Bug Report: Premature buffering stop during concurrent external reparenting can lead to query failures about vitess

[arthurschreiber]

@deepthi @vmg This wasn't an issue in v17 and earlier with --buffer_implementation=healthcheck - but that implementation was deprecated and removed in v18.

I'm a bit at a loss of how this could be fixed. Buffering starts because the vtgate notices that the vttablet is in read-only mode (but still serving), but keyspace events don't know about this and instead make decisions based solely on the serving state of the primary (which in this case is happily reporting that it's up and healthy even though it's in readonly mode during the external failover).