shellDBus: Add a real error reporting system to InstallExtensionRemote

Instead of using the 'extension-state-changed' signal to relay errors,
use DBus's native error mechanism to inform the method caller that the
call has failed. This requires making the method actually asynchronous
so that we don't block the browser, which is stuck waiting for a reply
from the browser plugin. To ensure this, we need to modify the browser
plugin API to ensure its extesion installation method is asynchronous.

Additionally, this lets us remove the awful, broken hacks that we used
when a user clicked the "Cancel" button, replacing it by a DBus return
value.

https://bugzilla.gnome.org/show_bug.cgi?id=679099

browser-plugin/browser-plugin.c

@@ -41,7 +41,7 @@
       "It can be used only by extensions.gnome.org"
 #define PLUGIN_MIME_STRING "application/x-gnome-shell-integration::Gnome Shell Integration Dummy Content-Type";
 
-#define PLUGIN_API_VERSION 4
+#define PLUGIN_API_VERSION 5
 
 typedef struct {
   GDBusProxy *proxy;
@@ -489,6 +489,12 @@ parse_args (const gchar     *format_str,
 
           *(gboolean *) arg_location = NPVARIANT_TO_BOOLEAN (arg);
           break;
+
+        case 'o':
+          if (!NPVARIANT_IS_OBJECT (arg))
+            goto out;
+
+          *(NPObject **) arg_location = NPVARIANT_TO_OBJECT (arg);
         }
     }
 
@@ -580,6 +586,53 @@ plugin_enable_extension (PluginObject    *obj,
   return ret;
 }
 
+typedef struct _AsyncClosure AsyncClosure;
+
+struct _AsyncClosure {
+  PluginObject *obj;
+  NPObject *callback;
+  NPObject *errback;
+};
+
+static void
+install_extension_cb (GObject      *proxy,
+                      GAsyncResult *async_res,
+                      gpointer      user_data)
+{
+  AsyncClosure *async_closure = (AsyncClosure *) user_data;
+  GError *error = NULL;
+  GVariant *res;
+  NPVariant args[1];
+  NPVariant result = { NPVariantType_Void };
+  NPObject *callback;
+
+  res = g_dbus_proxy_call_finish (G_DBUS_PROXY (proxy), async_res, &error);
+
+  if (res == NULL)
+    {
+      if (g_dbus_error_is_remote_error (error))
+        g_dbus_error_strip_remote_error (error);
+      STRINGZ_TO_NPVARIANT (error->message, args[0]);
+      callback = async_closure->errback;
+    }
+  else
+    {
+      char *string_result;
+      g_variant_get (res, "(&s)", &string_result);
+      STRINGZ_TO_NPVARIANT (string_result, args[0]);
+      callback = async_closure->callback;
+    }
+
+  funcs.invokeDefault (async_closure->obj->instance,
+                       callback, args, 1, &result);
+
+  funcs.releasevariantvalue (&result);
+
+  funcs.releaseobject (async_closure->callback);
+  funcs.releaseobject (async_closure->errback);
+  g_slice_free (AsyncClosure, async_closure);
+}
+
 static gboolean
 plugin_install_extension (PluginObject    *obj,
                           uint32_t         argc,
@@ -587,18 +640,25 @@ plugin_install_extension (PluginObject    *obj,
                           NPVariant       *result)
 {
   gchar *uuid;
+  NPObject *callback, *errback;
+  AsyncClosure *async_closure;
 
-  if (!parse_args ("u", argc, argv, &uuid))
+  if (!parse_args ("uoo", argc, argv, &uuid, &callback, &errback))
     return FALSE;
 
+  async_closure = g_slice_new (AsyncClosure);
+  async_closure->obj = obj;
+  async_closure->callback = funcs.retainobject (callback);
+  async_closure->errback = funcs.retainobject (errback);
+
   g_dbus_proxy_call (obj->proxy,
                      "InstallRemoteExtension",
                      g_variant_new ("(s)", uuid),
                      G_DBUS_CALL_FLAGS_NONE,
                      -1, /* timeout */
                      NULL, /* cancellable */
-                     NULL, /* callback */
-                     NULL /* user_data */);
+                     install_extension_cb,
+                     async_closure);
 
   g_free (uuid);