Expose a way to retrieve the underlying VM Runtime instance from JSI HermesRuntime (#1430) (#1430)

Summary:
Original Author: [email protected]
Original Git: 880b164
Original Reviewed By: avp
Original Revision: D61212063

I understand this change might be a bit contentious, but please hear me out.

Our Hermes integration does not use the JSI API, we integrate with the Hermes Runtime C++ API directly. There are a few reasons for this:
- We already have an API that is similar to JSI for which we have 4 different JS engines integrated with it. If we were integrating Hermes through JSI we would be implementing an abstraction on top of an abstraction, which adds performance and memory overhead. We want our Hermes integration to be as fast as it can be, and for that we need our bridge layer to have minimal overhead so that it can compete on a performance level against our other JS engine integrations that are also based on lower level JS engine primitives.
- There are APIs we need to integrate that can't be implemented with JSI.
- We don't have C++ exceptions enabled, and the JSI Hermes integration makes heavy use of them and forces its consumers to handle errors through exceptions.

The Hermes Debugger implementation is however based on the JSI API, so in order for us to leverage the Hermes Debugger, this change exposes a new method that returns the underlying VM Runtime instance from the JSI API. This makes it possible for us to keep our integration which is based on the VM Runtime API directly instead of JSI, and use the debugger API.

Pull Request resolved: #1430

Pulled By: neildhar

Reviewed By: neildhar

Differential Revision: D61574245

fbshipit-source-id: 3ce10e622233d2f75d788d9bb0d6b4b2cbdeed52

Author: dannysu

API/hermes/hermes.cpp

@@ -1426,6 +1426,10 @@ SHRuntime *HermesRuntime::getSHRuntime() noexcept {
   return vm::getSHRuntime(impl(this)->runtime_);
 }
 
+::hermes::vm::Runtime *HermesRuntime::getVMRuntimeUnsafe() const {
+  return impl(this)->rt_.get();
+}
+
 size_t HermesRuntime::rootsListLengthForTests() const {
   return impl(this)->hermesValues_.sizeForTests();
 }

API/hermes/hermes.h

@@ -27,6 +27,7 @@ struct SHRuntime;
 namespace hermes {
 namespace vm {
 class GCExecTrace;
+class Runtime;
 } // namespace vm
 } // namespace hermes
 
@@ -224,6 +225,12 @@ class HERMES_EXPORT HermesRuntime : public jsi::Runtime {
   /// Retrieve the underlying SHRuntime.
   SHRuntime *getSHRuntime() noexcept;
 
+  /// Returns the underlying low level Hermes VM runtime instance.
+  /// This function is considered unsafe and unstable.
+  /// Direct use of a vm::Runtime should be avoided as the lower level APIs are
+  /// unsafe and they can change without notice.
+  ::hermes::vm::Runtime *getVMRuntimeUnsafe() const;
+
  private:
   // Only HermesRuntimeImpl can subclass this.
   HermesRuntime() = default;