So that we can call cache_streamed_mutation::can_populate() before
we start reading from underlying. Will be needed in upcoming changes
which insert dummy entries when falling back to underlying.
database::make_sstable_reader() creates a reader which will need to
obtain a semaphore permit when invoked. Therefore, each read may
create at most one such reader in order to be guaranteed to make
progress. If the reader tries to create another reader, that may
deadlock (or for non-system tables, timeout), if enough number of such
readers tries to do the same thing at the same time.
Avoid the problem by dropping previous reader before creating a new
one.
Refs #2644.
Message-Id: <1501152454-4866-1-git-send-email-tgrabiec@scylladb.com>
The interaction will be as follows:
- Before creating cache_streamed_mutation for given partition, cache
mutation reader sets up read_context for current partition (in one
of two ways) so that the matching underlying streamed_mutation can
be accessed at any time by cached_stream_mutation.
- cache_streamed_mutation assumes that read_context is set up for
current partition and invokes fast_forward_to() and
get_next_fragment() to access the underlying
streamed_mutation.
When reading from incomplete partition entry, we may discover we need
to read something from the underlying mutation source. In such case we
will fast forward this reader to that partition. But we must do it
using a specific snapshot, the one we obtained when entering the
partition, not the latest one.
We will need to use this information later in yet another place, when
creating a reader for incomplete cache entry. This refactors the code
so that there is a single place which determines this fact.
This object stores all read relevant context required all
over the place. This leads to a cleaner code.
Signed-off-by: Piotr Jastrzebski <piotr@scylladb.com>
[tgrabiec:
- made read_context shareable to allow storing shared
mutable state later
- added range and cache getters
]
