Correct behaviour when encountering an unexpected enum on the wire

[Marc Gravell]

I'm doing some code maintenance on my protobuf library, and I have
encountered a test that is... confusing me. So before I go crazy (/
crazier)... what should an implementation do if during deserialization
it gets an enum it doesn't recognise?

- to explode in sparks?
- to ignore the data?
- to brutally coerce the data to the unexpected value?
- other?

Thanks in advance

Marc

[Jason Hsueh]

Ignore the data (sort of): The unknown value gets treated as an unknown field, leaving the enum field unset. In implementations that support propagation of unknown fields (non-lite C++, Java), the value is added to the UnknownFieldSet.

Implications:

- a sender may set a required enum field to a valid value, according to its defintion of the .proto. If the recipient doesn't know about that value, the message may fail to parse. It's best to avoid required enums

- by treating it as an unknown field, a message can be sent between two programs who understand a particular enum value through a middleman that doesn't, and there is no loss of data

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