AVRO-3155: [docs] Remove 'default' from map and array examples (and add to enum).

[HuwCampbell]

AVRO-3155

What is the purpose of the change

Default values have no special purpose for maps and arrays, and are only
valid for record fields and enums.

Having it in the examples is confusing, as people might have expectations
around it behaving in a certain way.

In the Java and Rust implementation, a default value listed here
will be packed into the custom props and attributes payloads, and
do not need to be of the same type; i.e., default: "no default" would
be valid.

Other implementations just ignore it and won't round trip it, for example:

MapType.prototype.toJSON = function () {
  return {type: 'map', values: this._values};
};

Verifying this change

I wrote a small test in rust to validate my understanding; unfortunately
using an assertion on Map would be confusing as the instance
for Eq for MapSchema ignores the attributes field.

#[test]
fn avro_test_map_default() -> TestResult {
    // string uuid, represents as native logical type.
    let schema = json!(
    {
        "type": "map",
        "values": "long",
        "default": "4"
    });
    let parse_result = Schema::parse(&schema)?;
    assert_eq!(parse_result, Schema::Uuid);

    Ok(())
}

Resulting in:

assertion `left == right` failed
  left: Map(MapSchema { types: Long, attributes: {"default": String("4")} })

For Java I just read the code.

[xxchan]

Originates from
#662 https://issues.apache.org/jira/browse/AVRO-2574 It doesn't mention any reason though.

cc @Fokko

[xxchan]

Looks correct to me

[xxchan]

BTW, I don't think Rust implementation is stable enough as a reference. Can you link to related Java code?

[HuwCampbell]

This is in Schema.java.

@Override
@Deprecated
void toJson(Set<String> knownNames, String currentNamespace, JsonGenerator gen) throws IOException {
  gen.writeStartObject();
  gen.writeStringField("type", "map");
  gen.writeFieldName("values");
  valueType.toJson(knownNames, currentNamespace, gen);
  writeProps(gen);
  gen.writeEndObject();
}

Props are written, but there's no default value to be seen.

On the parsing side, the code for the parser code also just reads the value field here.

[HuwCampbell]

Is there anything else required to get this merged? Like it's not critical, but it's weird to leave it hanging.

[martin-g]

@Fokko Do you have any comments about these defaults ?
They have been introduced by you with #662

[KalleOlaviNiemitalo]

This pull request seems to fix [AVRO-3155] Schema specification inconsistently describes default value for types.