contextify: tie lifetimes of context & sandbox

When the previous set of changes (bfff07b) it was possible to have the
context get garbage collected while sandbox was still live. We need to
tie the lifetime of the context to the lifetime of the sandbox.

This is a backport of #5786 to v5.x.

Ref: https://github.com/nodejs/node/pull/5786
PR-URL: https://github.com/nodejs/node/pull/5800
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/node_contextify.cc

@@ -207,7 +207,17 @@ class ContextifyContext {
 
     CHECK(!ctx.IsEmpty());
     ctx->SetSecurityToken(env->context()->GetSecurityToken());
+
+    // We need to tie the lifetime of the sandbox object with the lifetime of
+    // newly created context. We do this by making them hold references to each
+    // other. The context can directly hold a reference to the sandbox as an
+    // embedder data field. However, we cannot hold a reference to a v8::Context
+    // directly in an Object, we instead hold onto the new context's global
+    // object instead (which then has a reference to the context).
     ctx->SetEmbedderData(kSandboxObjectIndex, sandbox_obj);
+    sandbox_obj->SetHiddenValue(
+        FIXED_ONE_BYTE_STRING(env->isolate(), "_contextifyHiddenGlobal"),
+        ctx->Global());
 
     env->AssignToContext(ctx);
 

test/parallel/test-vm-create-and-run-in-context.js

@@ -1,4 +1,5 @@
 'use strict';
+// Flags: --expose-gc
 require('../common');
 var assert = require('assert');
 
@@ -18,3 +19,11 @@ console.error('test updating context');
 result = vm.runInContext('var foo = 3;', context);
 assert.equal(3, context.foo);
 assert.equal('lala', context.thing);
+
+// https://github.com/nodejs/node/issues/5768
+console.error('run in contextified sandbox without referencing the context');
+var sandbox = {x: 1};
+vm.createContext(sandbox);
+gc();
+vm.runInContext('x = 2', sandbox);
+// Should not crash.