Rework exit status capturing

example_app_generator/spec/support/default_preview_path

@@ -65,7 +65,7 @@ require_file_stub 'config/environment' do
   Rails.application.initialize!
 end
 
-exit if ENV['NO_ACTION_MAILER']
+exit(0) if ENV['NO_ACTION_MAILER']
 if ENV['DEFAULT_URL']
   puts ActionMailer::Base.default_url_options[:host]
 elsif defined?(::ActionMailer::Preview)
@@ -79,3 +79,4 @@ end
 # This will force the loading of ActionMailer settings to ensure we do not
 # accidentally set something we should not
 ActionMailer::Base.smtp_settings
+exit 0

example_app_generator/spec/verify_mailer_preview_path_spec.rb

@@ -17,9 +17,17 @@ def as_commandline(ops)
 
   def capture_exec(*ops)
     ops << { err: [:child, :out] }
-    io = IO.popen(ops)
+    lines = []
+
+    _process =
+      IO.popen(ops) do |io|
+        while (line = io.gets)
+          lines << line
+        end
+      end
+
     # Necessary to ignore warnings from Rails code base
-    out =  io.readlines
+    out = lines
               .reject { |line| line =~ /warning: circular argument reference/ }
               .reject { |line| line =~ /Gem::Specification#rubyforge_project=/ }
               .reject { |line| line =~ /DEPRECATION WARNING/ }