port conflicts when running tests concurrently on windows.

Hi,

With the meson patch applied the tests on windows run concurrently by
default. Unfortunately that fails semi-regularly. The reason for this
basically is that windows defaults to using TCP in tests, and that the
tap-test port determination is very racy:

> # When selecting a port, we look for an unassigned TCP port number,
> # even if we intend to use only Unix-domain sockets. This is clearly
> # necessary on $use_tcp (Windows) configurations, and it seems like a
> # good idea on Unixen as well.
> $port = get_free_port();
> ...
> =item get_free_port()
>
> Locate an unprivileged (high) TCP port that's not currently bound to
> anything. This is used by C<new()>, and also by some test cases that need to
> start other, non-Postgres servers.
>
> Ports assigned to existing PostgreSQL::Test::Cluster objects are automatically
> excluded, even if those servers are not currently running.
>
> XXX A port available now may become unavailable by the time we start
> the desired service.

I don't think there's an easy way to make this race-free. We'd need to teach
postmaster to use pre-opened socket or something like that.

An alternative to that would be to specify a base port number externally. In
the meson branch I already did that for the pg_regress style tests, since they
don't have the automatic port thing above. But for tap tests there's currently
no way to pass in a base-port that I can see.

Is it perhaps time to to use unix sockets on windows by default
(i.e. PG_TEST_USE_UNIX_SOCKETS), at least when on a new enough windows?

Greetings,

Andres Freund