PDGGK opened a new issue, #39730:
URL: https://github.com/apache/beam/issues/39730
### What happened?
`AttributeValueCoder.encode` cannot encode a DynamoDB attribute whose value
is an **empty list** or an **empty map** — it throws `CoderException("Unknown
Type")`.
The L and M branches are selected with a size check:
```java
} else if (value.ss() != null && value.ss().size() > 0) { // correct:
DynamoDB rejects empty sets
...
} else if (value.l() != null && value.l().size() > 0) { // wrong:
DynamoDB allows an empty L
} else if (value.m() != null && value.m().size() > 0) { // wrong:
DynamoDB allows an empty M
} else if (value.nul() != null) {
...
} else {
throw new CoderException("Unknown Type");
}
```
For an empty `L`, `l()` is non-null and `size()` is 0, so the guard fails;
`m()` and `nul()` do not match either, and it falls through to the terminal
`else`.
**The blast radius is the whole item, not one attribute.**
`MAP_ATTRIBUTE_CODER` re-enters this coder for every child, so a single empty
list nested anywhere inside an item makes that item unencodable:
```java
Map<String, AttributeValue> item = new HashMap<>();
item.put("name", AttributeValue.builder().s("widget").build());
item.put("tags", AttributeValue.builder().l(new ArrayList<>()).build());
AttributeValueCoder.of().encode(AttributeValue.builder().m(item).build(),
out);
// CoderException: Unknown Type
```
Empty `L` and `M` are valid DynamoDB attribute values and the service
returns them, unlike the `SS`/`NS`/`BS` set types where the `size() > 0` guard
is legitimate — DynamoDB rejects empty sets.
The `decode` side already copes: `case l:` and `case m:` delegate to
`ListCoder`/`MapCoder`, which round-trip empties fine. Only `encode` rejects
them.
### Issue Priority
Priority: 2 (default / most bugs should be filed as P2)
### Issue Components
- [x] Component: Java SDK
- [x] Component: IO connector
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