Fail when another window manager is already running

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