Calling a callback from a native addon asynchronously

[justinmchase]

I have a native module that does some threading and calls a javascript callback asynchronously using libuv. It used to work in node but the same code in iojs now fails. Specifically Isolate::GetCurrent() is returning null.

Isolate* isolate = Isolate::GetCurrent();
Local<Context> context = Context::New(isolate); // throws
Context::Scope context_scope(context);

I'm using libuv to put the callback into the same thread as before, and again, this used to work in node. Does anyone have any suggestions about what may be different in iojs?

[trevnorris]

Have a small test case we can help you with?

[justinmchase]

Thanks for the reply. I have been working on getting the repro down to the simplest form and I believe it is not actually an iojs bug :)

It turns out we are actually using http://nwjs.io, which runs with a particular build of iojs. When I run my repro from the commandline iojs it works fine, from inside of nwjs it crashes in the async callback after a few milliseconds. Meaning it works for a little while but it seems like the Isolate or Context is being garbage collected out from under us.

But anyway, I do have another question. How are people actually building native modules for iojs? In node you would use node-gyp, but that doesn't appear to actually build iojs compatible binaries. I had to fork and alter node-gyp to get it to work with iojs so it leaves me wondering how others are actually building their modules.

Also, I'm wondering if I'm doing it wrong and it just happens to work from the command line version accidentally? Here is my code I would appreciate any feedback:

#include <node.h>
#include "async.h"

using namespace v8;

static Persistent<Function, CopyablePersistentTraits<Function>> _cb;
Async* _async;

void AsyncCallback(void* data)
{
    int x = *((int*)data);
    auto isolate = Isolate::GetCurrent();
    auto context = isolate->GetCurrentContext(); // crashes nwjs here
    auto global = context->Global();

    const int argc = 1;
    Handle<Value> argv[argc];
    argv[0] = Number::New(isolate, (double)x);

    auto fn = Local<Function>::New(isolate, _cb);
    fn->Call(global, argc, argv);
}

DWORD Worker(LPVOID lparam)
{
    int x = 0;
    while(true) 
    {
        _async->send(&x);
        x++;
        Sleep(1000);
    }
    return S_OK;
}

void Start(const FunctionCallbackInfo<Value>& args)
{
    auto isolate = Isolate::GetCurrent();

    Handle<Function> arg0 = Handle<Function>::Cast(args[0]);
    Persistent<Function> cb(isolate, arg0);
    _cb = cb;

    _async = new Async(&AsyncCallback);
    CreateThread(nullptr, 0, Worker, nullptr, 0, 0);
}

void init(Handle<Object> exports, Handle<Object> module)
{
    NODE_SET_METHOD(exports, "start", Start);
}

NODE_MODULE(evolve_overlay, init)

The Async class is very similar to this one:
https://github.com/mapbox/node-sqlite3/blob/master/src/async.h

Then to call it from iojs:

$ iojs
> require('./build/Debug/test').start(function (x) {
>    console.log(x)
> }

This works as expected, but the exact same module compiled with nw-gyp running in nwjs fails in the function AsyncCallback after a few milliseconds. I will follow up with them there but I'm especially wondering about the call to isolate-GetCurrentContext(). Is this the right way to get the isolate and context when coming from an async callback?

Thanks.

[justinmchase]

I managed to figure it out. After adding a fatal error handler:

isolate->SetFatalErrorHandler(&OnFatalError);

I managed to get this error out of it:

Cannot create a handle without a HandleScope

Which lead me to realize I needed to create a HandleScope in my async callback and that is what fixed it.

int x = *((int*)data);
auto isolate = Isolate::GetCurrent();
auto context = isolate->GetCurrentContext(); // crashes nwjs here
auto global = context->Global();

Should actually be:

int x = *((int*)data);
auto isolate = Isolate::GetCurrent();
HandleScope scope(isolate);
auto context = isolate->GetCurrentContext(); // no longer crashes
auto global = context->Global();

Thanks.

[trevnorris]

Ah yes. That'll get you every time. Can't forget those HandleScopes :)