Fail with an error on Ruby 1.8 when posix-spawn is not present.

lib/foreman/cli.rb

@@ -34,6 +34,7 @@ class Foreman::CLI < Thor
   end
 
   def start(process=nil)
+    require_posix_spawn_for_ruby_18!
     check_procfile!
     load_environment!
     engine.load_procfile(procfile)
@@ -137,6 +138,14 @@ private ######################################################################
     end
   end
 
+  def require_posix_spawn_for_ruby_18!
+    begin
+      Kernel.require 'posix/spawn' # Use Kernel explicitly so we can mock the require call in the spec
+    rescue LoadError
+      error "foreman requires gem `posix-spawn` on Ruby #{RUBY_VERSION}. Please `gem install posix-spawn`."
+    end if Foreman.ruby_18?
+  end
+
   def procfile
     case
       when options[:procfile] then options[:procfile]

spec/foreman/cli_spec.rb

@@ -100,4 +100,12 @@ describe "Foreman::CLI", :fakefs do
     end
   end
 
+  describe "when posix-spawn is not present on ruby 1.8" do
+    it "should fail with an error" do
+      mock(Kernel).require('posix/spawn') { raise LoadError }
+      output = foreman("start -f #{resource_path("Procfile")}")
+      output.should == "ERROR: foreman requires gem `posix-spawn` on Ruby #{RUBY_VERSION}. Please `gem install posix-spawn`.\n"
+    end
+  end if running_ruby_18?
+
 end