Merge pull request #49 from tri-adam/opt-descriptors

feat: add OptWriteWithSpareDescriptorCapacity

pkg/sif/write.go

@@ -201,16 +201,46 @@ func numDescriptorsForIndex(ii v1.ImageIndex) (int64, error) {
 	return count + 1, nil
 }
 
+// writeOpts accumulates write options.
+type writeOpts struct {
+	spareDescriptors int64
+}
+
+// WriteOpt are used to specify write options.
+type WriteOpt func(*writeOpts) error
+
+// OptWriteWithSpareDescriptorCapacity specifies that the SIF should be created with n spare
+// descriptors.
+func OptWriteWithSpareDescriptorCapacity(n int64) WriteOpt {
+	return func(wo *writeOpts) error {
+		wo.spareDescriptors = n
+		return nil
+	}
+}
+
 // Write constructs a SIF at path from an ImageIndex.
-func Write(path string, ii v1.ImageIndex) error {
+//
+// By default, the SIF is created with the exact number of descriptors required to represent ii. To
+// include spare descriptor capacity, consider using OptWriteWithSpareDescriptorCapacity.
+func Write(path string, ii v1.ImageIndex, opts ...WriteOpt) error {
+	wo := writeOpts{
+		spareDescriptors: 0,
+	}
+
+	for _, opt := range opts {
+		if err := opt(&wo); err != nil {
+			return err
+		}
+	}
+
 	n, err := numDescriptorsForIndex(ii)
 	if err != nil {
 		return err
 	}
 
 	fi, err := sif.CreateContainerAtPath(path,
 		sif.OptCreateDeterministic(),
-		sif.OptCreateWithDescriptorCapacity(n),
+		sif.OptCreateWithDescriptorCapacity(n+wo.spareDescriptors),
 	)
 	if err != nil {
 		return err

pkg/sif/write_test.go

@@ -22,6 +22,7 @@ func TestWrite(t *testing.T) {
 	tests := []struct {
 		name string
 		ii   v1.ImageIndex
+		opts []sif.WriteOpt
 	}{
 		{
 			name: "DockerManifest",
@@ -35,12 +36,19 @@ func TestWrite(t *testing.T) {
 			name: "ManyLayers",
 			ii:   corpus.ImageIndex(t, "many-layers"),
 		},
+		{
+			name: "SpareDescriptor",
+			ii:   corpus.ImageIndex(t, "hello-world-docker-v2-manifest"),
+			opts: []sif.WriteOpt{
+				sif.OptWriteWithSpareDescriptorCapacity(1),
+			},
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			path := filepath.Join(t.TempDir(), "image.sif")
 
-			if err := sif.Write(path, tt.ii); err != nil {
+			if err := sif.Write(path, tt.ii, tt.opts...); err != nil {
 				t.Fatal(err)
 			}