Currently, partition snapshot destructor can throw which is a big no-no.
The solution is to ignore the exception and leave versions unmerged and
hope that subsequent reads will succeed at merging.
However, another problem is that the merge doesn't use allocating
sections which means that memory won't be reclaimed to satisfy its
needs. If the cache is full this may result in partition versions not
being merged for a very long time.
This patch introduces partition_snapshot::merge_partition_versions()
which contains all the version merging logic that was previously present
in the snapshot destructor. This function may throw so that it can be
used with allocating sections.
The actual merging and handling of potential erros is done from
partition_snapshot_reader destructor. It tries to merge versions under
the allocating section. Only if that fails it gives up and leaves them
unmerged.
Fixes#1578Fixes#1579.
Signed-off-by: Paweł Dziepak <pdziepak@scylladb.com>
Message-Id: <1471265544-23579-1-git-send-email-pdziepak@scylladb.com>
To ensure isolation of operation when streaming a mutation from a
mutable source (such as cache or memtable) MVCC is used.
Each entry in memtable or cache is actually a list of used versions of
that entry. Incoming writes are either applied directly to the last
verion (if it wasn't being read by anyone) or preprended to the list
(if the former head was being read by someone). When reader finishes it
tries to squash versions together provided there is no other reader that
could prevent this.
Signed-off-by: Paweł Dziepak <pdziepak@scylladb.com>
