src: expose ArrayBuffer version of Buffer::New()

This can be useful to create `Buffer` instances for already-existing
`ArrayBuffer`s, e.g. ones created manually from a backing store
with a free callback (of which our variant in the public API has
some limitations).

PR-URL: #30476
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/node_buffer.cc

@@ -231,6 +231,18 @@ MaybeLocal<Uint8Array> New(Environment* env,
   return ui;
 }
 
+MaybeLocal<Uint8Array> New(Isolate* isolate,
+                           Local<ArrayBuffer> ab,
+                           size_t byte_offset,
+                           size_t length) {
+  Environment* env = Environment::GetCurrent(isolate);
+  if (env == nullptr) {
+    THROW_ERR_BUFFER_CONTEXT_NOT_AVAILABLE(isolate);
+    return MaybeLocal<Uint8Array>();
+  }
+  return New(env, ab, byte_offset, length);
+}
+
 
 MaybeLocal<Object> New(Isolate* isolate,
                        Local<String> string,

src/node_buffer.h

@@ -65,6 +65,12 @@ NODE_EXTERN v8::MaybeLocal<v8::Object> New(v8::Isolate* isolate,
                                            char* data,
                                            size_t len);
 
+// Creates a Buffer instance over an existing ArrayBuffer.
+NODE_EXTERN v8::MaybeLocal<v8::Uint8Array> New(v8::Isolate* isolate,
+                                               v8::Local<v8::ArrayBuffer> ab,
+                                               size_t byte_offset,
+                                               size_t length);
+
 // This is verbose to be explicit with inline commenting
 static inline bool IsWithinBounds(size_t off, size_t len, size_t max) {
   // Asking to seek too far into the buffer

src/node_internals.h

@@ -160,7 +160,7 @@ v8::MaybeLocal<v8::Object> New(Environment* env,
                                char* data,
                                size_t length,
                                bool uses_malloc);
-// Creates a Buffer instance over an existing Uint8Array.
+// Creates a Buffer instance over an existing ArrayBuffer.
 v8::MaybeLocal<v8::Uint8Array> New(Environment* env,
                                    v8::Local<v8::ArrayBuffer> ab,
                                    size_t byte_offset,