Add JSI method for setting external memory size (facebook#41436)

Summary:

Add a JSI API for associating some native memory with a JS object. This
is intended to provide a mechanism to trigger more frequent garbage
collection when JS retains large external memory allocations, in order
to avoid memory buildup.

This diff just adds the JSI method, without any implementations.

Changelog:
[General][Added] - Added JSI method for reporting native memory to the GC.

Reviewed By: tmikov

Differential Revision: D50524912

packages/react-native/ReactCommon/jsc/JSCRuntime.cpp

@@ -233,6 +233,7 @@ class JSCRuntime : public jsi::Runtime {
   bool strictEquals(const jsi::String& a, const jsi::String& b) const override;
   bool strictEquals(const jsi::Object& a, const jsi::Object& b) const override;
   bool instanceOf(const jsi::Object& o, const jsi::Function& f) override;
+  void setExternalMemoryPressure(const jsi::Object&, size_t) override;
 
  private:
   // Basically convenience casts
@@ -1392,6 +1393,8 @@ bool JSCRuntime::instanceOf(const jsi::Object& o, const jsi::Function& f) {
   return res;
 }
 
+void JSCRuntime::setExternalMemoryPressure(const jsi::Object&, size_t) {}
+
 jsi::Runtime::PointerValue* JSCRuntime::makeSymbolValue(
     JSValueRef symbolRef) const {
 #ifndef NDEBUG

packages/react-native/ReactCommon/jsi/jsi/decorator.h

@@ -252,6 +252,10 @@ class RuntimeDecorator : public Base, private jsi::Instrumentation {
     plain_.setNativeState(o, state);
   }
 
+  void setExternalMemoryPressure(const Object& obj, size_t amt) override {
+    plain_.setExternalMemoryPressure(obj, amt);
+  }
+
   Value getProperty(const Object& o, const PropNameID& name) override {
     return plain_.getProperty(o, name);
   };

packages/react-native/ReactCommon/jsi/jsi/jsi-inl.h

@@ -227,6 +227,11 @@ inline void Object::setNativeState(
   runtime.setNativeState(*this, state);
 }
 
+inline void Object::setExternalMemoryPressure(Runtime& runtime, size_t amt)
+    const {
+  runtime.setExternalMemoryPressure(*this, amt);
+}
+
 inline Array Object::getPropertyNames(Runtime& runtime) const {
   return runtime.getPropertyNames(*this);
 }

packages/react-native/ReactCommon/jsi/jsi/jsi.h

@@ -387,6 +387,11 @@ class JSI_EXPORT Runtime {
 
   virtual bool instanceOf(const Object& o, const Function& f) = 0;
 
+  /// See Object::setExternalMemoryPressure.
+  virtual void setExternalMemoryPressure(
+      const jsi::Object& obj,
+      size_t amount) = 0;
+
   // These exist so derived classes can access the private parts of
   // Value, Symbol, String, and Object, which are all friends of Runtime.
   template <typename T>
@@ -834,6 +839,16 @@ class JSI_EXPORT Object : public Pointer {
   /// works.  I only need it in one place.)
   Array getPropertyNames(Runtime& runtime) const;
 
+  /// Inform the runtime that there is additional memory associated with a given
+  /// JavaScript object that is not visible to the GC. This can be used if an
+  /// object is known to retain some native memory, and may be used to guide
+  /// decisions about when to run garbage collection.
+  /// This method may be invoked multiple times on an object, and subsequent
+  /// calls will overwrite any previously set value. Once the object is garbage
+  /// collected, the associated external memory will be considered freed and may
+  /// no longer factor into GC decisions.
+  void setExternalMemoryPressure(Runtime& runtime, size_t amt) const;
+
  protected:
   void setPropertyValue(
       Runtime& runtime,