Adds a new diff plugin that can import image layers in the block CIM format using the new
block CIM layer writer added in hcsshim repo.
This commit also makes another important change in the way a diff is applied when using
CimFS based layer writers. Currently, the diff plugins call archive.Apply to apply a diff
and pass a function (that can actually apply the diff) as an argument (via
archive.ApplyOptions). This allows the callers to call archive.Apply with either a custom
applier function or if the caller doesn't pass such a function archive.Apply uses the
default naive diff applier.
However, there is drawback to this approach. The applier function passed to the
`archive.Apply` call needs to follow a specific signature. This signature expects it that
all parent layers are represented as an array of strings. In cases like CimFS, we can't
easily represent a set of layers as strings (unless we encode extra data in those strings
in a hacky way). To get around this problem, the diff plugins for CimFS based layers, skip
the archive.Apply call and directly call the layer writer instead.
Signed-off-by: Amit Barve <ambarve@microsoft.com>
hcsshim recently [updated](microsoft/hcsshim@1d406d0) the interface of APIs that are used
for importing OCI layers. It now expects that the CimFS snapshotter mounts contain the
full cim paths for parent layers. This change updates the cimfs differ & snapshotter to
use that new interface.
Signed-off-by: Amit Barve <ambarve@microsoft.com>
