cannot read pg_class without having selected a database / is this a bug?

Hi,

I've written a simple extension that limits number of connection by
IP/db/user, and I do receive this exception:

psql: FATAL: cannot read pg_class without having selected a database

I've found this happens because the extension defines a client auth hook
that reads pg_stat_activity. The really interesting thing is that this
happens only when I start several backends 'at the same time' right
after the cluster is started. From that time, everything works just fine.

So it seems like a race condition or something like that.

I've prepared a simple testcase to demonstrate this issue - see the
files attached. I've put there several 'sleep' to demonstrate the timing
error.

All you need to do is this:

1) compile the extension (make install)
2) add the extension to shared_preload_libraries
3) restart the cluster
4) start two backends at the same time (within a second or so)

Tomas