Bug 1206300 - Allow immutable prototypes to be disabled at compile time with a single-line change, but don't actually disable them. r=jorendorff

js/src/builtin/TestingFunctions.cpp

@@ -2463,6 +2463,15 @@ ByteSize(JSContext* cx, unsigned argc, Value* vp)
     return true;
 }
 
+static bool
+ImmutablePrototypesEnabled(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    args.rval().setBoolean(JS_ImmutablePrototypesEnabled());
+    return true;
+}
+
 static bool
 SetImmutablePrototype(JSContext* cx, unsigned argc, Value* vp)
 {
@@ -3215,6 +3224,11 @@ gc::ZealModeHelpText),
 "  Return the size in bytes occupied by |value|, or |undefined| if value\n"
 "  is not allocated in memory.\n"),
 
+    JS_FN_HELP("immutablePrototypesEnabled", ImmutablePrototypesEnabled, 0, 0,
+"immutablePrototypesEnabled()",
+"  Returns true if immutable-prototype behavior (triggered by setImmutablePrototype)\n"
+"  is enabled, such that modifying an immutable prototype will fail."),
+
     JS_FN_HELP("setImmutablePrototype", SetImmutablePrototype, 1, 0,
 "setImmutablePrototype(obj)",
 "  Try to make obj's [[Prototype]] immutable, such that subsequent attempts to\n"

js/src/jsfriendapi.h

@@ -70,6 +70,9 @@ JS_ObjectCountDynamicSlots(JS::HandleObject obj);
 extern JS_FRIEND_API(size_t)
 JS_SetProtoCalled(JSContext* cx);
 
+extern JS_FRIEND_API(bool)
+JS_ImmutablePrototypesEnabled();
+
 extern JS_FRIEND_API(size_t)
 JS_GetCustomIteratorCount(JSContext* cx);
 

js/src/jsobj.cpp

@@ -2387,6 +2387,19 @@ JSObject::reportNotExtensible(JSContext* cx, unsigned report)
                                  nullptr, nullptr);
 }
 
+// Our immutable-prototype behavior is non-standard, and it's unclear whether
+// it's shippable.  (Or at least it's unclear whether it's shippable with any
+// provided-by-default uses exposed to script.)  If this bool is true,
+// immutable-prototype behavior is enforced; if it's false, behavior is not
+// enforced, and immutable-prototype bits stored on objects are completely
+// ignored.
+static const bool ImmutablePrototypesEnabled = true;
+
+JS_FRIEND_API(bool)
+JS_ImmutablePrototypesEnabled()
+{
+    return ImmutablePrototypesEnabled;
+}
 
 /*** ES6 standard internal methods ***************************************************************/
 
@@ -2405,9 +2418,8 @@ js::SetPrototype(JSContext* cx, HandleObject obj, HandleObject proto, JS::Object
     }
 
     /* Disallow mutation of immutable [[Prototype]]s. */
-    if (obj->nonLazyPrototypeIsImmutable()) {
+    if (obj->nonLazyPrototypeIsImmutable() && ImmutablePrototypesEnabled)
         return result.fail(JSMSG_CANT_SET_PROTO);
-    }
 
     /*
      * Disallow mutating the [[Prototype]] on ArrayBuffer objects, which

js/src/tests/ecma_6/extensions/setImmutablePrototype.js

@@ -86,7 +86,9 @@ function checkPrototypeMutationFailure(obj, desc)
 
 function runNormalTests(global)
 {
-  if (typeof setImmutablePrototype !== "function")
+  if (typeof setImmutablePrototype !== "function" ||
+      (typeof immutablePrototypesEnabled === "function" &&
+       !immutablePrototypesEnabled()))
   {
     print("no usable setImmutablePrototype function available, skipping tests");
     return;