Currently all reader lifecycle policy implementations assume that
`semaphore()` will only be called after at least one call to
`make_reader()`. This assumption will soon not hold, so make sure
`semaphore()` can be called at any time, including before any calls are
made to `make_reader()`.
And use the reader_permit for this instead. This refactoring has
revealed a pre-existing bug in the `test_lifecycle_policy`, which is
also addressed in this patch. The bug is that said policy executes
reader destructions in the background, and these are not waited for. For
some reason, the semaphore -> permit transition pushes these races over
the edge and we start seeing some of these destruction fibers still
being unfinished when test scopes are exited, causing all sorts of
trouble. The solution is to introduce a special gate that tests can use
to wait for all background work to finish, before the test scope is
exited.
Detach test_multishard_combining_reader_as_mutation_source into
individual file.
This particular test runs ~13 minutes. What's left in the origin
completes a bit faster.
The split also requires moving the reader_lifecycle_policy and
the dummy_partitioner into lib/
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
