[jira] [Created] (AVRO-1898) C++ library cannot parse unions with default values

[Hua Zhang (JIRA)]

Hua Zhang created AVRO-1898:
-------------------------------

             Summary: C++ library cannot parse unions with default values
                 Key: AVRO-1898
                 URL: https://issues.apache.org/jira/browse/AVRO-1898
             Project: Avro
          Issue Type: Bug
          Components: c++
    Affects Versions: 1.8.1
            Reporter: Hua Zhang


I have a Avro file generated by the Java library, and when I tried to read it 
using the C++ library, I got:

Unexpected type for default value: Expected 6, but found 1

Which basically says the default value should be of type Object, but got type 
Boolean. After some investigation, I found the exception is thrown when parsing 
the default value of a union. Looks like the C++ library requires the default 
value of a union to be an object. This is different from what the Avro 
specification says, that the default value of a union should be of the first 
type in the union.

This field is causing the problem:
{
        "name" : "fake_name",
        "type" : [ "boolean", "null" ],
        "default" : false
}
>From the code the C++ library is expecting:
{
        "name" : "is_track_mau30",
        "type" : [ "boolean", "null" ],
        "default" : {"boolean": false}
}

The related code is in impl/Compiler.cc, line 284. If I change it to the 
following, the Avro file can be parsed:

        GenericUnion result(n);
        result.selectBranch(0);
        result.datum() = makeGenericDatum(n->leafAt(0), e, st);
        return GenericDatum(n, result);

Why is the C++ library behaves different than the specification? Can/should it 
be changed to follow the specification? It will be a breaking change though.

Thanks and I'll be happy to contribute this change if it's deemed appropriate.



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