string_bytes: Guarantee valid utf-8 output

Previously v8's WriteUtf8 function would produce invalid utf-8 output
when encountering unmatched surrogate code units [1]. The new
REPLACE_INVALID_UTF8 option fixes that by replacing invalid code points
with the unicode replacement character.

[1]: JS Strings are defined as arrays of 16 bit unsigned integers. There
is no unicode enforcement, so one can easily end up with invalid unicode
code unit sequences inside a string.

src/node.cc

@@ -176,6 +176,8 @@ static uv_async_t dispatch_debug_messages_async;
 // Declared in node_internals.h
 Isolate* node_isolate = NULL;
 
+int WRITE_UTF8_FLAGS = v8::String::HINT_MANY_WRITES_EXPECTED |
+                       v8::String::NO_NULL_TERMINATION;
 
 static void Spin(uv_idle_t* handle, int status) {
   assert((uv_idle_t*) handle == &tick_spinner);
@@ -3042,6 +3044,11 @@ static char **copy_argv(int argc, char **argv) {
 }
 
 int Start(int argc, char *argv[]) {
+  const char* replaceInvalid = getenv("NODE_INVALID_UTF8");
+
+  if (replaceInvalid == NULL)
+    WRITE_UTF8_FLAGS |= String::REPLACE_INVALID_UTF8;
+
   // Hack aroung with the argv pointer. Used for process.title = "blah".
   argv = uv_setup_args(argc, argv);
 

src/string_bytes.cc

@@ -199,7 +199,7 @@ size_t StringBytes::Write(char* buf,
       break;
 
     case UTF8:
-      len = str->WriteUtf8(buf, buflen, chars_written, flags);
+      len = str->WriteUtf8(buf, buflen, chars_written, WRITE_UTF8_FLAGS);
       break;
 
     case UCS2:

src/string_bytes.h

@@ -29,6 +29,8 @@
 
 namespace node {
 
+extern int WRITE_UTF8_FLAGS;
+
 using v8::Handle;
 using v8::Local;
 using v8::String;

test/simple/test-buffer.js

@@ -791,6 +791,18 @@ assert.equal(buf[3], 0xFF);
   assert.equal(buf[3], 0xFF);
 });
 
+// test unmatched surrogates not producing invalid utf8 output
+// ef bf bd = utf-8 representation of unicode replacement character
+// see https://codereview.chromium.org/121173009/
+buf = new Buffer('ab\ud800cd', 'utf8');
+assert.equal(buf[0], 0x61);
+assert.equal(buf[1], 0x62);
+assert.equal(buf[2], 0xef);
+assert.equal(buf[3], 0xbf);
+assert.equal(buf[4], 0xbd);
+assert.equal(buf[5], 0x63);
+assert.equal(buf[6], 0x64);
+
 // test for buffer overrun
 buf = new Buffer([0, 0, 0, 0, 0]); // length: 5
 var sub = buf.slice(0, 4);         // length: 4