Facter tests mutate the global logger state

[joshcooper]

Describe the Bug

While rearching #2664 and #2715 I noticed a few different classes of issues with Facter's logging system.

Tests often overwrite the global Logger with an instance_spy, but don't restore the original logger after each test:

before do
  Facter::Log.class_variable_set(:@@logger, logger_double)
end

This causes global state to leak across tests leading to rspec reporting:

 #<InstanceDouble(Facter::Log) (anonymous)> was originally created in one
    example but has leaked into another example and can no longer be used.

Tests also overwrite the global logger with an instance spy and restore it with a different logger instance instead of the original instance:

before do
  Facter::Log.class_variable_set(:@@logger, logger_double)
end

after do
  Facter::Log.class_variable_set(:@@logger, Logger.new(STDOUT))
end

A similar problem exists with resolvers as the each resolver class has a Facter::Log instance:

before do
  augeas.instance_variable_set(:@log, log_spy)
end

The problem is made worse, because some resolvers eagerly create their resolver:

class Uname < BaseResolver
  @log = Facter::Log.new(self)

While others rely on the BaseResolver to lazily create it:

class BaseResolver
  def self.log
    @log ||= Log.new(self)

As a result, you can never be sure if a resolver has a logger, and if so, who created it (the code or test).

Finally if aggregated facts are evaluated twice, they attempt to log a message to a nil logger.

Expected Behavior

Since Facter's Logger and Facter::Log are global state, they should be immutable.

[github-actions]

Migrated issue to FACT-3468