Add tryGetMutableBuffer API (#56156)

Summary:


Adds `ArrayBuffer::tryGetMutableBuffer` to JSI. This allows the user to
retrieve a `MutableBuffer` from the `ArrayBuffer`. The returned buffer 
will point directly into the same buffer as the runtime object, rather than
copying it out.

The data referencedby the `MutableBuffer` will be alive during the entirety
 of the `MutableBuffer` lifespan.

Addresses: facebook/hermes#1578

Changelog: [Internal]

Reviewed By: tmikov

Differential Revision: D91264901

Author: tsaichien

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

@@ -434,6 +434,11 @@ class RuntimeDecorator : public Base, private jsi::Instrumentation {
     return plain_.getRuntimeDataImpl(uuid);
   }
 
+  std::shared_ptr<MutableBuffer> tryGetMutableBuffer(
+      const jsi::ArrayBuffer& arrayBuffer) override {
+    return plain_.tryGetMutableBuffer(arrayBuffer);
+  }
+
   // Private data for managing scopes.
   Runtime::ScopeState* pushScope() override {
     return plain_.pushScope();
@@ -1000,6 +1005,12 @@ class WithRuntimeDecorator : public RuntimeDecorator<Plain, Base> {
     return RD::callAsConstructor(f, args, count);
   }
 
+  std::shared_ptr<MutableBuffer> tryGetMutableBuffer(
+      const jsi::ArrayBuffer& arrayBuffer) override {
+    Around around{with_};
+    return RD::tryGetMutableBuffer(arrayBuffer);
+  }
+
   // Private data for managing scopes.
   Runtime::ScopeState* pushScope() override {
     Around around{with_};

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

@@ -552,6 +552,11 @@ void Runtime::setPropertyValue(
   }
 }
 
+std::shared_ptr<MutableBuffer> Runtime::tryGetMutableBuffer(
+    const jsi::ArrayBuffer&) {
+  return nullptr;
+}
+
 Pointer& Pointer::operator=(Pointer&& other) noexcept {
   if (ptr_) {
     ptr_->invalidate();

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

@@ -628,6 +628,14 @@ class JSI_EXPORT IRuntime : public ICast {
       void* ctx,
       void (*cb)(void* ctx, bool ascii, const void* data, size_t num)) = 0;
 
+  /// If possible, returns the MutableBuffer representing \p arrayBuffer's
+  /// underlying data, else return a nullptr. Importantly, the returned
+  /// MutableBuffer directly points to \p arrayBuffer's data instead of copying
+  /// the data over. The data's lifetime is valid for the lifetime of
+  /// MutableBuffer, which is orthogonal from \p arrayBuffer.
+  virtual std::shared_ptr<MutableBuffer> tryGetMutableBuffer(
+      const jsi::ArrayBuffer& arrayBuffer) = 0;
+
  protected:
   virtual ~IRuntime() = default;
 };
@@ -713,6 +721,9 @@ class JSI_EXPORT Runtime : public IRuntime {
 
   size_t push(const Array&, const Value*, size_t) override;
 
+  std::shared_ptr<MutableBuffer> tryGetMutableBuffer(
+      const jsi::ArrayBuffer& arrayBuffer) override;
+
  protected:
   friend class Pointer;
   friend class PropNameID;
@@ -1456,6 +1467,10 @@ class JSI_EXPORT ArrayBuffer : public Object {
     return runtime.data(*this);
   }
 
+  std::shared_ptr<MutableBuffer> tryGetMutableBuffer(IRuntime& runtime) const {
+    return runtime.tryGetMutableBuffer(*this);
+  }
+
  private:
   friend class Object;
   friend class Value;