Bug 999297 - Simplify xpc::HasInstance. r=bz

js/xpconnect/src/XPCJSID.cpp

@@ -493,45 +493,24 @@ xpc::HasInstance(JSContext *cx, HandleObject objArg, const nsID *iid, bool *bp)
     if (!obj)
         return NS_OK;
 
-    if (IsDOMObject(obj)) {
-        // Not all DOM objects implement nsISupports. But if they don't,
-        // there's nothing to do in this HasInstance hook.
-        nsISupports *identity = UnwrapDOMObjectToISupports(obj);
-        if (!identity)
-            return NS_OK;;
-        nsCOMPtr<nsISupports> supp;
-        identity->QueryInterface(*iid, getter_AddRefs(supp));
-        *bp = supp;
-        return NS_OK;
-    }
-
     if (mozilla::jsipc::JavaScriptParent::IsCPOW(obj))
         return mozilla::jsipc::JavaScriptParent::InstanceOf(obj, iid, bp);
 
-    MOZ_ASSERT(IS_WN_REFLECTOR(obj));
-    XPCWrappedNative* other_wrapper = XPCWrappedNative::Get(obj);
-    if (!other_wrapper)
+    nsISupports *identity = UnwrapReflectorToISupports(obj);
+    if (!identity)
         return NS_OK;
 
-    // We'll trust the interface set of the wrapper if this is known
-    // to be an interface that the objects *expects* to be able to
-    // handle.
-    if (other_wrapper->HasInterfaceNoQI(*iid)) {
-        *bp = true;
-        return NS_OK;
-    }
+    nsCOMPtr<nsISupports> supp;
+    identity->QueryInterface(*iid, getter_AddRefs(supp));
+    *bp = supp;
 
-    // Otherwise, we'll end up Querying the native object to be sure.
-    XPCCallContext ccx(JS_CALLER, cx);
-
-    AutoMarkingNativeInterfacePtr iface(ccx);
-    iface = XPCNativeInterface::GetNewOrUsed(iid);
-
-    nsresult findResult = NS_OK;
-    if (iface && other_wrapper->FindTearOff(iface, false, &findResult))
-        *bp = true;
-    if (NS_FAILED(findResult) && findResult != NS_ERROR_NO_INTERFACE)
-        return findResult;
+    // Our old HasInstance implementation operated by invoking FindTearOff on
+    // XPCWrappedNatives, and various bits of chrome JS came to depend on
+    // |instanceof| doing an implicit QI if it succeeds. Do a drive-by QI to
+    // preserve that behavior. This is just a compatibility hack, so we don't
+    // really care if it fails.
+    if (IS_WN_REFLECTOR(obj))
+        (void) XPCWrappedNative::Get(obj)->FindTearOff(*iid);
 
     return NS_OK;
 }

js/xpconnect/src/XPCWrappedNative.cpp

@@ -1431,6 +1431,14 @@ XPCWrappedNative::FindTearOff(XPCNativeInterface* aInterface,
     return to;
 }
 
+XPCWrappedNativeTearOff*
+XPCWrappedNative::FindTearOff(const nsIID& iid) {
+    AutoJSContext cx;
+    AutoMarkingNativeInterfacePtr iface(cx);
+    iface = XPCNativeInterface::GetNewOrUsed(&iid);
+    return iface ? FindTearOff(iface) : nullptr;
+}
+
 nsresult
 XPCWrappedNative::InitTearOff(XPCWrappedNativeTearOff* aTearOff,
                               XPCNativeInterface* aInterface,

js/xpconnect/src/xpcprivate.h

@@ -2093,6 +2093,8 @@ public:
     XPCWrappedNativeTearOff* FindTearOff(XPCNativeInterface* aInterface,
                                          bool needJSObject = false,
                                          nsresult* pError = nullptr);
+    XPCWrappedNativeTearOff* FindTearOff(const nsIID& iid);
+
     void Mark() const
     {
         mSet->Mark();