Proto2 encode/decode issues after adding new message - protocol-buffers

Question

I'm fairly new to protocol buffers but have been trying to learn them as a means of sending data via MQTT. So far, I've been fine with creating proto messages and compiling them for python runtime, until I started to notice incompatibility between versions of my protobufs.
When I add a message type (no changes to existing messages/fields) to my server-side proto definitions without updating my client side proto definitions, decoding messages sent to the server give me non-deterministic results.
Here is an example of what I'm talking about:
Client proto:
message Wrapper {
optional uint32 id = 1;
optional string name = 2;
oneof payload {
Event event = 3;
Command command = 4;
}
}
message Event {
uint32 event_id = 1;
oneof event_payload {
LoginEvent login_event = 2;
LogoffEvent logoff_event = 3;
}
}
Server Proto:
message Wrapper {
optional uint32 id = 1;
optional string name = 2;
oneof payload {
Event event = 3;
Command command = 4;
}
message Event {
uint32 event_id = 1;
oneof event_payload {
LoginEvent login_event = 2;
LogoffEvent logoff_event = 3;
NewUserEvent new_user_event = 4;
}
}
I will encode and send a message from the client:
message Wrapper {
id = 12345;
name = John;
event = {
login_event = ...
}
}
And will decode the message on the server and get:
message Wrapper {
id = 12345;
name = John;
event = {
logoff_event = ...
}
}
NOTE: The decoded message type isn't deterministic and changes between messages
Can someone explain why adding an event type seems to screw up decode? Or any best-practices I should obey to improve version compatibility? Thanks in advance!

Answer 1

This may be a side-effect of the Python implementation rather than a bug.
You don't include the code you're using to (un)marshal the protobufs.
In your example, does logoff_event contain a LogoffEvent message type?
You say non-deterministic? Do you see different event_payload messages types on the server for the same message sent by the client?
What is returned by:
msg = Wrapper() # Your decoded message
assert msg.event.WhichOneof("event_payload")
# or
assert msg.event.HasField("login_event")
See Python Generated Code: OneOf
Update 210927
I'm unable to repro the behavior you observe; it works as expected for me.
Client:
import client_pb2
msg = client_pb2.Wrapper()
msg.id=0
msg.name="Test"
msg.event.event_id=1
msg.event.login_event.y="Hello Freddie"
print(msg)
f=open("message","wb")
f.write(msg.SerializeToString())
f.close()
Yields:
id: 0
name: "Test"
event {
event_id: 1
login_event {
y: "Hello Freddie"
}
}
Server:
import server_pb2
msg = server_pb2.Wrapper()
f=open("message","rb")
msg.ParseFromString(f.read())
f.close
assert msg.event.WhichOneof("event_payload")
assert msg.event.HasField("login_event")
print(msg)
Yields:
id: 0
name: "Test"
event {
event_id: 1
login_event {
y: "Hello Freddie"
}
}