[GitHub] tzulitai opened a new pull request #6875: [FLINK-9808] [state backends] Migrate state when necessary in state backends

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tzulitai opened a new pull request #6875: [FLINK-9808] [state backends] Migrate state when necessary in state backends
URL: https://github.com/apache/flink/pull/6875
 
 
   ## What is the purpose of the change
   
   This PR adds the procedure of migrating state (i.e., reading state bytes with the restored prior serializer, and then re-writing the deserialized object with the new serializer).
   
   This should only ever occur in the RocksDB state backend, since the restore / snapshotting of heap-based backends are already a migration process by nature.
   
   The requirement to perform a state migration is decided by the `CompatibilityResult` returned from compatibility checks against the new serializer using its prior serializer's snapshot.
   
   ## Brief change log
   
   - Add `migrateSerializedValue` to `AbstractRocksDBStateValue`
   - Use the new method to iterate through RocksDB bytes when state migration is necessary
   - Remove checks that are no longer relevant in the heap backends, since it is inherently already migrating state bytes as part of its restore / snapshot process
   - Add a `StateMigrationTestBase` that verifies serializers are being used as expected for a restore procedure
   
   ## Verifying this change
   
   The main new test coverage that covers this is in the `StateMigrationTestBase` class.
   
   ## Does this pull request potentially affect one of the following parts:
   
     - Dependencies (does it add or upgrade a dependency): (yes / **no**)
     - The public API, i.e., is any changed class annotated with `@Public(Evolving)`: (yes / **no**)
     - The serializers: (yes / **no** / don't know)
     - The runtime per-record code paths (performance sensitive): (**yes** / no / don't know)
     - Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (**yes** / no / don't know)
     - The S3 file system connector: (yes / **no** / don't know)
   
   ## Documentation
   
     - Does this pull request introduce a new feature? (**yes** / no)
     - If yes, how is the feature documented? (**not yet documented**)

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