Improve String::WriteAscii and add PRESERVE_ASCII_NULL option.

test/cctest/test-api.cc

Index: test/cctest/test-api.cc
diff --git a/test/cctest/test-api.cc b/test/cctest/test-api.cc
index 5a9df90e21fc8350c5e1d0dbc7bf5f8e8de9d1f4..26483e019c6a49071c465f217fec001dab26e2ee 100644
--- a/test/cctest/test-api.cc
+++ b/test/cctest/test-api.cc
@@ -5570,6 +5570,7 @@ THREADED_TEST(StringWrite) {
   v8::Handle<String> str = v8_str("abcde");
   // abc<Icelandic eth><Unicode snowman>.
   v8::Handle<String> str2 = v8_str("abc\303\260\342\230\203");
+  v8::Handle<String> str3 = v8::String::New("abc\0def", 7);
   const int kStride = 4;  // Must match stride in for loops in JS below.
   CompileRun(
       "var left = '';"
@@ -5780,6 +5781,28 @@ THREADED_TEST(StringWrite) {
   CHECK_NE(0, strcmp(utf8buf, "abc\303\260\342\230\203"));
   utf8buf[8] = '\0';
   CHECK_EQ(0, strcmp(utf8buf, "abc\303\260\342\230\203"));
+
+  memset(utf8buf, 0x1, sizeof(utf8buf));
+  utf8buf[5] = 'X';
+  len = str->WriteUtf8(utf8buf, sizeof(utf8buf), &charlen,
+                        String::NO_NULL_TERMINATION);
+  CHECK_EQ(5, len);
+  CHECK_EQ('X', utf8buf[5]);  // Test that the sixth character is untouched.
+  CHECK_EQ(5, charlen);
+  utf8buf[5] = '\0';
+  CHECK_EQ(0, strcmp(utf8buf, "abcde"));
+
+  memset(buf, 0x1, sizeof(buf));
+  len = str3->WriteAscii(buf);
+  CHECK_EQ(7, len);
+  CHECK_EQ(0, strcmp("abc def", buf));
+
+  memset(buf, 0x1, sizeof(buf));
+  len = str3->WriteAscii(buf, 0, -1, String::PRESERVE_ASCII_NULL);
+  CHECK_EQ(7, len);
+  CHECK_EQ(0, strcmp("abc", buf));
+  CHECK_EQ(0, buf[3]);
+  CHECK_EQ(0, strcmp("def", buf + 4));
 }