Those methods first create a neutral mutation_partition, and left-fold
it with the versions. The problem is that there is no neutral element
for static row continuity, the flag from the first addend always
wins. We have to copy the flag from the first version to preserve
the logical value.
Signed-off-by: Piotr Jastrzebski <piotr@scylladb.com>
[tgrabiec:
- extracted from a larger commit
- fix heap comparator in apply_incomplete_target to order versions properly
- extracted partition_version detaching into
partition_entry::with_detached_versions()
- dropped unnecessary rows_iterator::_version field
- dropped unnecessary allocation of rows_entry and key copies
in rows_iterator
- dropped row_pointer
- replaced apply_reversibly() with weaker and faster apply()
- added handling of dummy entries at any position
- fixed exception safety issue in apply_to_incomplete() which may
result in data loss. We cannot move data out of applied versions
into a new synthetic row and then apply it, because if exception
happens in the middle, the data which was moved from the source
will be lost. To fix that, row_iterator::consume_row() is
introduced which allows in-place consumption of data without
construction of temporary deletable_row.
]
This streamed mutation populates cache with
the rows requested by the read. It takes whatever
it can find in the cache and fetches the remainings
from underlying source.
Signed-off-by: Piotr Jastrzebski <piotr@scylladb.com>
[tgrabiec:
- fixed maybe_add_to_cache_and_update_continuity() leaking entries if
the key already exists in the snapshot
- fixed a problem where population race could result in a read
missing some rows, because cache_streamed_mutation was advancing
the cursor, then deferring, and then checking continuity. We
should check continuity atomically with advancing.
- fixed rows_handle.maybe_refresh() being accessed outside of update
section in read_from_underlying() (undefined behavior)
- fixed a problem in start_reading_from_underlying() where we would
use incorrect start if lower_bound ended with a range tombstone
starting before a key.
- range tombstone trimming in add_to_buffer() could create a
tombstone which has too low start bound if last_rt.end was a
prefix and had inclusive end. invert_kind(end_kind) should be used
instead of unconditional inc_start.
- range tombstone trimming incorrectly assumed it is fine to trim
the tombstone from underlying to the previous fragment's end and
emit such tombstone. That would mean the stream can't emit any
fragments which start before previous tombstone's end. Solve with
range_tombstone_stream.
- split add_to_buffer() into overloads for clustering_row, and
range_tombstone. Better than wrapping into mutation_fragment
before the call and having add_to_buffer() rediscover the
information.
- changed maybe_add_to_cache_and_update_continuity() to not set
continuity to false for existing entries, it's not necessary
- moved range tombstone trimming to range_tombstone class
- moved range tombstone slicing code to range_tombstone_list and partition_snapshot
- can_populate::can_use_cache was unused, dropped
- dropped assumption that dummy entries are only at the end
- renamed maybe_add_to_cache_and_update_continuity() to maybe_add_to_cache()
- dropped no longer needed lower_bound class
- extracted row_handle to a seaparate patch
- made the copy-from-cache loop preemptable
- split maybe_add_next_to_buffer_and_update_continuity(bool)
- dropped cache_populator
- replaced "underlying" class with use of read_context
- replaced can_populate class with a function
- simplified lsa_manager methods to avoid moves
]
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.
Currently scanning_and_populating_reader asks
just_cache_scanning_reader for the next partition from cache, together
with information if the range is continuous. If it's not, it saves the
partition it got from it and moves on to reading from the underlying
reader up to that partition. When that's done, it emits the stored
partition.
This approach won't work well with upcoming changes for storing
partial partitions. We won't have whole partitions any more, so
streamed_mutation returned for the entry needs to be prepared for
reading from the underlying mutation source. We want to reuse the same
underlying reader as much as possible, so all streamed_mutations for
given read (read_context) will share the state of the underlying
reader. Construction of a streamed_mutation will depend on the fact
that the shared state is set up for it, so we cannot have two
streamed_mutations prepared at the same time (one for entry from
primary, and one for the earlier entry being populated). This change
defers the creation of a streamed_mutation for the entry present in
cache until the whole reader reaches it to avoid this problem.
This will also have antoher potentially beneficial effect. Since we
defer the decision about which snapshot to use until we reach the
entry, there is a higher chance that the current snapshot of the entry
will match the one used last by the populating read, and that we will
be able to reuse the reader.
It's implemented by utilizing a stable partition cursor which tracks
its current position so that it's possible to revisit the cache entry
(if it's still there) after population ends. The functionality of
just_cache_scanning_reader was inlined into
scanning_and_populating_reader.
dht::token needs to be stored as a pointer now and not a reference so
that validity of old pointers is not impacted by in-place object
construction which would occur in the copy-assignment operator.
[1] says that old pointers can be used to access the new object only
if the type "does not contain any non-static data member whose type is
const-qualified or a reference type".
[1] http://en.cppreference.com/w/cpp/language/lifetime#Storage_reuse
It's needed before switching cache_entry ordering to rely solely on
cache_entry::position() so that invalidate_unwrapped() never removes
the dummy entry at the end. Currently if the range has upper bound
like this:
{ ring_position::max(), inclusive=true }
The code which selects entries for removal would include the dummy row
at the end. It uses upper_bound() to get the end iterator, and the
dummy entry has a position which is equal to the position in the
bound.
ring_position_view ranges are end-exclusive, so it's impossible to
create a partition range which would include a dummy entry.
The code is also simpler.
This will allow conversion from streamed_mutation that
supports fast forwarding to streamed_mutation that does not.
Signed-off-by: Piotr Jastrzebski <piotr@scylladb.com>
