Truncate an existing file in "w", "w+" modes.

lib/fakefs/file.rb

@@ -109,6 +109,7 @@ module FakeFS
       @open = true
 
       file_creation_mode? ? create_missing_file : check_file_existence!
+      truncate_file if truncation_mode?
     end
 
     def close
@@ -177,5 +178,13 @@ module FakeFS
     def write_only?
       @mode == WRITE_ONLY || @mode == APPEND_WRITE_ONLY
     end
+
+    def truncate_file
+      @file.content = ""
+    end
+
+    def truncation_mode?
+      @mode == READ_WRITE_TRUNCATE || @mode == WRITE_ONLY
+    end
   end
 end

test/fakefs_test.rb

@@ -156,6 +156,22 @@ class FakeFSTest < Test::Unit::TestCase
     end
   end
 
+  def test_file_in_write_mode_truncates_existing_file
+    File.open("foo", "w") { |f| f << "contents" }
+
+    f = File.open("foo", "w")
+
+    assert_equal "", File.read("foo")
+  end
+
+  def test_file_in_read_write_truncation_mode_truncates_file
+    File.open("foo", "w") { |f| f << "foo" }
+
+    f = File.open("foo", "w+")
+
+    assert_equal "", File.read("foo")
+  end
+
   def test_file_in_append_write_only_raises_error_when_reading
     FileUtils.touch("foo")