Morgan Parry created AVRO-3770:
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Summary: SpecificDefaultReader and PreresolvingDatumReader don't
handle aliases properly
Key: AVRO-3770
URL: https://issues.apache.org/jira/browse/AVRO-3770
Project: Apache Avro
Issue Type: Bug
Components: csharp
Affects Versions: 1.11.1
Reporter: Morgan Parry
Both of these classes will read from an aliased value correctly but then will
go on to try and insert a default value reader, because they're also still
looking for the new name in the writer schema.
For example, with a field like
```json
{
"name": "i",
"type": "int",
"aliases": "j"
}
```
and some old serialised data that was still writing the field `j` instead, the
readers will read that `j` value into `i` (as expected) but then also add some
fallback handling for the 'missing' `i` field. This is due to the following
loop in `PreresolvingDatumReader`, for example:
```csharp
foreach (Field rf in readerSchema)
{
if (writerSchema.Contains(rf.Name)) continue;
```
That condition isn't sufficient in the case of aliasing.
I believe it's also the case that `SpecificDefaultReader` doesn't handle
aliases at all, since it uses `TryGetField()` rather than `TryGetFieldAlias()`.
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