Add ReaderAt support to content store

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

content/readerat.go

@@ -0,0 +1,26 @@
+package content
+
+import (
+	"io"
+	"os"
+)
+
+// readerat implements io.ReaderAt in a completely stateless manner by opening
+// the referenced file for each call to ReadAt.
+type readerAt struct {
+	f string
+}
+
+func (ra readerAt) ReadAt(p []byte, offset int64) (int, error) {
+	fp, err := os.Open(ra.f)
+	if err != nil {
+		return 0, err
+	}
+	defer fp.Close()
+
+	if _, err := fp.Seek(offset, io.SeekStart); err != nil {
+		return 0, err
+	}
+
+	return fp.Read(p)
+}