cross-version upgrade testing

I've had a buildfarm module testing cross-version pg_upgrade for a while
now. It's fairly stable - it considered there's a success when the diffs
from the original and the converted dumps match the following perl table:

#target source
my $expected_difflines = {
HEAD => {
REL9_0_STABLE => 1715,
REL9_1_STABLE => 643,
REL9_2_STABLE => 741,
},
REL9_2_STABLE => {
REL9_1_STABLE => 0,
REL9_0_STABLE => 1085,
},
REL9_1_STABLE => {
REL9_0_STABLE => 1085,
}
};

A compressed copy of the actual diffs it finds is attached for those who
are interested.

However, code changes can obviously change the numbers, so I don't want
to hardcode them in a Module.

I was thinking that a file in the pg_upgrade directory containing theses
numbers would be a sensible3 way to go. The file would just contain
lines like (for master):

REL9_0_STABLE 1715
REL9_1_STABLE 643
REL9_2_STABLE 741

When we branch the REL9_3_STABLE branch we'll add a line like this to
the master branch file:

REL9_3_STABLE 0

The buildfarm module could parse this file and test accordingly. If we
change things then all we'd need to do to get the buildfarm green again
would be to adjust this file accordingly.

Thoughts? Better suggestions?

cheers

andrew