Incorrect encoding of nested empty messages.

[kfir-drivenets]

protobuf.js version: 6.3.0

protobuf js isn't encoding correctly messages with nested empty messages.

protobuf used:

syntax = 'proto2';          
                            
message Empty {             
                            
}                           
                            
message A {                 
    optional int32 a = 1;   
}                           
                            
message B {                 
    oneof child {           
        A a = 1;            
        Empty b = 2;        
    }                       
}

Snippet to reproduce:

root = protobuf.parse(protoString).root;
var Empty = root.lookup("Empty");
var A = root.lookup("A");
var B = root.lookup("B");
var msg = B.from({
	b: {}
});
console.log("msg", msg);
var buffer = B.encode(msg).finish();
console.log(buffer); // Buffer is empty!, should be [18, 00]

console.log(B.decode(new Uint8Array(18, 0))); // Should be equal to msg

jsfiddle link: https://jsfiddle.net/13c0re75/

Using the python binding for protobuf I got a different result.
Note: I used SetInParent - http://stackoverflow.com/questions/29643295/how-to-set-a-protobuf-field-which-is-an-empty-message-in-python

import example_pb2
import binascii

msg = example_pb2.B()
msg.b.SetInParent()
print "msg: ", msg
print "msg buffer: (hex string)", binascii.hexlify(msg.SerializeToString())

The result of the previous snippet is

msg:  b {
}

msg buffer: (hex string) 1200

I've verified the same behavior with another binding of protobuf (https://github.com/protobuf-c/protobuf-c) and got the same results.

[dcodeIO]

Let me know if this fixes it. The test case is somewhat simplified but should still catch it, theoretically.

[kfir-drivenets]

Works like a charm. Thanks 👍