Add a special ProtobufError for truncated message

[luser]

I'm using rust-protobuf to send and receive protobuf messages over sockets. The server is using mio, and it tries to parse the message with whatever bytes it has received so far by using parse_length_delimited_from_bytes. Unfortunately, if the entire message hasn't been received yet, the only way to tell is to check that the returned error is a ProtobufError::WireError("truncated message"):
https://github.com/luser/sccache2/blob/1ad02689a9829502cd09b2b7381cec431a01e5a7/src/server.rs#L332

If would be nice if there was a separate ProtobufError::MessageTruncated so that I didn't have to do string comparison to catch this fairly likely case.

Maybe I'm missing something, if so I'd appreciate any info.

[stepancheg]

Protobuf messages are not self-delimited. Protobuf reads messages until EOF (or until explicitly specified limit).

When transferring messages over network, messages are usually prepended with length (with function like write_length_delimited_to). So when you read a message from network, first you need to read the length. After that you read the message only after buffer contains length bytes, and truncated message error does not occur.

ProtobufError::MessageTruncated should be added, but I think it is not the way you handle input from network.

See the explanation in protobuf manual.

[luser]

Right, and that's what I'm doing, but it's pretty awkward to manually read the length like that. I'm doing that on the client side of my app currently, since parse_length_delimited_from doesn't work on a TcpStream if the other end doesn't shutdown its write handle after writing data:
https://github.com/luser/sccache2/blob/1ad02689a9829502cd09b2b7381cec431a01e5a7/src/client.rs#L51

It seems like at the very least the parse_length_delimited_ functions could pretty easily return a MessageTruncated error, which would make them a lot easier to use.

[luser]

Related: #157 (the other issue I mentioned)

[stepancheg]

Closing due to old age.