PookieBuns commented on issue #383:
URL: https://github.com/apache/avro-rs/issues/383#issuecomment-4147374006
Hi guys. I noticed another issue, specifically wrt Unions when performing
schema resolution
```
fn incorrect_schema_resolution_with_reader() {
use apache_avro::{Reader, Schema, Writer, types::Record};
let schema_str = r#"
[
{
"type": "record",
"name": "foo",
"fields": [
{"name": "a", "type": "string", "default": "a"},
{"name": "b", "type": "string", "default": "b"},
{"name": "c", "type": "string", "default": "c"}
]
},
{
"type": "record",
"name": "bar",
"fields": [
{"name": "d", "type": "string"}
]
}
]
"#;
let schema = apache_avro::Schema::parse_str(schema_str).unwrap();
apache_avro::schema_compatibility::SchemaCompatibility::can_read(&schema,
&schema).unwrap();
let value = apache_avro::types::Value::Union(
1,
Box::new(apache_avro::types::Value::Record(vec![(
"d".to_string(),
apache_avro::types::Value::String("d".to_string()),
)])),
);
let encoded = apache_avro::to_avro_datum(&schema,
value.clone()).unwrap();
let without_reader =
apache_avro::from_avro_datum(&schema, &mut
std::io::Cursor::new(encoded.clone()), None)
.unwrap();
assert_eq!(value, without_reader);
let with_reader =
apache_avro::from_avro_datum(&schema, &mut
std::io::Cursor::new(encoded), Some(&schema))
.unwrap();
assert_eq!(value, with_reader);
}
```
In this, if you use a reader's schema, it ends up converting your avro value
into record Foo because of how resolution is currently implemented.
https://github.com/apache/avro-rs/blob/48dfe5b3576944b7c6936981759df58e779d93f7/avro/src/schema/union.rs#L87
this function currently will instantly succeed any Value in the first branch,
because defaults were all provided. I haven't looked into deeply how java
avoids this (maybe somewhere in
https://github.com/apache/avro/blob/main/lang/java/avro/src/main/java/org/apache/avro/Resolver.java#L722)
but I will take a deeper dive tmr and see if the currently proposed solution
above can address it (by attaching writer schema in schema resolution)
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