Implement WCOW parentless active snapshots and view snapshots

The WCOW layer support does not support creating sandboxes with no
parent.  Instead, parentless scratch layers must be layed out as a
directory containing only a directory named 'Files', and all data stored
inside 'Files'. At commit-time, this will be converted in-place into a
read-only layer suitable for use as a parent layer.

The WCOW layer support also does not deal with making read-only layers,
i.e. layers that are prepared to be parent layers, visible in a
read-only manner. A bind-mount or junction point cannot be made
read-only, so a view must instead be a small sandbox layer that we can
mount via WCOW, and discard later, to protect the layer against
accidental or deliberate modification.

Signed-off-by: Paul "TBBle" Hampson <Paul.Hampson@Pobox.com>

diff/windows/windows.go

@@ -320,10 +320,13 @@ func mountsToLayerAndParents(mounts []mount.Mount) (string, []string, error) {
 		return "", nil, fmt.Errorf("number of mounts should always be 1 for Windows layers: %w", errdefs.ErrInvalidArgument)
 	}
 	mnt := mounts[0]
-	if mnt.Type != "windows-layer" {
+
+	if mnt.Type != "windows-layer" && mnt.Type != "bind" {
 		// This is a special case error. When this is received the diff service
 		// will attempt the next differ in the chain which for Windows is the
 		// lcow differ that we want.
+		// TODO: Is there any situation where we actually wanted a "bind" mount to
+		// fall through to the lcow differ?
 		return "", nil, fmt.Errorf("windowsDiff does not support layer type %s: %w", mnt.Type, errdefs.ErrNotImplemented)
 	}
 
@@ -332,6 +335,30 @@ func mountsToLayerAndParents(mounts []mount.Mount) (string, []string, error) {
 		return "", nil, err
 	}
 
+	isView := false
+	for _, o := range mnt.Options {
+		if o == "ro" {
+			isView = true
+			break
+		}
+	}
+
+	if isView {
+		if mnt.Type == "bind" && len(parentLayerPaths) != 0 {
+			return "", nil, fmt.Errorf("unexpected bind-mount View with parents: %w", errdefs.ErrInvalidArgument)
+		} else if mnt.Type == "bind" {
+			// rootfs.CreateDiff creates a new, empty View to diff against,
+			// when diffing something with no parent.
+			// This makes perfect sense for a walking Diff, but for WCOW,
+			// we have to recognise this as "diff against nothing"
+			return "", nil, nil
+		} else if len(parentLayerPaths) == 0 {
+			return "", nil, fmt.Errorf("unexpected windows-layer View with no parent: %w", errdefs.ErrInvalidArgument)
+		}
+		// Ignore the dummy sandbox.
+		return parentLayerPaths[0], parentLayerPaths[1:], nil
+	}
+
 	return mnt.Source, parentLayerPaths, nil
 }
 
@@ -346,8 +373,16 @@ func mountPairToLayerStack(lower, upper []mount.Mount) ([]string, error) {
 		return nil, fmt.Errorf("Upper mount invalid: %w", err)
 	}
 
+	lowerLayer, lowerParentLayerPaths, err := mountsToLayerAndParents(lower)
+	if errdefs.IsNotImplemented(err) {
+		// Upper was a windows-layer or bind, lower is not. We can't handle that.
+		return nil, fmt.Errorf("windowsDiff cannot diff a windows-layer against a non-windows-layer: %w", errdefs.ErrInvalidArgument)
+	} else if err != nil {
+		return nil, fmt.Errorf("Lower mount invalid: %w", err)
+	}
+
 	// Trivial case, diff-against-nothing
-	if len(lower) == 0 {
+	if lowerLayer == "" {
 		if len(upperParentLayerPaths) != 0 {
 			return nil, fmt.Errorf("windowsDiff cannot diff a layer with parents against a null layer: %w", errdefs.ErrInvalidArgument)
 		}
@@ -358,14 +393,6 @@ func mountPairToLayerStack(lower, upper []mount.Mount) ([]string, error) {
 		return nil, fmt.Errorf("windowsDiff cannot diff a layer with no parents against another layer: %w", errdefs.ErrInvalidArgument)
 	}
 
-	lowerLayer, lowerParentLayerPaths, err := mountsToLayerAndParents(lower)
-	if errdefs.IsNotImplemented(err) {
-		// Upper was a windows-layer, lower is not. We can't handle that.
-		return nil, fmt.Errorf("windowsDiff cannot diff a windows-layer against a non-windows-layer: %w", errdefs.ErrInvalidArgument)
-	} else if err != nil {
-		return nil, fmt.Errorf("Lower mount invalid: %w", err)
-	}
-
 	if upperParentLayerPaths[0] != lowerLayer {
 		return nil, fmt.Errorf("windowsDiff cannot diff a layer against a layer other than its own parent: %w", errdefs.ErrInvalidArgument)
 	}