[jira] [Created] (FLINK-11591) Restoring LockableTypeSerializer's snapshot from 1.6 and below requires pre-processing before snapshot is valid to use

[Shang Yuanchun (Jira)]

Tzu-Li (Gordon) Tai created FLINK-11591:
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             Summary: Restoring LockableTypeSerializer's snapshot from 1.6 and below requires pre-processing before snapshot is valid to use
                 Key: FLINK-11591
                 URL: https://issues.apache.org/jira/browse/FLINK-11591
             Project: Flink
          Issue Type: Bug
          Components: CEP, Type Serialization System
            Reporter: Tzu-Li (Gordon) Tai
            Assignee: Igal Shilman


In 1.6 and below, the {{LockableTypeSerializer}} incorrectly returns directly the element serializer's snapshot instead of wrapping it within an independent snapshot class:
https://github.com/apache/flink/blob/release-1.6/flink-libraries/flink-cep/src/main/java/org/apache/flink/cep/nfa/sharedbuffer/Lockable.java#L188

This results in the fact that the the written state information for this would be {{(LockableTypeSerializer, SomeArbitrarySnapshot)}}.

The problem occurs when restoring this in Flink 1.7+, since compatibility checks are now performed by providing the new serializer to the snapshot, what would happen is:
{{SomeArbitrarySnapshot.resolveSchemaCompatibility(newLockableTypeSerializer)}}, which would not work since the arbitrary snapshot does not recognize the {{LockableTypeSerializer}}.

To fix this, we essentially need to preprocess that arbitrary snapshot when restoring from <= 1.6 version snapshots.

A proposed fix would be to have the following interface:
{code}
public interface RequiresLegacySerializerSnapshotPreprocessing<T> {
    TypeSerializerSnapshot<T> preprocessLegacySerializerSnapshot(TypeSerializerSnapshot<T> legacySnapshot)
}
{code}

The {{LockableTypeSerializer}} would then implement this interface, and in the {{preprocessLegacySerializerSnapshot}} method, properly wrap that arbitrary element serializer snapshot into a {{LockableTypeSerializerSnapshot}}.

In general, this interface is useful to preprocess any problematic snapshot that was returned pre 1.7.

The point-in-time to check if a written serializer in <= 1.6 savepoints implements this interface and preprocesses the read snapshot would be:
https://github.com/apache/flink/blob/a567a1ef628eadad21e11864ec328481cd6d7898/flink-core/src/main/java/org/apache/flink/api/common/typeutils/TypeSerializerSerializationUtil.java#L218



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