Proper dealing with protobuf3 decoding

[soareschen]

Crate

ibc

Summary

Protobuf3 has the unfortunate semantics of allowing all fields to be optional, and leaves no way to mark a field as being required. (protocolbuffers/protobuf#2497) The prost crate follows this semantic, and decodes missing fields into default values such as 0 and "". This can cause unintended bugs when the relayer logic expects a given field to be present. In particular, the semantics allow empty buffers to be decoded successfully, producing nonsense values such as in #1042.

On Rust side, we want to use the Option type to properly denote the absence of values, and have proper distinction between values not being present and the default values. One strategy to deal with this is to mark all scalar fields in the raw prost structs as being optional, so that prost will properly decode them into Option value when the field is not present in the raw buffer. The conversion from raw prost structs into the proper domain types should then return error when the required fields have value of None.

We may need to modify the proto-compiler crate so that it properly marks all scalar fields as being optional in the generated Rust code.

Acceptance Criteria

All scalar fields of the raw prost structs in proto/src/prost should have the Option type.

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