JET-2518: Log keyed-state key and state name on RocksDB migration failures

[rafael-sotelo]

Summary

When a savepoint is restored with an evolved POJO schema, Flink runs state migration: RocksDBKeyedStateBackend.migrateStateValues iterates the raw RocksDB bytes, deserializes each value with the old serializer, and re-serializes with the new one. The migration loop never decodes the key (it copies iterator.key() verbatim) and never calls setCurrentKey, so when a value fails to deserialize, PojoSerializer's error log shows key=null with no state name — leaving no way to identify which keyed entry holds the corrupt/incompatible state.

This change decodes the composite key during migration and threads it (plus the state name) into DeserializationContext, so the existing PojoSerializer failure log now prints [state=<name>, key=<key>].

Changes

• In migrateStateValues, best-effort decode the composite key (key group + key) from each iterator entry using the existing CompositeKeySerializationUtils.
• Wrap the per-entry migrateSerializedValue call with DeserializationContext.set(key, stateName) / clear() — the same try/finally pattern used by RocksDBValueState.value().
• Key decoding is best-effort: a key that fails to decode leaves key=null and never masks the real value-migration failure.
• No change to migration behavior or the bytes written.

Effect

Failure logs change from Deserialization failure: ... to Deserialization failure [state=<stateName>, key=<realKey>]: ..., pinpointing the offending entry.

Verification

• flink-statebackend-rocksdb module compiles cleanly (./mvnw / Maven 3.8.6).

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✨ PR Description

Purpose: Add diagnostic logging of keyed-state keys and state names during RocksDB migration failures to improve error attribution in state deserialization.

Main changes:

• Import DeserializationContext and thread keyed-state key/name through migration loop for error diagnostics
• Wrap migrateSerializedValue call with best-effort key decoding and DeserializationContext population/cleanup
• Add PoisonV1TestTypeSerializer test utility and comprehensive migration failure test verifying context propagation

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✨ PR Review

LGTM

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flink-state-backends/flink-statebackend-rocksdb/src/main/java/org/apache/flink/contrib/streaming/state/RocksDBKeyedStateBackend.java

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flink-state-backends/flink-statebackend-rocksdb/src/test/java/org/apache/flink/contrib/streaming/state/RocksDBStateBackendMigrationTest.java

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