v8::String::Concat must not throw.

R=dcarney@chromium.org
BUG=chromium:420240
LOG=Y
API=v8::String::Concat may return empty handle on overflow.

Review URL: https://codereview.chromium.org/735763002

Cr-Commit-Position: refs/heads/master@{#25402}

src/api.cc

@@ -5550,7 +5550,11 @@ Local<String> v8::String::Concat(Handle<String> left, Handle<String> right) {
   LOG_API(isolate, "String::New(char)");
   ENTER_V8(isolate);
   i::Handle<i::String> right_string = Utils::OpenHandle(*right);
-  // We do not expect this to fail. Change this if it does.
+  // If we are steering towards a range error, do not wait for the error to be
+  // thrown, and return the null handle instead.
+  if (left_string->length() + right_string->length() > i::String::kMaxLength) {
+    return Local<String>();
+  }
   i::Handle<i::String> result = isolate->factory()->NewConsString(
       left_string, right_string).ToHandleChecked();
   return Utils::ToLocal(result);

test/cctest/test-api.cc

@@ -742,23 +742,28 @@ THREADED_TEST(UsingExternalOneByteString) {
 }
 
 
-class DummyResource : public v8::String::ExternalStringResource {
+class RandomLengthResource : public v8::String::ExternalStringResource {
  public:
+  explicit RandomLengthResource(int length) : length_(length) {}
   virtual const uint16_t* data() const { return string_; }
-  virtual size_t length() const { return 1 << 30; }
+  virtual size_t length() const { return length_; }
 
  private:
   uint16_t string_[10];
+  int length_;
 };
 
 
-class DummyOneByteResource : public v8::String::ExternalOneByteStringResource {
+class RandomLengthOneByteResource
+    : public v8::String::ExternalOneByteStringResource {
  public:
+  explicit RandomLengthOneByteResource(int length) : length_(length) {}
   virtual const char* data() const { return string_; }
-  virtual size_t length() const { return 1 << 30; }
+  virtual size_t length() const { return length_; }
 
  private:
   char string_[10];
+  int length_;
 };
 
 
@@ -767,7 +772,7 @@ THREADED_TEST(NewExternalForVeryLongString) {
     LocalContext env;
     v8::HandleScope scope(env->GetIsolate());
     v8::TryCatch try_catch;
-    DummyOneByteResource r;
+    RandomLengthOneByteResource r(1 << 30);
     v8::Local<v8::String> str = v8::String::NewExternal(CcTest::isolate(), &r);
     CHECK(str.IsEmpty());
     CHECK(try_catch.HasCaught());
@@ -779,7 +784,7 @@ THREADED_TEST(NewExternalForVeryLongString) {
     LocalContext env;
     v8::HandleScope scope(env->GetIsolate());
     v8::TryCatch try_catch;
-    DummyResource r;
+    RandomLengthResource r(1 << 30);
     v8::Local<v8::String> str = v8::String::NewExternal(CcTest::isolate(), &r);
     CHECK(str.IsEmpty());
     CHECK(try_catch.HasCaught());
@@ -24232,3 +24237,17 @@ TEST(InvalidCacheData) {
   TestInvalidCacheData(v8::ScriptCompiler::kConsumeParserCache);
   TestInvalidCacheData(v8::ScriptCompiler::kConsumeCodeCache);
 }
+
+
+TEST(StringConcatOverflow) {
+  v8::V8::Initialize();
+  v8::HandleScope scope(CcTest::isolate());
+  RandomLengthOneByteResource* r =
+      new RandomLengthOneByteResource(i::String::kMaxLength);
+  v8::Local<v8::String> str = v8::String::NewExternal(CcTest::isolate(), r);
+  CHECK(!str.IsEmpty());
+  v8::TryCatch try_catch;
+  v8::Local<v8::String> result = v8::String::Concat(str, str);
+  CHECK(result.IsEmpty());
+  CHECK(!try_catch.HasCaught());
+}