layer: add filesystem MediaType

To enable mounting of node filesystems, a new MediaType,
'application/x-filesystem' is introduced.
As the fileystem does not provide a ReaderAt, the Reader function has
been updated with a check to return an error if the layer
does not have a ReaderAt.

Signed-off-by: Matthias Meidinger <[email protected]>
Signed-off-by: Piotr Rygielski <[email protected]>

layer.go

@@ -112,6 +112,13 @@ func (l *Layer) Init(ctx context.Context, desc *LayerDescription, r io.ReaderAt)
 			return fmt.Errorf("claircore: layer %v: unable to create fs.FS: %w", desc.Digest, err)
 		}
 		l.sys = sys
+	case `application/vnd.claircore.filesystem`:
+		if desc.URI == "" {
+			return fmt.Errorf("claircore: layer %v: unable to create fs.FS: no URI provided", desc.Digest)
+		}
+		sys := os.DirFS(desc.URI)
+		l.sys = sys
+		l.rd = nil // The reader cannot be used for a filesystem
 	default:
 		return fmt.Errorf("claircore: layer %v: unknown MediaType %q", desc.Digest, desc.MediaType)
 	}
@@ -196,6 +203,9 @@ func (l *Layer) Reader() (ReadAtCloser, error) {
 	if !l.init {
 		return nil, errors.New("claircore: unable to return Reader: uninitialized Layer")
 	}
+	if l.rd == nil {
+		return nil, errors.New("claircore: unable to return Reader: ReaderAt is nil")
+	}
 	// Some hacks for making the returned ReadAtCloser implements as many
 	// interfaces as possible.
 	switch rd := l.rd.(type) {

layer_test.go

@@ -48,6 +48,21 @@ func TestLayer(t *testing.T) {
 		return &l
 	}
 
+	goodFilesystem := func(t *testing.T) *claircore.Layer {
+		t.Helper()
+		var l claircore.Layer
+		desc := claircore.LayerDescription{
+			Digest:    "sha256:" + strings.Repeat("00c0ffee", 8),
+			MediaType: `application/vnd.claircore.filesystem`,
+			URI:       t.TempDir(),
+		}
+
+		if err := l.Init(ctx, &desc, nil); err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		return &l
+	}
+
 	t.Run("Init", func(t *testing.T) {
 		t.Run("Checksum", func(t *testing.T) {
 			var l claircore.Layer
@@ -94,6 +109,25 @@ func TestLayer(t *testing.T) {
 				t.Error("unexpected success")
 			}
 		})
+		t.Run("Filesystem", func(t *testing.T) {
+			l := goodFilesystem(t)
+			if err := l.Close(); err != nil {
+				t.Errorf("close error: %v", err)
+			}
+		})
+		t.Run("FilesystemNoURI", func(t *testing.T) {
+			var l claircore.Layer
+			desc := claircore.LayerDescription{
+				Digest:    "sha256:" + strings.Repeat("00c0ffee", 8),
+				MediaType: `application/vnd.claircore.filesystem`,
+				URI:       "",
+			}
+
+			if err := l.Init(ctx, &desc, nil); err == nil {
+				t.Error("unexpected success")
+			}
+		})
+
 	})
 	t.Run("Close", func(t *testing.T) {
 		t.Run("Success", func(t *testing.T) {
@@ -177,6 +211,19 @@ func TestLayer(t *testing.T) {
 				t.Error("unexpected error")
 			}
 		})
+		t.Run("FilesystemSuccess", func(t *testing.T) {
+			l := goodFilesystem(t)
+			t.Cleanup(func() {
+				if err := l.Close(); err != nil {
+					t.Errorf("close error: %v", err)
+				}
+			})
+			_, err := l.FS()
+			t.Logf("error: %v", err)
+			if err != nil {
+				t.Error("unexpected error")
+			}
+		})
 	})
 	t.Run("Reader", func(t *testing.T) {
 		t.Run("Fail", func(t *testing.T) {
@@ -208,5 +255,17 @@ func TestLayer(t *testing.T) {
 				t.Errorf("unexpected error: read %d bytes, got error: %v", n, err)
 			}
 		})
+		t.Run("Filesystem", func(t *testing.T) {
+			l := goodFilesystem(t)
+			t.Cleanup(func() {
+				if err := l.Close(); err != nil {
+					t.Errorf("close error: %v", err)
+				}
+			})
+			_, err := l.Reader()
+			if err == nil {
+				t.Error("unexpected success")
+			}
+		})
 	})
 }