Re: "caught_up" status in walsender

I wrote:
> On further contemplation, it seems like the protocol needs another field
> besides that: each record should also carry a boolean indicating whether
> walsender.c thinks it is currently "caught up", ie the record carries
> all WAL data up to the current end of WAL.

Actually, there's a better way to do that: let's have the record carry
not just a boolean but the actual current end-of-WAL LSN.  The receiver
could then not just determine "am I behind" but find out *how far*
behind it is, and thereby perhaps adjust its behavior in more subtle
ways than just a binary on/off fashion.

(Actually doing anything like that is material for future work, of
course, but I think we should try to get the SR protocol right now.)
        regards, tom lane