TCP option assign hook doesn't work well if option not supported

macOS does not support the socket option TCP_USER_TIMEOUT. Yet, I can
start a server with postgres -D ... --tcp-user-timeout=100 without a
diagnostic. Only when I connect I get a log entry

LOG: setsockopt(TCP_USER_TIMEOUT) not supported

Perhaps the logic in pq_settcpusertimeout() should be changed like this:

int
pq_settcpusertimeout(int timeout, Port *port)
{
+#ifdef TCP_USER_TIMEOUT
if (port == NULL || IS_AF_UNIX(port->laddr.addr.ss_family))
return STATUS_OK;

-#ifdef TCP_USER_TIMEOUT
if (timeout == port->tcp_user_timeout)
return STATUS_OK;

So that the #else branch that is supposed to check this will also be run
in the postmaster (where port == NULL).

Or perhaps there should be a separate GUC check hook that just does

#ifndef TCP_USER_TIMEOUT
if (val != 0)
return false;
#endif
return true;

The same considerations apply to the various TCP keepalive settings, but
since those are widely supported the unsupported code paths probably
haven't gotten much attention.

--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services