Race condition in signal processing

When a devices changes its state after a signal is emitted, only the new state will be visible when processing the signal. If the device becomes unavailable its properties cannot be accessed at all, resulting in dbus-errors leading to suppressed notifications.

udisks2 solves this problem by communicating the updated state of devices, such that the dbus client will always see a consistent state. It should be thought about implementing a similar behaviour within the udisks1 client. This will also make the udisks1 module behave more consistently with the planned udisks2 module.