Re: allow online change primary_conninfo

Hello

> So, I'd like to propose to move the stuff to the second switch().
> (See the attached incomplete patch.) This is rather similar to
> Sergei's previous proposal, but the structure of the state
> machine is kept.

Very similar to my v4 proposal (also move RequestXLogStreaming call), but closer to currentSource reading. No objections from me, attached patch is changed this way.
I renamed start_wal_receiver to startWalReceiver - this style looks more consistent to near code.

> + /*
> + * Else, check if WAL receiver is still active.
> + */
> + else if (!WalRcvStreaming())

I think we still need wait WalRcvStreaming after RequestXLogStreaming call. So I remove else branch and leave separate condition.

> In ProcessStartupSigHup, conninfo and slotname don't need to be
> retained until the end of the function.

Agreed, I move pfree

> The log message in the function seems to be too detailed. On the
> other hand, if we changed primary_conninfo to '' (stop) or vise
> versa (start), the message (restart) looks strange.

I have no strong opinion here. These messages was changed many times during this thread lifetime, can be changed anytime. I think this is not issue since we have no consensus about overall design.
Write detailed messages was proposed here: https://www.postgresql.org/message-id/20190216151025.GJ2240%40paquier.xyz

> or vise versa (start)

I explicitly check currentSource and WalRcvRunning, so we have no such messages if user had no walreceiver before.

regards, Sergei