process: add `process.memoryUsage.external`

PR-URL: https://github.com/nodejs/node/pull/9587
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/process.md

@@ -1175,6 +1175,7 @@ added: v0.1.16
     * `rss` {Integer}
     * `heapTotal` {Integer}
     * `heapUsed` {Integer}
+    * `external` {Integer}
 
 The `process.memoryUsage()` method returns an object describing the memory usage
 of the Node.js process measured in bytes.
@@ -1191,11 +1192,14 @@ Will generate:
 {
   rss: 4935680,
   heapTotal: 1826816,
-  heapUsed: 650472
+  heapUsed: 650472,
+  external: 49879
 }
 ```
 
 `heapTotal` and `heapUsed` refer to V8's memory usage.
+`external` refers to the memory usage of C++ objects bound to JavaScript
+objects managed by V8.
 
 ## process.nextTick(callback[, ...args])
 <!-- YAML

src/env.h

@@ -103,6 +103,7 @@ namespace node {
   V(exponent_string, "exponent")                                              \
   V(exports_string, "exports")                                                \
   V(ext_key_usage_string, "ext_key_usage")                                    \
+  V(external_string, "external")                                              \
   V(external_stream_string, "_externalStream")                                \
   V(family_string, "family")                                                  \
   V(fatal_exception_string, "_fatalException")                                \

src/node.cc

@@ -2247,11 +2247,15 @@ void MemoryUsage(const FunctionCallbackInfo<Value>& args) {
       Number::New(env->isolate(), v8_heap_stats.total_heap_size());
   Local<Number> heap_used =
       Number::New(env->isolate(), v8_heap_stats.used_heap_size());
+  Local<Number> external_mem =
+      Number::New(env->isolate(),
+                  env->isolate()->AdjustAmountOfExternalAllocatedMemory(0));
 
   Local<Object> info = Object::New(env->isolate());
   info->Set(env->rss_string(), Number::New(env->isolate(), rss));
   info->Set(env->heap_total_string(), heap_total);
   info->Set(env->heap_used_string(), heap_used);
+  info->Set(env->external_string(), external_mem);
 
   args.GetReturnValue().Set(info);
 }

test/parallel/test-memory-usage.js

@@ -6,3 +6,4 @@ var r = process.memoryUsage();
 assert.ok(r.rss > 0);
 assert.ok(r.heapTotal > 0);
 assert.ok(r.heapUsed > 0);
+assert.ok(r.external > 0);