lib: add setFlagsFromString() to tracing module

Expose v8::V8::SetFlagsFromString() on tracing.v8 in lib/tracing.js.

PR-URL: node-forward/node#62
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-by: Trevor Norris <trev.norris@gmail.com>

doc/api/tracing.markdown

@@ -58,6 +58,21 @@ Returns an object with the following properties
 }
 ```
 
+### setFlagsFromString()
+
+Set additional V8 command line flags.  Use with care; changing settings
+after the VM has started may result in unpredictable behavior, including
+crashes and data loss.  Or it may simply do nothing.
+
+Usage:
+
+```
+// Print GC events to stdout for one minute.
+var v8 = require('tracing').v8;
+v8.setFlagsFromString('--trace_gc');
+setTimeout(function() { v8.setFlagsFromString('--notrace_gc'); }, 60e3);
+```
+
 
 # Async Listeners
 

lib/tracing.js

@@ -31,6 +31,7 @@ exports._nodeInitialization = function nodeInitialization(pobj) {
 
   // Finish setting up the v8 Object.
   v8.getHeapStatistics = v8binding.getHeapStatistics;
+  v8.setFlagsFromString = v8binding.setFlagsFromString;
 
   // Part of the AsyncListener setup to share objects/callbacks with the
   // native layer.

src/node_v8.cc

@@ -41,7 +41,9 @@ using v8::Local;
 using v8::Null;
 using v8::Number;
 using v8::Object;
+using v8::String;
 using v8::Uint32;
+using v8::V8;
 using v8::Value;
 using v8::kGCTypeAll;
 using v8::kGCTypeMarkSweepCompact;
@@ -204,6 +206,12 @@ void StopGarbageCollectionTracking(const FunctionCallbackInfo<Value>& args) {
 }
 
 
+void SetFlagsFromString(const FunctionCallbackInfo<Value>& args) {
+  String::Utf8Value flags(args[0]);
+  V8::SetFlagsFromString(*flags, flags.length());
+}
+
+
 void InitializeV8Bindings(Handle<Object> target,
                           Handle<Value> unused,
                           Handle<Context> context) {
@@ -215,6 +223,7 @@ void InitializeV8Bindings(Handle<Object> target,
                  "stopGarbageCollectionTracking",
                  StopGarbageCollectionTracking);
   env->SetMethod(target, "getHeapStatistics", GetHeapStatistics);
+  env->SetMethod(target, "setFlagsFromString", SetFlagsFromString);
 }
 
 }  // namespace node

test/simple/test-v8-flags.js

@@ -0,0 +1,26 @@
+// Copyright (c) 2014, StrongLoop Inc.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+// ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+// ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+// OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+var common = require('../common');
+var assert = require('assert');
+var v8 = require('tracing').v8;
+var vm = require('vm');
+
+v8.setFlagsFromString('--allow_natives_syntax');
+assert(eval('%_IsSmi(42)'));
+assert(vm.runInThisContext('%_IsSmi(42)'));
+
+v8.setFlagsFromString('--noallow_natives_syntax');
+assert.throws(function() { eval('%_IsSmi(42)') }, SyntaxError);
+assert.throws(function() { vm.runInThisContext('%_IsSmi(42)') }, SyntaxError);