Use Emscripten with STANDALONE_WASM=1.

[PiotrSikora]

This PR is a bit overloaded and includes a lot of changes,
but it would be impossible to split them up:

1. Emscripten: update to 1.38.48.
2. Emscripten: use "upstream" instead of "fastcomp".
3. Emscripten: use STANDALONE_WASM=1.
4. Emscripten: use __wasm_global_ctors.
5. Emscripten: use WASI calls.
6. Emscripten: drop underscore prefix.
7. Emscripten: remove support for the JavaScript-flavored Emscripten.
8. Runtimes: throw WasmException for function call errors.
9. Tests: catch DivByZero (which is now an runtime error).
10. WAVM: update to 2019-10-15.
11. WAVM: stop using WAVM's Emscripten implementation, since we have our own.
12. SDK: distribute static libraries instead of the bytecode.

Fixes #77, #212.

Signed-off-by: Piotr Sikora piotrsikora@google.com

[PiotrSikora]

@AndrewScheidecker WAVM isn't running as part of CI, so you cannot see it here, but when updating it from 2019-10-01 (WAVM/WAVM@6146b4e) to 2019-10-15 (WAVM/WAVM@1ec06cd), one of our tests started failing because tryCollectCompartment(std::move(compartment_)) started returning false in a test where WAVM compartment is cloned.

I've tried changing existing code:

std::unique_ptr<WasmVm> Wavm::clone() {
  auto wavm = std::make_unique<Wavm>();
  wavm->compartment_ = WAVM::Runtime::cloneCompartment(compartment_);
  wavm->memory_ = WAVM::Runtime::remapToClonedCompartment(memory_, wavm->compartment_);
  wavm->memory_base_ = WAVM::Runtime::getMemoryBaseAddress(wavm->memory_);
  wavm->context_ = WAVM::Runtime::createContext(wavm->compartment_);
  for (auto& p : intrinsic_module_instances_) {
    wavm->intrinsic_module_instances_.emplace(
        p.first, WAVM::Runtime::remapToClonedCompartment(p.second, wavm->compartment_));
  }
  wavm->module_instance_ =
      WAVM::Runtime::remapToClonedCompartment(module_instance_, wavm->compartment_);
  return wavm;
}

to clone the context instead, i.e.:

 std::unique_ptr<WasmVm> Wavm::clone() {
   auto wavm = std::make_unique<Wavm>();
   wavm->compartment_ = WAVM::Runtime::cloneCompartment(compartment_);
+  wavm->context_ = WAVM::Runtime::cloneContext(context_, wavm->compartment_);
   wavm->memory_ = WAVM::Runtime::remapToClonedCompartment(memory_, wavm->compartment_);
   wavm->memory_base_ = WAVM::Runtime::getMemoryBaseAddress(wavm->memory_);
-  wavm->context_ = WAVM::Runtime::createContext(wavm->compartment_);
   for (auto& p : intrinsic_module_instances_) {
     wavm->intrinsic_module_instances_.emplace(
         p.first, WAVM::Runtime::remapToClonedCompartment(p.second, wavm->compartment_));

but that didn't make any difference.

I've looked at all the commits, but nothing obvious jumps out... any ideas?

cc @jplevyak

[AndrewScheidecker]

I've looked at all the commits, but nothing obvious jumps out... any ideas?

I think what must be happening is:

• Your initial Wavm object that you used as the source for the clone is still around, along with its Emscripten::Instance.
• I added some GCPointer<Function> to Emscripten::Instance here.
• The WAVM Function objects are strange objects that may be shared between cloned compartments: they are just pointers to a header that is emitted before the machine code for the function.
• The GCPointer<Function> is treated as a GC root in all compartments that contain the function, and so none of the compartments that were cloned from the original can be collected until the GCPointer<Function> in the Emscripten::Instance is destroyed.

I should perhaps get rid of the GCPointer<Function> in Emscripten::Instance, but I think that there are likely other problems lurking here due to the Emscripten::Instance being shared between cloned compartments. To solve this I could add an API like this:

std::shared_ptr<Emscripten::Instance> remapToClonedCompartment(Emscripten::Instance&,
                                                               Compartment*)

However, I am wondering if you need the Emscripten::Instance at all. It looks like you have your own Emscripten emulation layer, so is it possible to remove WAVM's?

[PiotrSikora]

However, I am wondering if you need the Emscripten::Instance at all. It looks like you have your own Emscripten emulation layer, so is it possible to remove WAVM's?

That's an excellent question! We don't need it anymore, and removing it fixes the issue.

Thanks a lot!

[jplevyak]

Just curious what caused these changes?

[PiotrSikora]

I removed libglib2.0-dev which wasn't necessary (it used to be needed by V8, but it's not anymore, and I'm not sure why it was here in the first place), so I had to add pkg-config and zlib1g-dev directly, since those are needed and were previously installed as transitive dependencies of libglib2.0-dev.

[jplevyak]

We are going to have to upstream this.

[PiotrSikora]

Yup, V8 isn't merged yet, so it's fine I think.