On Tue, 20 Aug 2024 at 15:48, Jacob Champion <jacob.champ...@enterprisedb.com> wrote: > Put another way: for a middlebox on the connection (which may be > passively observing, but also maybe actively adding new messages to > the stream), what is guaranteed to remain the same in the protocol > across a minor version bump? Hopefully the answer isn't "nothing"?
I think primarily we do a minor version bump because a major version bump would cause existing Postgres servers to throw an error for the connection attempt (and we don't want that). While for a minor version bump they will instead send a NegotiateProtocolVersion message. In practical terms I think that means for a minor version bump the format of the StartupMessage cannot be changed. Changing anything else is fair game for a minor protocol version bump. I think we probably would not want to change the format of ErrorResponse and NoticeResponse, since those can be sent by the server before the NegotiateProtocolVersion message. But I don't even think that is strictly necessary, as long as clients would be able to parse both the old and new versions. Note that this definition arises from code and behaviour introduced in ae65f6066dc3 in 2017. And PQprotocolVersion was introduced in efc3a25bb0 in 2003. So anyone starting to use the PQprotocolVersion function in between 2003 and 2017 had no way of knowing that there would ever be a thing called a "minor" version, in which anything about the protocol could be changed except for the StartupMessage.
