v8: add cachedDataVersionTag

Adds `v8.cachedDataVersionTag()`, which returns an integer
representing the version tag for `cachedData` for the current V8
version & flags.

PR-URL: #11515
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

doc/api/v8.md

@@ -9,6 +9,16 @@ const v8 = require('v8');
 
 *Note*: The APIs and implementation are subject to change at any time.
 
+## v8.cachedDataVersionTag()
+<!-- YAML
+added: REPLACEME
+-->
+
+Returns an integer representing a "version tag" derived from the V8 version,
+command line flags and detected CPU features. This is useful for determining
+whether a [`vm.Script`][] `cachedData` buffer is compatible with this instance
+of V8.
+
 ## v8.getHeapSpaceStatistics()
 <!-- YAML
 added: v6.0.0
@@ -144,5 +154,6 @@ setTimeout(function() { v8.setFlagsFromString('--notrace_gc'); }, 60e3);
 ```
 
 [V8]: https://developers.google.com/v8/
+[`vm.Script`]: vm.html#vm_new_vm_script_code_options
 [here]: https://github.com/thlorenz/v8-flags/blob/master/flags-0.11.md
 [`GetHeapSpaceStatistics`]: https://v8docs.nodesource.com/node-5.0/d5/dda/classv8_1_1_isolate.html#ac673576f24fdc7a33378f8f57e1d13a4

lib/v8.js

@@ -59,6 +59,7 @@ exports.getHeapStatistics = function() {
   };
 };
 
+exports.cachedDataVersionTag = v8binding.cachedDataVersionTag;
 exports.setFlagsFromString = v8binding.setFlagsFromString;
 
 exports.getHeapSpaceStatistics = function() {

src/node_v8.cc

@@ -13,10 +13,12 @@ using v8::Context;
 using v8::FunctionCallbackInfo;
 using v8::HeapSpaceStatistics;
 using v8::HeapStatistics;
+using v8::Integer;
 using v8::Isolate;
 using v8::Local;
 using v8::NewStringType;
 using v8::Object;
+using v8::ScriptCompiler;
 using v8::String;
 using v8::Uint32;
 using v8::V8;
@@ -53,6 +55,15 @@ static const size_t kHeapSpaceStatisticsPropertiesCount =
 static size_t number_of_heap_spaces = 0;
 
 
+void CachedDataVersionTag(const FunctionCallbackInfo<Value>& args) {
+  Environment* env = Environment::GetCurrent(args);
+  Local<Integer> result =
+      Integer::NewFromUnsigned(env->isolate(),
+                               ScriptCompiler::CachedDataVersionTag());
+  args.GetReturnValue().Set(result);
+}
+
+
 void UpdateHeapStatisticsArrayBuffer(const FunctionCallbackInfo<Value>& args) {
   Environment* env = Environment::GetCurrent(args);
   HeapStatistics s;
@@ -99,6 +110,8 @@ void InitializeV8Bindings(Local<Object> target,
                           Local<Context> context) {
   Environment* env = Environment::GetCurrent(context);
 
+  env->SetMethod(target, "cachedDataVersionTag", CachedDataVersionTag);
+
   env->SetMethod(target,
                  "updateHeapStatisticsArrayBuffer",
                  UpdateHeapStatisticsArrayBuffer);

test/parallel/test-v8-version-tag.js

@@ -0,0 +1,19 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+const v8 = require('v8');
+
+const versionTag1 = v8.cachedDataVersionTag();
+assert.strictEqual(typeof versionTag1, 'number');
+assert.strictEqual(v8.cachedDataVersionTag(), versionTag1);
+
+// The value of cachedDataVersionTag is derived from the command line flags and
+// detected CPU features. Test that the value does indeed update when flags
+// are toggled.
+v8.setFlagsFromString('--allow_natives_syntax');
+
+const versionTag2 = v8.cachedDataVersionTag();
+assert.strictEqual(typeof versionTag2, 'number');
+assert.strictEqual(v8.cachedDataVersionTag(), versionTag2);
+
+assert.notStrictEqual(versionTag1, versionTag2);