Fail when another window manager is already running

.travis.yml

@@ -6,7 +6,7 @@ rvm:
   - 2.1
 before_install:
   - sudo apt-get update -qq
-  - sudo apt-get install -qq xdotool
+  - sudo apt-get install -qq twm xdotool
 matrix:
   allow_failures:
     - rvm: ruby-head

features/manager/check_other_wm.feature

@@ -0,0 +1,8 @@
+Feature: checking if another window manager is running
+
+  @other_wm_running
+  Scenario: fails when another window manager is running
+    Given another window manager is running
+    When I start uhwm
+    Then the exit status must be 70
+    And the current output must match /error.+other.+window.+manager/i

features/steps/output_steps.rb

@@ -2,7 +2,7 @@ def uhwm_wait_output message, timeout: 1
   Timeout.timeout(timeout) do
     loop do
       break if case message
-        when Regexp then @process.read_stdout =~ message
+        when Regexp then @process.stdout + @process.stderr =~ message
         when String then assert_partial_output_interactive message
       end
       sleep 0.1
@@ -28,6 +28,10 @@ options:
   eoh
 end
 
+Then /^the output must match \/([^\/]+)\/([a-z]*)$/ do |pattern, options|
+  expect(@process.stdout).to match Regexp.new(pattern, options)
+end
+
 Then /^the current output must match \/([^\/]+)\/([a-z]*)$/ do |pattern, options|
   uhwm_wait_output Regexp.new(pattern, options)
 end

features/steps/run_steps.rb

@@ -9,6 +9,10 @@ def uhwm_run_wait_ready
   uhwm_wait_output 'Connected to'
 end
 
+Given /^another window manager is running$/ do
+  expect(@other_wm).to be_alive
+end
+
 Given /^uhwm is running$/ do
   uhwm_run_wait_ready
 end

features/support/env.rb

@@ -14,3 +14,10 @@ Headless.new.start
 After do |scenario|
   @process and @process.terminate
 end
+
+Around '@other_wm_running' do |scenario, block|
+  @other_wm = ChildProcess.build('twm')
+  @other_wm.start
+  block.call
+  @other_wm.stop
+end

lib/uh/wm.rb

@@ -14,5 +14,6 @@ module Uh
     Error               = Class.new(StandardError)
     RuntimeError        = Class.new(RuntimeError)
     ArgumentError       = Class.new(Error)
+    OtherWMRunningError = Class.new(RuntimeError)
   end
 end

lib/uh/wm/manager.rb

@@ -1,6 +1,8 @@
 module Uh
   module WM
     class Manager
+      INPUT_MASK = Events::SUBSTRUCTURE_REDIRECT_MASK
+
       attr_reader :display
 
       def initialize events, display = Display.new
@@ -11,6 +13,13 @@ module Uh
       def connect
         @events.emit :connecting, args: @display
         @display.open
+        Display.on_error do
+          fail OtherWMRunningError, 'another window manager is already running'
+        end
+        @display.listen_events INPUT_MASK
+        @display.sync false
+        Display.on_error { |*args| handle_error *args }
+        @display.sync false
         @events.emit :connected, args: @display
       end
 
@@ -27,6 +36,13 @@ module Uh
         when :key_press then @events.emit :key, event.key.to_sym
         end
       end
+
+
+      private
+
+      def handle_error *args
+        @dispatcher.emit :error, args: args
+      end
     end
   end
 end

spec/uh/wm/manager_spec.rb

@@ -11,14 +11,11 @@ module Uh
         end
       end
 
-      describe '#connect' do
+      describe '#connect', :xvfb do
         let(:block) { proc { } }
 
-        # Prevent Manager to open a real display.
-        before { allow(display).to receive :open }
-
         it 'opens the display' do
-          expect(manager.display).to receive :open
+          expect(manager.display).to receive(:open).and_call_original
           manager.connect
         end