"
This series fixes the "LCS data-loss bug" where full scans (and
everything that uses them) would miss some small percentage (> 0.001%)
of the keys. This could easily lead to permanent data-loss as compaction
and decomission both use full scans.
aeffbb673 worked around this bug by disabling the incremental reader
selectors (the class identified as the source of the bug) altogether.
This series fixes the underlying issue and reverts aeffbb673.
The root cause of the bug is that the `incremental_reader_selector` uses
the current read position to poll for new readers using
`sstable_set::incremental_selector::select()`. This means that when the
currently open sstables contain no partitions that would intersect with
some of the yet unselected sstables, those sstables would be ignored.
Solve the problem by not calling `select()` with the current read
position and always pass the `next_position` returned in the previous
call. This means that the traversal of the sstable-set happens at a pace
defined by the sstable-set itself and this guarantees that no sstable
will be jumped over. When asked for new readers the
`incremental_reader_selector` will now iteratively call `select()` using
the `next_position` from the previous `select()` call until it either
receives some new, yet unselected sstables, or `next_position` surpasses
the read position (in which case `select()` will be tried again later).
The `sstable_set::incremental_selector` was not suitable in its present
state to support calling `select()` with the `next_position` from a
previous call as in some cases it could not make progress due to
inclusiveness related ambiguities. So in preparation to the above fix
`sstable_set` was updated to work in terms of ring-position instead of
tokens. Ring-position can express positions in a much more fine-grained
way then token, including positions after/before tokens and keys. This
allows for a clear expression of `next_position` such that calling
`select()` with it guarantees forward progress in the token-space.
Tests: unit(release, debug)
Refs: #3513
"
* 'leveled-missing-keys/v4' of https://github.com/denesb/scylla:
tests/mutation_reader_test: combined_mutation_reader_test: use SEASTAR_THREAD_TEST_CASE
tests/mutation_reader_test: refactor combined_mutation_reader_test
tests/mutation_reader_test: fix reader_selector related tests
Revert "database: stop using incremental selectors"
incremental_reader_selector: don't jump over sstables
mutation_reader: reader_selector: use ring_position instead of token
sstables_set::incremental_selector: use ring_position instead of token
compatible_ring_position: refactor to compatible_ring_position_view
dht::ring_position_view: use token_bound from ring_position
i_partitioner: add free function ring-position tri comparator
mutation_reader_merger::maybe_add_readers(): remove early return
mutation_reader_merger: get rid of _key
f4caa418ff