Re: Server won't start with fallback setting by initdb.

I wrote:
> Therefore, the condition that actually ought to be getting enforced here
> is either "ReservedBackends + max_wal_senders < MaxConnections", or
> "ReservedBackends + max_wal_senders <= MaxConnections", depending on
> whether you think it's appropriate to require at least one not-reserved-
> for-superusers connection slot to remain available if the walsenders
> slots are fully populated.

I propose the first attached patch to do that. (I failed to resist the
temptation to copy-edit some nearby docs and comments, too.)

> My proposal is to default max_wal_senders to perhaps 3, and leave
> initdb's logic alone.

... and then the second attached patch to do that.

I noticed that a lot of our TAP tests are setting max_wal_senders
and max_replication_slots to random values around 4 or 5. Probably
we could drop all that now, and let them just use the defaults.
I've not done that here, except for adjusting 010_pg_basebackup.pl
which would fail for no very good reason with minimal max_connections.

regards, tom lane