This doesn't introduce additional work for single-partition queries: the
key is copied anyway on consume_end_of_stream().
Multi-partition reads and compaction are not that sensitive to
additional copy added.
This change fixes a bug in the compacting_reader: currently the reader
passes _last_uncompacted_partition_start.key() to the compactor's
consume_new_partition(). When the compactor emits enough content for this
partition, _last_uncompacted_partition_start is moved from to emit the
partition start, this makes the key reference passed to the compaction
corrupt (refer to moved-from value). This in turn means that subsequent
GC checks done by the compactor will be done with a corrupt key and
therefore can result in tombstone being garbage-collected while they
still cover data elsewhere (data resurrection).
The compacting reader is violating the API contract and normally the bug
should be fixed there. We make an exception here because doing the fix
in the mutation compactor better aligns with our future plans:
* The fix simplifies the compactor (gets rid of _last_dk).
* Prepares the way to get rid of the consume API used by the compactor.
c2518cdf1a