pg_upgrade old cluster delete script

You might remember this pg_upgrade bug report where the user complained
that user-defined tablespaces _inside_ the old cluster directory were
deleted by the old cluster delete script:

http://www.postgresql.org/message-id/[email protected]

and my reply that we don't want to fix this:

http://www.postgresql.org/message-id/[email protected]

However, creating a script that deletes data is never a good idea so I
have created the attached patch to avoid creating the script in these
cases, and issue a message to the user.

The test isn't perfect but it is probably close enough. I don't think
it is worth unlinking a possibly matching filename in such cases.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +