WebAssembly.instantiate didn't call then nor catch in v8 embedded

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Jian Guo

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Jun 3, 2019, 11:29:36 AM6/3/19
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I tried to ship WebAssembly feature in v8 7.2 for my Android Project. I have successfully imported v8 as a static library. But I came across an issue that WebAssembly didn't call either then nor catch callback. Here is my code below:


std::unique_ptr<v8::Platform> platform;
v8::Isolate *isolate;
v8::Persistent<v8::Context> persistentContext;
void runMain();
void runScript();
void _log(const v8::FunctionCallbackInfo<v8::Value>& info) {
  v8::String::Utf8Value utf(isolate, info[0].As<v8::String>());
  __android_log_print(ANDROID_LOG_DEBUG, "V8Native", "%s",*utf);
}

void JNICALL
Java_com_hustunique_v8demoapplication_MainActivity_initV8(JNIEnv *env, jobject /* this */) {
  // Initialize V8.
  v8::V8::InitializeICU();
  platform = v8::platform::NewDefaultPlatform();
  v8::V8::InitializePlatform(&(*platform.get()));
  v8::V8::Initialize();
  runMain();
}

void runMain() {
  // Create a new Isolate and make it the current one.

  v8::Isolate::CreateParams create_params;
  create_params.array_buffer_allocator = v8::ArrayBuffer::Allocator::NewDefaultAllocator();
  isolate = v8::Isolate::New(create_params);
//  isolate->Enter();
  v8::Isolate::Scope isolate_scope(isolate);
  v8::HandleScope scope(isolate);


  auto global_template = v8::ObjectTemplate::New(isolate);
  global_template->Set(v8::String::NewFromUtf8(isolate, "log"), v8::FunctionTemplate::New(isolate, _log));   // set log function here, as it is used in my sample javascript code
  // Enter the context for compiling and running the sample script.
  v8::Local<v8::Context> context = v8::Context::New(isolate, nullptr, global_template);
  persistentContext.Reset(isolate, context);

  // Run the script to get the result.
  runScript();

}

void runScript() {
  // sample wasm javascript code here.
  const char *csource = R"(
    WebAssembly.instantiate(new Uint8Array([0,97,115,109,1,0,0,0,1,8,2,96,1,127,0,96,0,0,2,8,1,2,106,
      115,1,95,0,0,3,2,1,1,8,1,1,10,9,1,7,0,65,185,10,16,0,11]),
      {js:{_:console.log('Called from WebAssembly Hello world')}}).then(function(obj) {
        log('Called with instance ' + obj);
      }).catch(function(err) {
        log('Called with error ' + err);
      });
  )"; // should call my Hello World log and trigger the error or return the instance successfully

  v8::HandleScope handle_scope(isolate);
  auto ctx = persistentContext.Get(isolate);
  v8::Context::Scope context_scope(ctx);
  v8::TryCatch try_catch(isolate);
  v8::Local<v8::String> source = v8::String::NewFromUtf8(isolate, csource,
                                                         v8::NewStringType::kNormal).ToLocalChecked();

  v8::Local<v8::Script> script =
      v8::Script::Compile(ctx, source).ToLocalChecked();
  v8::Local<v8::Value> result;
  if (!script->Run(ctx).ToLocal(&result)) {
    ReportException(isolate, &try_catch); // report exception, ignore the implementation here
    return;
  }
  // Convert the result to an UTF8 string and print it.
  v8::String::Utf8Value utf8(isolate, result);
  __android_log_print(ANDROID_LOG_INFO, "V8Native", "%s\n", *utf8);

}



In the demo above, I got the output with Called from WebAssembly Hello world as excepted, but I couldn't get the error message nor the instance info.

I made a simple example on the website compared with my demo above, here is the output in the website, which can be reproduced easily I think:


Called from WebAssembly Hello world

Called with error LinkError: WebAssembly.instantiate(): Import #0 module="js" function="_" error: function import requires a callable



It seems that in my demo, neither resolve nor reject was called from WebAssembly's returning promise. After checking the type of v8::Local<v8::Value> result in runScript method, v8 runtime confirms that it is a promise object.

I have tried several things here but none of them works:

  1. 1. call v8::Isolate::RunMicroTasks(). Nothing happened
  2. 2. cast result to v8::Local<v8::Promise> at the end of runScript method, then run with:

  4. auto resolver = v8::Resolver::New(context)->toLocalChecked();
    while (promise->State() == v8::PromiseState::kPending) {
        isolate->RunMicroTasks();
    }
    if (promise->State() == v8::PromiseState::kFullfilled) {
        resolver->Resolve(context, promise->Result());
    }
    if (promise->State() == v8::PromiseState::kRejected) {
        resolver->Reject(context, promise->Result());
    }

This snippet didn't work either, besides, it stuck at the kPending status.

I searched for something like flush the promise queue, but didn't get any solutions. What am I missing here?

aimaomao wo

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Dec 31, 2019, 4:19:11 AM12/31/19
to v8-users
I also encountered the same problem in v6.5 version, did you find the answer

在 2019年6月3日星期一 UTC+8下午11:29:36，Jian Guo写道：

Darin Dimitrov

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Feb 21, 2020, 6:01:28 AM2/21/20
to v8-users
I had the same problem in my Android project and used a workaround. I replaced the global.WebAssembly object with a Proxy and intercept the "compile" and "instantiate" method calls. When this method is called I start a new background thread which is polling the main looper to pump messages and run the pending microtasks. When the promise is resolved the background thread is destroyed.

Clemens Backes

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Feb 21, 2020, 6:06:21 AM2/21/20
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