[AVRO-1723] Add support for forward declarations in avro IDL - ASF JIRA

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Release Note:

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With this patch recursive data structures will be supported in IDL:

record SampleNode {
  int count = 0;
  array<SamplePair> samples = [];
}
record SamplePair {
string name;
SampleNode node;
}

Also the order of declaration of named types will not matter anymore. (similar with other languages like java)

Compatibility:

All schemas that are compilable with older versions of AVRO will continue working without issues.
IDLs that will take advantage of the the features introduced with this patch, will not be compilable with older versions of AVRO
please see the dependent JIRAs for more detail on related issues.

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With this patch recursive data structures will be supported in IDL: record SampleNode {   int count = 0;   array<SamplePair> samples = []; } record SamplePair { string name; SampleNode node; } Also the order of declaration of named types will not matter anymore. (similar with other languages like java) Compatibility: All schemas that are compilable with older versions of AVRO will continue working without issues. IDLs that will take advantage of the the features introduced with this patch, will not be compilable with older versions of AVRO please see the dependent JIRAs for more detail on related issues.
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Patch

Currently Recursive data structures like:

record SampleNode {
int count = 0;
array<SamplePair> samples = [];
}

record SamplePair {
string name;
SampleNode node;
}

It is not possible to declare in IDL,
however it is possible to declare in avsc (with fix from https://issues.apache.org/jira/browse/AVRO-1667 )

It is actually not complicated to implement, here is some detail on a possible implementation:

This would close a capability gap with google protocol buffers...

is blocked by

AVRO-1997 Avro Field.defaultVal broken for Fixed fields.

requires

AVRO-1667 Parser symbol tree flattening is broken for recursive schemas

links to

GitHub Pull Request #79