Currently, replica::tablet_map_to_mutation generates a mutation having a row per tablet.
With enough tablets (10s of thousands) in the table we observe reactor stalls when freezing / unfreezing such large mutations, as seen in https://github.com/scylladb/scylladb/pull/18095#issuecomment-2029246954, and I assume we would see similar stalls also when converting those mutation into canonical_mutation and back, as they are similar to frozen_mutation, and bit more expensive since they also save the column mappings.
This series takes a different approach than allowing freeze to yield.
`tablet_map_to_mutation` is changed to `tablet_map_to_mutations`, able to generate multiple split mutations, that when squashed together are equivalent to the previously large mutation. Those mutations are fed into a `process_mutation` callback function, provided by the caller, which may add those mutation to a vector for further processing, and/or process them inline by freezing or making a canonical mutation.
In addition, split the large mutations would also prevent hitting the commitlog maximum mutation size.
Closesscylladb/scylladb#18162
* github.com:scylladb/scylladb:
schema_tables: convert_schema_to_mutations: simplify check for system keyspace
tablets: read_tablet_mutations: use unfreeze_and_split_gently
storage_service: merge_topology_snapshot: freeze snp.mutations gently
mutation: async_utils: add unfreeze_and_split_gently
mutation: add for_each_split_mutation
tablets: tablet_map_to_mutations: maybe split tablets mutation
tablets: tablet_map_to_mutations: accept process_func
perf-tablets: change default tables and tablets-per-table
perf-tablets: abort on unhandled exception
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