Add reader option to local content reader at

Allows optimized copying from a local content file into another file.

Signed-off-by: Derek McGowan <derek@mcg.dev>

content/helpers.go

@@ -37,10 +37,16 @@ var bufPool = sync.Pool{
 	},
 }
 
+type reader interface {
+	Reader() io.Reader
+}
+
 // NewReader returns a io.Reader from a ReaderAt
 func NewReader(ra ReaderAt) io.Reader {
-	rd := io.NewSectionReader(ra, 0, ra.Size())
-	return rd
+	if rd, ok := ra.(reader); ok {
+		return rd.Reader()
+	}
+	return io.NewSectionReader(ra, 0, ra.Size())
 }
 
 // ReadBlob retrieves the entire contents of the blob from the provider.

content/local/readerat.go

@@ -18,6 +18,7 @@ package local
 
 import (
 	"fmt"
+	"io"
 	"os"
 
 	"github.com/containerd/containerd/content"
@@ -65,3 +66,7 @@ func (ra sizeReaderAt) Size() int64 {
 func (ra sizeReaderAt) Close() error {
 	return ra.fp.Close()
 }
+
+func (ra sizeReaderAt) Reader() io.Reader {
+	return io.LimitReader(ra.fp, ra.size)
+}