[PostgreSQL-Hackers] TODO item for pg_ctl and server detection

Well, rereading that old thread makes me realize that what you just
implemented is still pretty far short of what was discussed. In
particular, this implementation entirely fails to cope with the
possibility that a Windows postmaster is using a specialized
listen_addresses setting that has to be taken into account in order to
get a TCP connection. I wonder whether we should revert this patch and
have another go at the idea of a separate postmaster.ports status file
with a line for each active port.

The business with a magic number can't be implemented unless we actually
add a new separate pg_ping protocol. PQping() has removed a lot of the
pressure to have that, namely all the authentication-failure problem
cases. I'm not sure that the case where you're looking at an inactive
data directory but there's a live postmaster someplace else with the
same port number is important enough to justify new protocol all by
itself.

regards, tom lane

I had forgotten about having to use TCP and needing to honor
listen_address restrictions. We only need one valid listen_address so I
went ahead and added a line to the postmaster.pid file.

I am not sure what a separate file will buy us except additional files
to open/manage.
Yes, that was my calculus too. I realized that we create session ids by
merging the process id and backend start time, so I went ahead and added
the postmaster start time epoch to the postmaster.pid file. While there
is no way to pass back the postmaster start time from PQping, I added
code to pg_ctl to make sure the time in the postmaster.pid file is not
_before_ pg_ctl started running. We only check PQping() after we have
started the postmaster ourselves, so it fits our needs.

Patch attached.

Tom suggested that there might be clock skew between pg_ctl and the
postmaster, so I added a 2-second slop in checking the postmaster start
time. Tom also wanted the connection information to be output all at
once, but that causes a problem with detecting pre-9.1 servers so I
avoided it.

Updated patch attached.

Patch applied, and TODO item removed because patch mostly detects if a
stale postmaster created the postmaster.pid file. The TODO was:

Allow pg_ctl to work properly with configuration files located outside
the PGDATA directory)

Please fix this new compiler warning:

pg_ctl.c:1787: warning: implicit declaration of function ‘time’

Thanks, done.