Add external ArrayBuffers to JSI (#793)

Summary:
X-link: facebook/hermes#793

Add a JSI API for constructing `ArrayBuffer`s from a user provided
`jsi::MutableBuffer`.

Changelog: [General][Added] Added ability to construct ArrayBuffers from existing memory buffers.

Reviewed By: jpporto

Differential Revision: D37744467

fbshipit-source-id: 9d9ece00d1dbde341846c45fa30c935b5fa81e9a

ReactCommon/jsi/JSCRuntime.cpp

@@ -206,6 +206,8 @@ class JSCRuntime : public jsi::Runtime {
   jsi::Value lockWeakObject(jsi::WeakObject &) override;
 
   jsi::Array createArray(size_t length) override;
+  jsi::ArrayBuffer createArrayBuffer(
+      std::shared_ptr<jsi::MutableBuffer> buffer) override;
   size_t size(const jsi::Array &) override;
   size_t size(const jsi::ArrayBuffer &) override;
   uint8_t *data(const jsi::ArrayBuffer &) override;
@@ -1087,6 +1089,11 @@ jsi::Array JSCRuntime::createArray(size_t length) {
   return createObject(obj).getArray(*this);
 }
 
+jsi::ArrayBuffer JSCRuntime::createArrayBuffer(
+    std::shared_ptr<jsi::MutableBuffer> buffer) {
+  throw std::logic_error("Not implemented");
+}
+
 size_t JSCRuntime::size(const jsi::Array &arr) {
   return static_cast<size_t>(
       getProperty(arr, createPropNameID(getLengthString())).getNumber());

ReactCommon/jsi/jsi/decorator.h

@@ -302,6 +302,10 @@ class RuntimeDecorator : public Base, private jsi::Instrumentation {
   Array createArray(size_t length) override {
     return plain_.createArray(length);
   };
+  ArrayBuffer createArrayBuffer(
+      std::shared_ptr<MutableBuffer> buffer) override {
+    return plain_.createArrayBuffer(std::move(buffer));
+  };
   size_t size(const Array& a) override {
     return plain_.size(a);
   };
@@ -701,6 +705,10 @@ class WithRuntimeDecorator : public RuntimeDecorator<Plain, Base> {
     Around around{with_};
     return RD::createArray(length);
   };
+  ArrayBuffer createArrayBuffer(
+      std::shared_ptr<MutableBuffer> buffer) override {
+    return RD::createArrayBuffer(std::move(buffer));
+  };
   size_t size(const Array& a) override {
     Around around{with_};
     return RD::size(a);

ReactCommon/jsi/jsi/jsi.cpp

@@ -66,6 +66,8 @@ Value callGlobalFunction(Runtime& runtime, const char* name, const Value& arg) {
 
 Buffer::~Buffer() = default;
 
+MutableBuffer::~MutableBuffer() = default;
+
 PreparedJavaScript::~PreparedJavaScript() = default;
 
 Value HostObject::get(Runtime&, const PropNameID&) {

ReactCommon/jsi/jsi/jsi.h

@@ -31,6 +31,10 @@ class FBJSRuntime;
 namespace facebook {
 namespace jsi {
 
+/// Base class for buffers of data or bytecode that need to be passed to the
+/// runtime. The buffer is expected to be fully immutable, so the result of
+/// size(), data(), and the contents of the pointer returned by data() must not
+/// change after construction.
 class JSI_EXPORT Buffer {
  public:
   virtual ~Buffer();
@@ -52,6 +56,18 @@ class JSI_EXPORT StringBuffer : public Buffer {
   std::string s_;
 };
 
+/// Base class for buffers of data that need to be passed to the runtime. The
+/// result of size() and data() must not change after construction. However, the
+/// region pointed to by data() may be modified by the user or the runtime. The
+/// user must ensure that access to the contents of the buffer is properly
+/// synchronised.
+class JSI_EXPORT MutableBuffer {
+ public:
+  virtual ~MutableBuffer();
+  virtual size_t size() const = 0;
+  virtual uint8_t* data() = 0;
+};
+
 /// PreparedJavaScript is a base class representing JavaScript which is in a
 /// form optimized for execution, in a runtime-specific way. Construct one via
 /// jsi::Runtime::prepareJavaScript().
@@ -336,6 +352,8 @@ class JSI_EXPORT Runtime {
   virtual Value lockWeakObject(WeakObject&) = 0;
 
   virtual Array createArray(size_t length) = 0;
+  virtual ArrayBuffer createArrayBuffer(
+      std::shared_ptr<MutableBuffer> buffer) = 0;
   virtual size_t size(const Array&) = 0;
   virtual size_t size(const ArrayBuffer&) = 0;
   virtual uint8_t* data(const ArrayBuffer&) = 0;
@@ -915,6 +933,9 @@ class JSI_EXPORT ArrayBuffer : public Object {
   ArrayBuffer(ArrayBuffer&&) = default;
   ArrayBuffer& operator=(ArrayBuffer&&) = default;
 
+  ArrayBuffer(Runtime& runtime, std::shared_ptr<MutableBuffer> buffer)
+      : ArrayBuffer(runtime.createArrayBuffer(std::move(buffer))) {}
+
   /// \return the size of the ArrayBuffer, according to its byteLength property.
   /// (C++ naming convention)
   size_t size(Runtime& runtime) const {