(SERVER-2470) Draft implementation for list_all_transports

To test out, run `rake spec_prep` and use the following setup in pry:

```
require 'puppet'
require 'puppet/resource_api/transport'
require 'puppet/settings'

Puppet.initialize_settings
Puppet[:modulepath] = 'spec/fixtures/modules'

# env = Puppet.lookup(:environments).get!('production')
# files = Puppet::Util::Autoload.files_to_load('puppet/transport/schema', env)

Puppet::ResourceApi::Transport.list_all_transports('production')
```

lib/puppet/resource_api/transport.rb

@@ -20,11 +20,35 @@ def register(schema)
   module_function :register # rubocop:disable Style/AccessModifierDeclarations
 
   # retrieve a Hash of transport schemas, keyed by their name.
+  # Only already loaded transports are returned.
   def list
     Marshal.load(Marshal.dump(transports))
   end
   module_function :list # rubocop:disable Style/AccessModifierDeclarations
 
+  # retrieve a Hash of transport schemas, keyed by their name.
+  # This uses the Puppet autoloader, provide a  environment name as `force_environment`
+  # to choose where to load from.
+  # @api private
+  def list_all_transports(force_environment)
+    env = Puppet.lookup(:environments).get!(force_environment)
+    Puppet.override({ current_environment: env }, 'current env for list_all_transports') do
+      load_all_schemas
+      Marshal.load(Marshal.dump(transports))
+    end
+  end
+  module_function :list_all_transports # rubocop:disable Style/AccessModifierDeclarations
+
+  # Loads all schemas using the Puppet Autoloader.
+  def self.load_all_schemas
+    require 'puppet'
+    require 'puppet/settings'
+    require 'puppet/util/autoload'
+    autoloader = Puppet::Util::Autoload.new(self, 'puppet/transport/schema')
+    autoloader.loadall(Puppet.lookup(:current_environment))
+  end
+  private_class_method :load_all_schemas
+
   def connect(name, connection_info)
     validate(name, connection_info)
     require "puppet/transport/#{name}"

spec/integration/resource_api/transport_spec.rb

@@ -0,0 +1,26 @@
+require 'spec_helper'
+
+RSpec.describe 'Resource API Transport integration tests:' do
+  after(:each) do
+    # reset registered transports between tests to reduce cross-test poisoning
+    Puppet::ResourceApi::Transport.instance_variable_set(:@transports, nil)
+  end
+
+  describe '#list_all_transports' do
+    subject(:transports) { Puppet::ResourceApi::Transport.list_all_transports('production') }
+
+    it 'loads all transports' do
+      expect(transports).to have_key 'test_device'
+      expect(transports).to have_key 'test_device_sensitive'
+      expect(transports['test_device']).to be_a Puppet::ResourceApi::TransportSchemaDef
+      expect(transports['test_device'].definition).to include(name: 'test_device')
+    end
+
+    it 'can be called twice' do
+      expect {
+        Puppet::ResourceApi::Transport.list_all_transports('production')
+        Puppet::ResourceApi::Transport.list_all_transports('production')
+      }.not_to raise_error
+    end
+  end
+end

spec/spec_helper.rb

@@ -46,6 +46,11 @@
   # override legacy default from puppetlabs_spec_helper
   config.mock_with :rspec
 
+  config.before(:suite) do
+    # set a modulepath to the fixtures directory
+    Puppet[:modulepath] = File.join(Dir.pwd, 'spec', 'fixtures', 'modules')
+  end
+
   # reset the warning suppression count
   config.before(:each) do
     Puppet::ResourceApi.warning_count = 0