Raise an Errno::ENOENT error when opening a file which does not exist

lib/fakefs/file.rb

@@ -107,6 +107,8 @@ module FakeFS
       @mode = mode
       @file = FileSystem.find(path)
       @open = true
+
+      check_file_existence! if read_only?
     end
 
     def close
@@ -145,6 +147,12 @@ module FakeFS
 
   private
 
+    def check_file_existence!
+      unless @file
+        raise Errno::ENOENT, "No such file or directory - #{@file}"
+      end
+    end
+
     def read_only?
       @mode == READ_ONLY
     end

test/fakefs_test.rb

@@ -114,6 +114,12 @@ class FakeFSTest < Test::Unit::TestCase
     end
   end
 
+  def test_raises_error_when_cannot_find_file_in_read_mode
+    assert_raises(Errno::ENOENT) do
+      File.open("does_not_exist", "r")
+    end
+  end
+
   def test_can_read_files_once_written
     path = '/path/to/file.txt'
     File.open(path, 'w') do |f|