libchromeos: dbus: unregister DBus object synchronously

When DBus object is unregistered explicitly, it allows its path to be
reused.

BUG=chromium:518026
TEST=emerge-$BOARD libchromeos shill
TEST=Verify using shill with chromeos dbus

Reviewed-on: https://chromium-review.googlesource.com/291841
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Zeping Qiu <zqiu@chromium.org>
Commit-Queue: Zeping Qiu <zqiu@chromium.org>

(cherry-picked from
 https://chromium.googlesource.com/chromiumos/platform2 at
 5688fbaf4aeb195c6915d3f362b888e549b7a010)

Change-Id: I77d88089b687df85afd78ebe0178040f24be8a31

2 files changed, 22 insertions, 0 deletions

diff --git a/chromeos/dbus/dbus_object.cc b/chromeos/dbus/dbus_object.cc
index eaed1b9..16539e2 100644
--- a/chromeos/dbus/dbus_object.cc
+++ b/chromeos/dbus/dbus_object.cc
@@ -251,6 +251,21 @@ void DBusObject::RegisterAndBlock() {
   }
 }
 
+void DBusObject::UnregisterAsync() {
+  VLOG(1) << "Unregistering D-Bus object '" << object_path_.value() << "'.";
+  CHECK(exported_object_ != nullptr) << "Object not registered.";
+
+  // This will unregister the object path from the bus.
+  exported_object_->Unregister();
+  // This will remove |exported_object_| from bus's object table. This function
+  // will also post a task to unregister |exported_object_| (same as the call
+  // above), which will be a no-op since it is already done by then.
+  // By doing both in here, the object path is guarantee to be reusable upon
+  // return from this function.
+  bus_->UnregisterExportedObject(object_path_);
+  exported_object_ = nullptr;
+}
+
 bool DBusObject::SendSignal(dbus::Signal* signal) {
   if (exported_object_) {
     exported_object_->SendSignal(signal);
diff --git a/chromeos/dbus/dbus_object.h b/chromeos/dbus/dbus_object.h
index 441df5b..10894e6 100644
--- a/chromeos/dbus/dbus_object.h
+++ b/chromeos/dbus/dbus_object.h
@@ -534,6 +534,13 @@ class CHROMEOS_EXPORT DBusObject {
   // will block until the object and all of its interfaces are registered.
   virtual void RegisterAndBlock();
 
+  // Unregister the object instance with D-Bus.  This will unregister the
+  // |exported_object_| and its path from the bus.  The destruction of
+  // |exported_object_| will be deferred in an async task posted by the bus.
+  // It is guarantee that upon return from this call a new DBusObject with the
+  // same object path can be created/registered.
+  virtual void UnregisterAsync();
+
   // Returns the ExportedObjectManager proxy, if any. If DBusObject has been
   // constructed without an object manager, this method returns an empty
   // smart pointer (containing nullptr).