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migrator: allow removing settings in migrations #644

Merged
merged 4 commits into from
Feb 1, 2020
Merged

migrator: allow removing settings in migrations #644

merged 4 commits into from
Feb 1, 2020

Commits on Feb 1, 2020

  1. migrator: allow removing settings in migrations 

    Migrations weren't able to remove settings because we started with a copy of
the existing data store and wrote out any changes key-by-key, meaning a
"removal" would just leave the old copied key in place on disk.

With this change, we no longer copy the data store, we instead give migrations
the source data store and the target data store, and (via migration-helpers)
the input data is read from the source instead of a copy.  This is more
efficient because it avoids a needless copy, and allows the migration to
determine exactly what's left after it runs.
    @tjkirch
    tjkirch committed Feb 1, 2020
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  2. apiserver: make MemoryDataStore public 

    This allows for DataStore-based testing in other crates.
    @tjkirch
    tjkirch committed Feb 1, 2020
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  3. apiserver: add DataStore methods to unset keys and metadata 

    We've gotten by without this because settings are effectively all required, but
there are now some advanced cases (migrator interface change) that require
removing keys from an existing data store.  This probably shouldn't be exposed
in a higher-level API, though, unless we account for optional settings.
    @tjkirch
    tjkirch committed Feb 1, 2020
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  4. migration-helpers: back-compat: remove datastore keys removed in migr… 

    …ation

The old migrator interface used a single data store as source and target.
After a migration changes the MigrationData, the resulting values are written
out to the data store.  Since this data store also served as the source, it had
all of the old keys on disk already, and nothing explicitly removed them; we'd
only see new keys overwrite old.

To allow downgrading to versions with the old migrator interface, we need
migrations to explicitly remove keys from the data store.  This change to
migration-helpers detects keys that were removed from MigrationData by a
migration and tells the data store to explicitly remove them.  As long as
migrations are rebuilt against this library, they'll work in the old or new
migrator.

This workaround can be removed when we no longer support versions with the old
migrator interface.
    @tjkirch
    tjkirch committed Feb 1, 2020
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