vm: add isContext; prevent double-contextifying

Previously, calling `vm.createContext(o)` repeatedly on the same `o`
would cause new C++ `ContextifyContext`s to be created and stored on
`o`, while the previous resident went off into leaked-memory limbo.
Now, repeatedly trying to contextify a sandbox will do nothing after
the first time.

To detect this, an independently-useful `vm.isContext(sandbox)` export
was added.

1 files changed, 5 insertions, 1 deletions

diff --git a/lib/vm.js b/lib/vm.js
index c71bb81add..8fee40a8f2 100644
--- a/lib/vm.js
+++ b/lib/vm.js
@@ -28,6 +28,7 @@ var util = require('util');
 //   - runInThisContext()
 //   - runInContext(sandbox, [timeout])
 // - makeContext(sandbox)
+// - isContext(sandbox)
 // From this we build the entire documented API.
 
 Script.prototype.runInNewContext = function(initSandbox, timeout, disp) {
@@ -44,10 +45,11 @@ exports.createScript = function(code, filename, disp) {
 exports.createContext = function(initSandbox) {
   if (util.isUndefined(initSandbox)) {
     initSandbox = {};
+  } else if (binding.isContext(initSandbox)) {
+    return initSandbox;
   }
 
   binding.makeContext(initSandbox);
-
   return initSandbox;
 };
 
@@ -65,3 +67,5 @@ exports.runInThisContext = function(code, filename, timeout, disp) {
   var script = exports.createScript(code, filename, disp);
   return script.runInThisContext(timeout, disp);
 };
+
+exports.isContext = binding.isContext;