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Schema digest is calculated by querying for mutations of all schema tables, then compacting them so that all tombstones in them are dropped. However, even if the mutation becomes empty after compaction, we still feed its partition key. If the same mutations were compacted prior to the query, because the tombstones expire, we won't get any mutation at all and won't feed the partition key. So schema digest will change once an empty partition of some schema table is compacted away. That's not a problem during normal cluster operation because the tombstones will expire at all nodes at the same time, and schema digest, although changes, will change to the same value on all nodes at about the same time. This fix changes digest calculation to not feed any digest for partitions which are empty after compaction. The digest returned by schema_mutations::digest() is left unchanged by this patch. It affects the table schema version calculation. It's not changed because the version is calculated on boot, where we don't yet know all the cluster features. It's possible to fix this but it's more complicated, so this patch defers that. Refs #4485. Asd