Introduce strategy for runner behavior

The runner model is only a class responsible for storing information
now. Based on the configuration it picks a strategy for the runner
management. The Poseidon strategy is already implemented and tested.
The Docker strategy will follow.

lib/runner/strategy/poseidon.rb

@@ -0,0 +1,103 @@
+# frozen_string_literal: true
+
+class Runner::Strategy::Poseidon < Runner::Strategy
+  HEADERS = {'Content-Type' => 'application/json'}.freeze
+  ERRORS = %w[NOMAD_UNREACHABLE NOMAD_OVERLOAD NOMAD_INTERNAL_SERVER_ERROR UNKNOWN].freeze
+
+  ERRORS.each do |error|
+    define_singleton_method :"error_#{error.downcase}" do
+      error
+    end
+  end
+
+  def self.request_from_management(environment)
+    url = "#{Runner::BASE_URL}/runners"
+    body = {executionEnvironmentId: environment.id, inactivityTimeout: Runner::UNUSED_EXPIRATION_TIME}
+    response = Faraday.post(url, body.to_json, HEADERS)
+
+    case response.status
+      when 200
+        response_body = parse response
+        runner_id = response_body[:runnerId]
+        runner_id.presence || raise(Runner::Error::Unknown.new('Poseidon did not send a runner id'))
+      when 404
+        raise Runner::Error::NotFound.new('Execution environment not found')
+      else
+        handle_error response
+    end
+  end
+
+  def self.handle_error(response)
+    case response.status
+      when 400
+        response_body = parse response
+        raise Runner::Error::BadRequest.new(response_body[:message])
+      when 401
+        raise Runner::Error::Unauthorized.new('Authentication with Poseidon failed')
+      when 404
+        raise Runner::Error::NotFound.new('Runner not found')
+      when 500
+        response_body = parse response
+        error_code = response_body[:errorCode]
+        if error_code == error_nomad_overload
+          raise Runner::Error::NotAvailable.new("Poseidon has no runner available (#{error_code}): #{response_body[:message]}")
+        else
+          raise Runner::Error::InternalServerError.new("Poseidon sent #{response_body[:errorCode]}: #{response_body[:message]}")
+        end
+      else
+        raise Runner::Error::Unknown.new("Poseidon sent unexpected response status code #{response.status}")
+    end
+  end
+
+  def self.parse(response)
+    JSON.parse(response.body).deep_symbolize_keys
+  rescue JSON::ParserError => e
+    # Poseidon should not send invalid json
+    raise Runner::Error::Unknown.new("Error parsing response from Poseidon: #{e.message}")
+  end
+
+  def copy_files(files)
+    url = "#{runner_url}/files"
+    body = {copy: files.map {|filename, content| {path: filename, content: Base64.strict_encode64(content)} }}
+    response = Faraday.patch(url, body.to_json, HEADERS)
+    self.class.handle_error response unless response.status == 204
+  end
+
+  def attach_to_execution(command)
+    starting_time = Time.zone.now
+    websocket_url = execute_command(command)
+    EventMachine.run do
+      socket = Runner::Connection.new(websocket_url)
+      yield(socket) if block_given?
+    end
+    Time.zone.now - starting_time # execution duration
+  end
+
+  def destroy_at_management
+    response = Faraday.delete runner_url
+    self.class.handle_error response unless response.status == 204
+  end
+
+  private
+
+  def execute_command(command)
+    url = "#{runner_url}/execute"
+    body = {command: command, timeLimit: @execution_environment.permitted_execution_time}
+    response = Faraday.post(url, body.to_json, HEADERS)
+    if response.status == 200
+      response_body = self.class.parse response
+      websocket_url = response_body[:websocketUrl]
+      if websocket_url.present?
+        return websocket_url
+      else
+        raise Runner::Error::Unknown.new('Poseidon did not send websocket url')
+      end
+    end
+
+    self.class.handle_error response
+  end
+
+  def runner_url
+    "#{Runner::BASE_URL}/runners/#{@runner_id}"
+  end
+end