations.
Basically, they should be used for loading sstables with hard coded generation numbers stored under `test/resource/sstables`.
For all the rest, the tests should use existing and mechanisms introduced in this series such as generation_factory, sst_factory and smart make_sstable methods in sstable_test_env and table_for_tests to generate new sstables with a unique generation, and use the abstract sst->generation() method to get their generation if needed, without resorting the the actual value it may hold.
Closes#12994
* github.com:scylladb/scylladb:
everywhere: use sstables::generation_type
test: sstable_test_env: use make_new_generation
sstable_directory::components_lister::process: fixup indentation
sstables: make highest_generation_seen return optional generation
replica: table: add make_new_generation function
replica: table: move sstable generation related functions out of line
test: sstables: use generation_type::int_t
sstables: generation_type: define int_t
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