Introduce a Fence class which models a synchronization primitive that
allows to notify the availability of a resource.
The Fence is modeled as a wrapper of a UniqueFD instance where
read events are used to signal the Fence. The class can be later
extended to support additional signalling mechanisms.
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
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