santiagomed opened a new pull request, #3414:
URL: https://github.com/apache/thrift/pull/3414
## Problem
When a struct contains a union-typed field and the wire data has a union
variant (field ID) not present in the local IDL, the generated Rust code fails
to deserialize the **entire struct** — not just the unknown union field.
The root cause: the union`s `read_from_in_protocol` correctly returns
`Err("received empty union")` for unknown variants (fixed in THRIFT-5926 / PR
#3336). But the parent struct`s generated deserializer propagates this error
via `?`, causing the whole struct to fail.
This breaks Thrift`s forward-compatibility guarantee. When a server adds a
new union variant, all clients running an older IDL version fail to parse any
message containing that variant — even though the client doesn`t need to
understand the new variant to process the rest of the struct.
Java, Go, Python, and C++ Thrift all handle this gracefully: unknown union
variants are silently skipped, and the struct field is left unset.
## Fix
In `render_struct_sync_read`, detect struct fields whose type is a union.
For those fields, wrap the `read_from_in_protocol` call in a `match` that
catches the `"received empty union"` error and treats it as `None` (field
absent):
```rust
// Before (all fields):
let val = FooUnion::read_from_in_protocol(i_prot)?;
f_N = Some(val);
// After (union fields only):
match FooUnion::read_from_in_protocol(i_prot) {
Ok(val) => { f_N = Some(val); },
Err(thrift::Error::Protocol(ref e))
if e.message.contains("received empty union") => {
// forward compatibility: unknown union variant skipped
},
Err(e) => return Err(e),
}
```
Non-union fields are unchanged. The union`s own `TSerializable` impl is
unchanged — it still returns `Err` for unknown variants, preserving the
contract for callers that read unions directly.
## Impact
- Fixes forward compatibility for all Rust Thrift users with evolving union
schemas
- No changes to the `TSerializable` trait or the thrift runtime library
- No changes to the union`s own serialization/deserialization
- Struct fields with union types that receive unknown variants get `None`
instead of causing parse failure
- All other errors (malformed data, I/O, multiple union fields) still
propagate normally
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