(SERVER-2500) Use DNF modules to install postgres on RHEL 8

[Magisus]

The postgresql module added support for DNF modules to install postgres
at specific versions on RHEL 8. However in order to install a
non-default version, like we need to test Puppet Platform 6, which
only supports Postgres 9.6, manage_dnf_module must be set to true for
the puppetlabs-postgresql module. This commit ensures that mode is set
when isntalling on RHEL 8.

Note that when the default version is being installed (postgres 10 on
RHEL8), the code path that this flag opts into is already being used.

[puppet-community-rangefinder]

puppetdb::database::postgresql is a class

that may have no external impact to Forge modules.

puppetdb::params is a class

that may have no external impact to Forge modules.

This module is declared in 33 of 578 indexed public Puppetfiles.

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[Magisus]

For the postgresql module change, see puppetlabs/puppetlabs-postgresql#1239.

This resolves the following issue seen when trying to enable install PDB with this module on RHEL 8 in our acceptance tests:

Error: Execution of '/usr/bin/dnf -d 0 -e 1 -y install postgresql96-server' returned 1: Error: Unable to find a match

This is because the package string is invalid for dnf version selection, we need to be using the top branch in this conditional. When using the module with this change, after install PDB, I am able to see

=> postgres --version
postgres (PostgreSQL) 9.6.22

[Magisus]

I wasn't sure if this should be exposed to users or not.

[Magisus]

I tested this on 7.x and was surprised to see us still trying to install postgres 9.6 there... then I realized this module hasn't been released in a while, so we were not getting 3ea57a5. I think this is necessary even after that goes in, though, since we're still using a non-default version (11 instead of 10).

[Magisus]

On further investigation, I think this may not be what we want. DNF modules can't be used to install Postgres 11, which means with this change we are even more broken in Puppet 7 land.

It seems what we really want is something that follows these instructions:

# Install the repository RPM:
sudo dnf install -y https://download.postgresql.org/pub/repos/yum/reporpms/EL-8-x86_64/pgdg-redhat-repo-latest.noarch.rpm

# Disable the built-in PostgreSQL module:
sudo dnf -qy module disable postgresql

# Install PostgreSQL:
sudo dnf install -y postgresql11-server

Not sure yet if there's a way to force the postgresql module to do that.

[bschonec]

I'm returning to the world of Puppet after a four year break. Is there any reason this project is tied to Postgresql 9.6 and 11? I've installed PSQL 10, 11 and 12 and (so far) haven't had any issues.

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[jhoblitt]

psql 11 is EOL and 12 is the oldest version that we need to worry about supporting.

[jhoblitt]

This should be changed to check for either any version >= 8 or match EL8 & EL9.