This is a revival of #7490.
Quoting #7490:
The managed_bytes class now uses implicit linearization: outside LSA, data is never fragmented, and within LSA, data is linearized on-demand, as long as the code is running within with_linearized_managed_bytes() scope.
We would like to stop linearizing managed_bytes and keep it fragmented at all times, since linearization can require large contiguous chunks. Large contiguous allocations are hard to satisfy and cause latency spikes.
As a first step towards that, we remove all implicitly linearizing accessors and replace them with an explicit linearization accessor, with_linearized().
Some of the linearization happens long before use, by creating a bytes_view of the managed_bytes object and passing it onwards, perhaps storing it for later use. This does not work with with_linearized(), which creates a temporary linearized view, and does not work towards the longer term goal of never linearizing. As a substitute a managed_bytes_view class is introduced that acts as a view for managed_bytes (for interoperability it can also be a view for bytes and is compatible with bytes_view).
By the end of the series, all linearizations are temporary, within the scope of a with_linearized() call and can be converted to fragmented consumption of the data at leisure.
This has limited practical value directly, as current uses of managed_bytes are limited to keys (which are limited to 64k). However, it enables converting the atomic_cell layer back to managed_bytes (so we can remove IMR) and the CQL layer to managed_bytes/managed_bytes_view, removing contiguous allocations from the coordinator.
Closes#7820
* github.com:scylladb/scylla:
test: add hashers_test
memtable: fix accounting of managed_bytes in partition_snapshot_accounter
test: add managed_bytes_test
utils: fragment_range: add a fragment iterator for FragmentedView
keys: update comments after changes and remove an unused method
mutation_test: use the correct preferred_max_contiguous_allocation in measuring_allocator
row_cache: more indentation fixes
utils: remove unused linearization facilities in `managed_bytes` class
misc: fix indentation
treewide: remove remaining `with_linearized_managed_bytes` uses
memtable, row_cache: remove `with_linearized_managed_bytes` uses
utils: managed_bytes: remove linearizing accessors
keys, compound: switch from bytes_view to managed_bytes_view
sstables: writer: add write_* helpers for managed_bytes_view
compound_compat: transition legacy_compound_view from bytes_view to managed_bytes_view
types: change equal() to accept managed_bytes_view
types: add parallel interfaces for managed_bytes_view
types: add to_managed_bytes(const sstring&)
serializer_impl: handle managed_bytes without linearizing
utils: managed_bytes: add managed_bytes_view::operator[]
utils: managed_bytes: introduce managed_bytes_view
utils: fragment_range: add serialization helpers for FragmentedMutableView
bytes: implement std::hash using appending_hash
utils: mutable_view: add substr()
utils: fragment_range: add compare_unsigned
utils: managed_bytes: make the constructors from bytes and bytes_view explicit
utils: managed_bytes: introduce with_linearized()
utils: managed_bytes: constrain with_linearized_managed_bytes()
utils: managed_bytes: avoid internal uses of managed_bytes::data()
utils: managed_bytes: extract do_linearize_pure()
thrift: do not depend on implicit conversion of keys to bytes_view
clustering_bounds_comparator: do not depend on implicit conversion of keys to bytes_view
cql3: expression: linearize get_value_from_mutation() eariler
bytes: add to_bytes(bytes)
cql3: expression: mark do_get_value() as static
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