matriv opened a new pull request #19249:
URL: https://github.com/apache/flink/pull/19249
## What is the purpose of the change
Previously, when the Json aggregation is taking place, and id
JsonOnNull.NULL is selected, which means that we still want to emit
a `null` JSON node, .i.e `{.... "myField" : null ... }` when no values
get accumulated, we used a null `StringData` object. When
`state.backend.changelog.enabled` is enabled, the contents of the map
accumulating the aggregated records, gets serialized leading to NPE,
since `null` is not supported by `StringDataSerilizer`.
To solve this, we instead create a StringData with an empty `byte[]`,
which denotes the null, and when the aggregation ends and we create
the final JSON result, we check for a `byte[]` of `length` `0` in
order to write the JSON `null` node.
## Verifying this change
This change added tests and can be verified as follows:
- Change the parent class: `BuiltInAggregateFunctionTestBase` of
`JsonAggregationFunctionsITCase` to run the whole set of tests with
`state.backend.changelog.enabled` both `true` and `false`.
## Does this pull request potentially affect one of the following parts:
- Dependencies (does it add or upgrade a dependency): **no**
- The public API, i.e., is any changed class annotated with
`@Public(Evolving)`: **no**
- The serializers: **no**
- The runtime per-record code paths (performance sensitive): **no**
- Anything that affects deployment or recovery: JobManager (and its
components), Checkpointing, Kubernetes/Yarn, ZooKeeper: **no**
- The S3 file system connector: **no**
## Documentation
- Does this pull request introduce a new feature? **no**
- If yes, how is the feature documented? **not applicable**
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