The shard readers of the multishard reader assumed that the positions in
the data stream are strictly monotonous. This assumption is invalid.
Range tombstones can have positions that they can share with other range
tombstones and/or a clustering row. The effect of this false assumption
was that when the shard reader was evicted such that the last seen
fragment was a range tombstone, when recreated it would skip any unseen
fragments that have the same position as that of the last seen range
tombstone.
Fixes: #4418
Branches: master, 3.0, 2019.1
Tests: unit(dev)
* https://github.com/denesb/scylla.git
multishard_reader_handle_non_strictly_monotonous_positions/v4:
multishard_combining_reader: shard_reader::remote_reader extract
fill-buffer logic into do_fill_buffer()
mutlishard_combining_reader: reorder
shard_reader::remote_reader::do_fill_buffer() code
position_in_partition_view: add region() accessor
multishard_combining_reader: fix handling of non-strictly monotonous
positions
flat_mutation_reader: add flat_mutation_reader_from_mutations()
overload with range and slice
flat_mutation_reader: add make_flat_mutation_reader_from_fragments()
overload with range and slice
tests: add unit test for multishard reader correctly handling
non-strictly monotonous positions
1adcb3637e