This separates out the DeviceSet logic a bit better from the raw
device mapper operations.
devicemapper: Serialize addess to the devicemapper deviceset
This code is not safe to run in multiple threads at the same time,
and neither is libdevmapper.
DeviceMapper: Move deactivate into UnmountDevice
This way the deactivate is atomic wrt othe device mapper operations
and will not fail with EBUSY if someone else starts a devicemapper
operation inbetween unmount and deactivate.
devmapper: Fix loopback mounting regression
Some changes were added to attach_loop_device which added
a perror() in a place that caused it to override errno so that
a later errno != EBUSY failed. This fixes that and cleans up
the error reporting a bit.
devmapper: Build on old kernels without LOOP_CTL_GET_FREE define
Right now this does nothing but add a new layer, but it means
that all DeviceMounts are paired with DeviceUnmounts so that we
can track (and cleanup) active mounts.
This wraps an existing DeviceSet and just adds a prefix to all ids in
it. This will be useful for reusing a single DeviceSet for all the tests
(but with separate ids)
This interface matches the device-mapper implementation (DeviceSetDM)
but is free from any dependencies. This allows core docker code
to refer to a DeviceSet without having an explicit dependency on
the devmapper package.
This is important, because the devmapper package has external
dependencies which are not wanted in the docker client app, as it
needs to run with minimal dependencies in the docker image.
