The mapping between a base table update and a view update is schema
dependent, so we need to ensure the view schema versions match the
base schema version. For example, we match base columns to view
columns by name, so we need to ensure the base and view schemas we're
using for writting are isolated with respect to a previous alter
table statement.
We thus need to match base schema versions with view schema versions,
and we need to so atomically to ensure that when one fiber sees a
schema, it also sees the complete set of corresponding view schemas.
This series ensures the schemas modified as a result of an alter
table statement are published atomically, under the schema lock. This
way, all the schemas referenced by the database are consistent with
each other when they are observed by other fibers.
Finally, we upgrade the mutation schema before generating the view
updates, to ensure it matches the most recent view schemas the base
replica knows about, registered in the database.
The db::view::view class was replaced by a set of non-member
functions, with its state, which used to reflect only the most recent
schema version, being moved to a new view_info class.
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