src: don't overwrite non-writable vm globals

Check that the property doesn't have the read-only flag set before
overwriting it.

This is Ben Noordhuis previous commit, but keeping
is_contextual_store. is_contextual_store describes whether this.foo = 42 or
foo = 42 was called. The second is contextual and will fail
in strict mode if foo is used without declaration. Therefore only do an
early return if it is a contextual store. In particular,
don't do an early return for Object.defineProperty(this, ...).

Fixes: #10223
Refs: #10227

src/node_contextify.cc

@@ -383,19 +383,28 @@ class ContextifyContext {
     if (ctx->context_.IsEmpty())
       return;
 
+    auto attributes = PropertyAttribute::None;
     bool is_declared =
-        ctx->global_proxy()->HasRealNamedProperty(ctx->context(),
-                                                  property).FromJust();
+        ctx->global_proxy()->GetRealNamedPropertyAttributes(ctx->context(),
+                                                            property)
+        .To(&attributes);
+    bool read_only =
+        static_cast<int>(attributes) &
+        static_cast<int>(PropertyAttribute::ReadOnly);
+
+    if (is_declared && read_only)
+      return;
+
+    // true for x = 5
+    // false for this.x = 5
+    // false for Object.defineProperty(this, 'foo', ...)
+    // false for vmResult.x = 5 where vmResult = vm.runInContext();
     bool is_contextual_store = ctx->global_proxy() != args.This();
 
-    bool set_property_will_throw =
-        args.ShouldThrowOnError() &&
-        !is_declared &&
-        is_contextual_store;
+    if (!is_declared && args.ShouldThrowOnError() && is_contextual_store)
+      return;
 
-    if (!set_property_will_throw) {
-      ctx->sandbox()->Set(property, value);
-    }
+    ctx->sandbox()->Set(property, value);
   }
 
 

test/parallel/test-vm-context.js

@@ -75,3 +75,16 @@ assert.throws(function() {
 // https://github.com/nodejs/node/issues/6158
 ctx = new Proxy({}, {});
 assert.strictEqual(typeof vm.runInNewContext('String', ctx), 'function');
+
+// https://github.com/nodejs/node/issues/10223
+ctx = vm.createContext();
+vm.runInContext('Object.defineProperty(this, "x", { value: 42 })', ctx);
+assert.strictEqual(ctx.x, 42);
+assert.strictEqual(vm.runInContext('x', ctx), 42);
+
+vm.runInContext('x = 0', ctx);                      // Does not throw but x...
+assert.strictEqual(vm.runInContext('x', ctx), 42);  // ...should be unaltered.
+
+assert.throws(() => vm.runInContext('"use strict"; x = 0', ctx),
+              /Cannot assign to read only property 'x'/);
+assert.strictEqual(vm.runInContext('x', ctx), 42);