src: export InitializeV8Platform function

[jc-lab]

Export InitializeV8Platform to allow use node as a shared library.

For example:
https://github.com/jc-lab/node-app

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• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

[addaleax]

LGTM

However, I would very much encourage using CreatePlatform()/FreePlatform() rather than using the global instance.

[jc-lab]

@addaleax Thanks for comment.

I used CreatePlatform. However, the call to GetMainThreadMultiIsolatePlatform (Call from NewIsolate) used by worker_threads fails.

Initialization of per_process::v8_platform seems to only happen on InitializeV8Platform.

node/src/api/environment.cc

Lines 353 to 355 in 2bdeb88

	MultiIsolatePlatform* GetMainThreadMultiIsolatePlatform() {
	return per_process::v8_platform.Platform();
	}

<=

node/src/node_v8_platform-inl.h

Lines 83 to 99 in 2bdeb88

	inline void Initialize(int thread_pool_size) {
	tracing_agent_ = std::make_unique<tracing::Agent>();
	node::tracing::TraceEventHelper::SetAgent(tracing_agent_.get());
	node::tracing::TracingController* controller =
	tracing_agent_->GetTracingController();
	trace_state_observer_ =
	std::make_unique<NodeTraceStateObserver>(controller);
	controller->AddTraceStateObserver(trace_state_observer_.get());
	tracing_file_writer_ = tracing_agent_->DefaultHandle();
	// Only start the tracing agent if we enabled any tracing categories.
	if (!per_process::cli_options->trace_event_categories.empty()) {
	StartTracingAgent();
	}
	// Tracing must be initialized before platform threads are created.
	platform_ = new NodePlatform(thread_pool_size, controller);
	v8::V8::InitializePlatform(platform_);
	}

[bnoordhuis]

Export InitializeV8Platform to allow use node as a shared library.

You already can with node::CreatePlatform() because conceptually all InitializeV8Platform() does is this:

auto platform = node::CreatePlatform(thread_pool_size, some_stock_tracing_controller);
v8::V8::InitializePlatform(platform);
return platform;

I don't see a good reason to export it. Can you elaborate on what it enables that you can't do with node::CreatePlatform()?

edit: comments just crossed.

[jc-lab]

@bnoordhuis See the comments above.

[bnoordhuis]

Initialization of per_process::v8_platform seems to only happen on InitializeV8Platform.

Right, so what is probably missing is an API to call V8Platform::Initialize() with a custom MultiIsolatePlatform.

(I won't block this PR but it seems like the wrong approach.)

[addaleax]

I used CreatePlatform. However, the call to GetMainThreadMultiIsolatePlatform (Call from NewIsolate) used by worker_threads fails.

@jc-lab That sounds an awful lot like the bug that was fixed by 6db45bf … what version of Node.js are you using?

Initialization of per_process::v8_platform seems to only happen on InitializeV8Platform.

Right, so what is probably missing is an API to call V8Platform::Initialize() with a custom MultiIsolatePlatform.

I’d prefer moving away from the global V8Platform object altogether, rather than exposing even more hooks into it.

[jc-lab]

@addaleax Oops...
I didn't know it was fixed. I used node version 12.4.
Thanks for letting me know.

[jc-lab]

I have a question about node::CreateEnvironment.

node/src/api/environment.cc

Lines 301 to 322 in 2551a21

	Environment* CreateEnvironment(IsolateData* isolate_data,
	Local<Context> context,
	int argc,
	const char* const* argv,
	int exec_argc,
	const char* const* exec_argv) {
	Isolate* isolate = context->GetIsolate();
	HandleScope handle_scope(isolate);
	Context::Scope context_scope(context);
	// TODO(addaleax): This is a much better place for parsing per-Environment
	// options than the global parse call.
	std::vector<std::string> args(argv, argv + argc);
	std::vector<std::string> exec_args(exec_argv, exec_argv + exec_argc);
	// TODO(addaleax): Provide more sensible flags, in an embedder-accessible way.
	Environment* env = new Environment(
	isolate_data,
	context,
	args,
	exec_args,
	static_cast<Environment::Flags>(Environment::kIsMainThread |
	Environment::kOwnsProcessState |
	Environment::kOwnsInspector));

->

node/src/env.cc

Lines 333 to 337 in 2551a21

	if (tracing::AgentWriterHandle* writer = GetTracingAgentWriter()) {
	trace_state_observer_ = std::make_unique<TrackingTraceStateObserver>(this);
	TracingController* tracing_controller = writer->GetTracingController();
	tracing_controller->AddTraceStateObserver(trace_state_observer_.get());
	}

->

node/src/node_v8_platform-inl.h

Lines 135 to 137 in 2551a21

	inline tracing::AgentWriterHandle* GetTracingAgentWriter() {
	return &tracing_file_writer_;
	}

Here, V8Platform :: Initialize must be called for tracing_file_writer_ to be initialized when NODE_USE_V8_PLATFORM is 1.
How can I solve this?

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[addaleax]

@jc-lab Are you okay with us closing this in favour of #31245?

[jc-lab]

@addaleax Yes, I close this PR.
I tested it with #31245 commit. Have are a few more things to fix, but #31245 work well in limited situations.