Radai Rosenblatt created AVRO-3512:
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Summary: aliases to the null namespace do not work as expected
Key: AVRO-3512
URL: https://issues.apache.org/jira/browse/AVRO-3512
Project: Apache Avro
Issue Type: Bug
Components: java, spec
Affects Versions: 1.11.0
Reporter: Radai Rosenblatt
the avro spec allows for the "null namespace" (when no namespace is specified
anywhere). it also has [the
following|https://avro.apache.org/docs/current/spec.html#Aliases] to say about
aliases:
```
if a type named "a.b" has aliases of "c" and "x.y", then the fully qualified
names of its aliases are "a.c" and "x.y"
```
which means a "simple" alias ("c" above) inherits any namespace defined on the
declaring type.
now suppose i was to use aliases on a namespaced schema to be able to read data
written using a schema that is in the null namespace (has no namespace).
here are my writer schema:
```
{
"type": "record",
"name": "AncientSchema",
"fields": [
{
"name" : "enumField",
"type" : {
"type" : "enum",
"name" : "AncientEnum",
"symbols" : [ "THE", "SPEC", "IS", "A", "LIE" ]
}
}
]
}
```
and reader schema:
```
{
"type": "record",
"namespace": "much.namespace",
"name": "ModernRecord",
"fields": [
{
"name" : "enumField",
"type" : {
"type" : "enum",
"name" : "ModernEnum",
"symbols" : [ "THE", "SPEC", "IS", "A", "LIE" ],
"aliases": [
".AncientEnum"
]
}
}
],
"aliases": [
".AncientSchema"
]
}
```
notice the dots used in the aliases. as far as i understand the spec this
should be the only legal way to do this. and it does indeed work .... to a
point.
when testing this i found multiple issues with avro's handling of such aliases,
dating back to late avro 1.7.*
# without these aliases, decoding does fail, but it fails over the nested
enum, whereas it should have failed "immediately" on the fullname mismatch on
the top level record schema. in fact, on further testing i think avro (at least
in java) doesnt bother comparing the fullnames on the top level writer vs
reader schemas at all?
# while the schema with the aliases parse()es fine, Schema.toString() strips
out the dots from the aliases, thereby creating a "monsanto terminator schema"
- once printed and parsed again the aliases would become "simple aliases" and
stop working
# the spec doesnt explicitly talk about how to use aliases to "target" the
null namespace. if this is an intentional specification I think the spec should
be expanded a little to cover it?
i have code to reproduce all these issues in
[https://github.com/radai-rosenblatt/avro/blob/aliasing-to-null-namespace/lang/java/avro/src/test/java/org/apache/avro/TestAliasToNullNamespace.java]
(coded against master)
i also have code to reproduce all the above against multiple older avro
versions in
[https://github.com/linkedin/avro-util/blob/master/helper/tests/helper-tests-allavro/src/test/java/com/linkedin/avroutil1/compatibility/AvroTypeAliasesTest.java]
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