(PUP-11328) Get the environment's name without loading the environment

The previous commit added back the node request as a fallback, but it was
calling `Node#environment`, which will try to load the environment from disk.
Since the server-assigned environment likely doesn't exist on the agent, the run
printed the following warning and continued using "production"

    Warning: Could not find a directory environment named 'xxx' anywhere in the
    path ... Does the directory exist?

To complicate matters, node termini set the environment on the node differently.
The :rest terminus returns a node with an environment_name, but not an instance.
The :plain terminus returns a node with an environment instance, but not an
environment_name. To avoid breaking things, retrieve the environment name the
same way we did before[1].

[1] https://github.com/puppetlabs/puppet/blob/6.24.0/lib/puppet/configurer.rb#L315-L319

lib/puppet/configurer.rb

@@ -513,7 +513,17 @@ def current_server_specified_environment(current_environment, configured_environ
                                              :transaction_uuid => @transaction_uuid,
                                              :fail_on_404 => true)
 
-        @server_specified_environment = node.environment.name.to_s
+        # The :rest node terminus returns a node with an environment_name, but not an
+        # environment instance. Attempting to get the environment instance will load
+        # it from disk, which will likely fail. So create a remote environment.
+        #
+        # The :plain node terminus returns a node with an environment, but not an
+        # environment_name.
+        if !node.has_environment_instance? && node.environment_name
+          node.environment = Puppet::Node::Environment.remote(node.environment_name)
+        end
+
+        @server_specified_environment = node.environment.to_s
 
         if @server_specified_environment != @environment
           Puppet.notice _("Local environment: '%{local_env}' doesn't match server specified node environment '%{node_env}', switching agent to '%{node_env}'.") % { local_env: @environment, node_env: @server_specified_environment }

spec/integration/application/agent_spec.rb

@@ -643,7 +643,14 @@ def with_another_agent_running(&block)
 
   context "environment convergence" do
     it "falls back to making a node request if the last server-specified environment cannot be loaded" do
-      server.start_server do |port|
+      mounts = {}
+      mounts[:node] = -> (req, res) {
+        node = Puppet::Node.new('test', environment: Puppet::Node::Environment.remote('doesnotexistonagent'))
+        res.body = formatter.render(node)
+        res['Content-Type'] = formatter.mime
+      }
+
+      server.start_server(mounts: mounts) do |port|
         Puppet[:serverport] = port
         Puppet[:log_level] = 'debug'
 

spec/unit/configurer_spec.rb

@@ -2,6 +2,8 @@
 require 'puppet/configurer'
 
 describe Puppet::Configurer do
+  include PuppetSpec::Files
+
   before do
     Puppet[:server] = "puppetmaster"
     Puppet[:report] = true
@@ -10,6 +12,17 @@
     allow_any_instance_of(described_class).to(
       receive(:valid_server_environment?).and_return(true)
     )
+
+    Puppet[:lastrunfile] = file_containing('last_run_summary.yaml', <<~SUMMARY)
+    ---
+    version:
+      config: 1624882680
+      puppet: #{Puppet.version}
+    application:
+      initial_environment: #{Puppet[:environment]}
+      converged_environment: #{Puppet[:environment]}
+      run_mode: agent
+    SUMMARY
   end
 
   let(:node_name) { Puppet[:node_name_value] }
@@ -1099,7 +1112,6 @@ def expects_neither_new_or_cached_catalog
   end
 
   describe "when selecting an environment" do
-    include PuppetSpec::Files
     include PuppetSpec::Settings
 
     describe "when the last used environment is available" do
@@ -1205,10 +1217,12 @@ def expects_neither_new_or_cached_catalog
     describe "when the last used environment is not available" do
       describe "when the node request succeeds" do
         let(:node_environment) { Puppet::Node::Environment.remote(:salam) }
-        let(:node) { double(Puppet::Node, :environment => node_environment) }
+        let(:node) { Puppet::Node.new(Puppet[:node_name_value]) }
         let(:last_server_specified_environment) { 'development' }
 
         before do
+          node.environment = node_environment
+
           allow(Puppet::Node.indirection).to receive(:find)
           allow(Puppet::Node.indirection).to receive(:find)
             .with(anything, hash_including(:ignore_cache => true, :fail_on_404 => true))