Add null strategy for runners

* This is the default strategy used when the runner management is disabled. It might be replaced with a generic Docker strategy in the future (without pooling). For now, it allows normal "operation" of CodeOcean without any runner management. However, as no runner system is configured, no command can be executed.

app/models/runner.rb

@@ -11,10 +11,12 @@ class Runner < ApplicationRecord
   attr_accessor :strategy
 
   def self.strategy_class
-    @strategy_class ||= begin
-      strategy_name = CodeOcean::Config.new(:code_ocean).read[:runner_management][:strategy]
-      "runner/strategy/#{strategy_name}".camelize.constantize
-    end
+    @strategy_class ||= if Runner.management_active?
+                          strategy_name = CodeOcean::Config.new(:code_ocean).read[:runner_management][:strategy]
+                          "runner/strategy/#{strategy_name}".camelize.constantize
+                        else
+                          Runner::Strategy::Null
+                        end
   end
 
   def self.management_active?

lib/runner/strategy/null.rb

@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+# This strategy allows normal operation of CodeOcean even when the runner management is disabled.
+# However, as no command can be executed, all execution requests will fail.
+class Runner::Strategy::Null < Runner::Strategy
+  def self.initialize_environment; end
+
+  def self.sync_environment(_environment); end
+
+  def self.request_from_management(_environment)
+    SecureRandom.uuid
+  end
+
+  def destroy_at_management; end
+
+  def copy_files(_files); end
+
+  def attach_to_execution(command, event_loop)
+    socket = Connection.new(nil, self, event_loop)
+    # We don't want to return an error if the execution environment is changed
+    socket.status = :terminated_by_codeocean if command == ExecutionEnvironment::VALIDATION_COMMAND
+    yield(socket)
+    socket
+  end
+
+  def self.available_images
+    []
+  end
+
+  def self.config; end
+
+  def self.release
+    'N/A'
+  end
+
+  def self.pool_size
+    {}
+  end
+
+  def self.websocket_header
+    {}
+  end
+
+  class Connection < Runner::Connection
+    def decode(event_data)
+      event_data
+    end
+
+    def encode(data)
+      data
+    end
+
+    def active?
+      false
+    end
+  end
+end