This patch is a bag of fixes/cleanups that were omitted from the reader
memory tracking series due to contributor error. It contains the
following changes:
* Get rid of unused `increase()` and `decrease()` methods.
* Make all constructors and assignment operators `noexcept`.
* Make move assignment operator safe w.r.t. self assignment.
* `reset()`: consume the new amount before releasing the old amount,
to prevent a transient window where new readers might be admitted.
Signed-off-by: Botond Dénes <bdenes@scylladb.com>
Message-Id: <20200206143007.633069-1-bdenes@scylladb.com>
Previously `tracking_file_impl::make_tracked_buf()`. In the next patches
we plan on using this outside `tracking_file_impl`, so make it public
and templatize on the char type.
Similar to `seastar::semaphore_units`, this allows consuming and
releasing memory via an RAII object. In addition to that, it also allows
tracking changing values. This feature was designed to be used for
tracking the ever changing memory consumption of the buffers of
`flat_mutation_reader`:s.
This is now the only supported way of consuming memory from a permit.
In the next patches we will replace `reader_resource_tracker` and have
code use the `reader_permit` directly. In subsequent patches, the
`reader_permit` will get even more usages as we attempt to make the
tracking of reader resource more accurate by tracking more parts of it.
So the grand plan is that the current `reader_concurrency_semaphore.hh`
is split into two headers:
* `reader_concurrency_semaphore.hh` - containing the semaphore proper.
* `reader_permit.hh` - a very lightweight header, to be used by
components which only want to track various parts of the resource
consumption of reads.
