>> I assume what would happen is the slave would PANIC upon seeing a WAL >> record code it didn't recognize.
> I wonder if we should for the future have the START_REPLICATION command (or > the IDENTIFY_SYSTEM would probably make more sense - or even adding a new > command like IDENTIFY_CLIENT. The point is, something in the replication > protocol) have walreceiver include it's version sent to the master. That > way we could have the walsender identify a walreceiver that's too old and > disconnect it right away - with a much nicer error message than a PANIC.
Meh. That only helps for the case of streaming replication, and not for the thirty-seven other ways that some WAL might arrive at something that wants to replay it.
It might be worth doing anyway, but I can't get excited about it for this scenario.
It does, but I bet it's one of the by far most common cases. I'd say it's that one and restore-from-backup that would cover a huge majority of all cases. If we can cover those, we don't have to be perfect - so unless it turns out to be ridiculously complicated, I think it would be worthwhile having.
