add multireader

Signed-off-by: Avi Deitcher <avi@deitcher.net>

pkg/content/multireader.go

@@ -0,0 +1,40 @@
+package content
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/containerd/containerd/content"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+)
+
+// MultiReader store to read content from multiple stores. It finds the content by asking each underlying
+// store to find the content, which it does based on the hash.
+//
+// Example:
+//        fileStore := NewFileStore(rootPath)
+//        memoryStore := NewMemoryStore()
+//        // load up content in fileStore and memoryStore
+//        multiStore := MultiReader([]content.Provider{fileStore, memoryStore})
+//
+// You now can use multiStore anywhere that content.Provider is accepted
+type MultiReader struct {
+	stores []content.Provider
+}
+
+// AddStore add a store to read from
+func (m *MultiReader) AddStore(store ...content.Provider) {
+	m.stores = append(m.stores, store...)
+}
+
+// ReaderAt get a reader
+func (m MultiReader) ReaderAt(ctx context.Context, desc ocispec.Descriptor) (content.ReaderAt, error) {
+	for _, store := range m.stores {
+		r, err := store.ReaderAt(ctx, desc)
+		if r != nil && err == nil {
+			return r, nil
+		}
+	}
+	// we did not find any
+	return nil, fmt.Errorf("not found")
+}