Actually it turns out it was my error, misusing HandleScope. It turns out
the function that constructed the callback data had a HandleScope and the
returned handle didn't survive the scope closure.
I think the Embedder's Guide should go into more detail about the design of
HandleScope as it seems critical to understanding how to use the library
properly. In particular, you need to be very careful that any handle you
construct doesn't outlive the HandleScope it is constructed in, and how
EscapableHandleScope solves the problem of wanting to return a handle from
a function that has a HandleScope.
-- ryan
On Monday, May 16, 2016 at 3:22:34 PM UTC-7, Ryan Ingram wrote:
>
> What am I doing wrong here? Does FunctionTemplate only support primitives
> for callback data? (Its type should suggest that, if so...)
>
> I'm trying to register native functions to be called back in my embedder,
> but my function callback data is getting corrupted. I tried saving off the
> data passed to FunctionTemplate::New and I'm not getting the same data in
> my callback!
>
> Here's a code snippet:
>
> v8::Persistent<v8::Value> gDebugData;
>
> void RegisterTestFunction( v8::Handle<v8::String> name, v8::Isolate*
> isolate, v8::Context* context )
> {
> v8::Local<v8::Object> data = v8::Object::New(isolate); // In the
> actual code this is SomeFunctionTemplate->InstanceTemplate()->NewInstance()
> gDebugData.Reset(isolate, data); // save off pointer for debugging
>
> v8::Local<v8::FunctionTemplate> template =
> FunctionTemplate::New(isolate, &TestFunctionCallback, data);
> context->Enter();
> context->Global()->Set( name, template->GetFunction() );
> context->Exit();
> }
>
> void TestFunctionCallback(const v8::FunctionCallbackInfo<v8::Value>
> &arguments)
> {
> v8::Isolate* isolate = arguments.GetIsolate();
>
> // Let's get our context
> v8::Local<v8::Value> dataValue = arguments.Data();
> assert( dataValue->IsObject() ); // Fails??
>
> // Hmm, let's look at our original context variable...
> v8::Local<v8::Value> expectedDataValue = v8::Local<v8::Value>::New(
> isolate, gDebugData );
> assert( dataValue == expectedDataValue ); // Also fails...
>
> assert( expectedDataValue->IsObject() ); // The original pointer is
> still valid...
>
> // Lets find out what we are getting passed
> v8::TryCatch tryCatch;
>
> // This function crashes...
> v8::Local<v8::String> whatIsIt = dataValue->ToString();
>
> if(tryCatch.HasCaught() { tryCatch.ReThrow(); return; }
> }
>
> Crash callstack:
> 3eb50f46()
> 3eb483e1()
> 3eb2fa55()
> 3eb2b8ea()
> v8.dll!v8::internal::Invoke(bool is_construct,
> v8::internal::Handle<v8::internal::JSFunction> function,
> v8::internal::Handle<v8::internal::Object> receiver, int argc,
> v8::internal::Handle<v8::internal::Object> * args) Line 108 C++
> v8.dll!v8::internal::Execution::Call(v8::internal::Isolate * isolate,
> v8::internal::Handle<v8::internal::Object> callable,
> v8::internal::Handle<v8::internal::Object> receiver, int argc,
> v8::internal::Handle<v8::internal::Object> * argv, bool convert_receiver)
> Line 157 + 0x13 bytes C++
> v8.dll!v8::internal::Execution::ToString(v8::internal::Isolate *
> isolate, v8::internal::Handle<v8::internal::Object> obj) Line 517 + 0x95
> bytes C++
> v8.dll!v8::Value::ToString() Line 2594 + 0x9 bytes C++
>
> If I use "expectedDataValue" in my callback, everything works as desired.
> But that's not getting passed through to the FunctionCallbackInfo.
>
> I am on a relatively old version of v8 (3.30). Is there a way to tell
> what is inside of dataValue's handle more safely? In the debugger it has a
> pointer value, although it doesn't look particularly valid...
>
> dataValue
> v8::Handle<v8::Value> {val_=0x0023d608 }
> expectedDataValue
> v8::Handle<v8::Value> {val_=0x128daab0 }
> gDebugData
> v8::PersistentBase<v8::Value> {val_=0x128d6a90 }
>
--
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