diff --git a/backend/s3/cont_reader.go b/backend/s3/cont_reader.go
new file mode 100644
index 000000000..da0ef4000
--- /dev/null
+++ b/backend/s3/cont_reader.go
@@ -0,0 +1,23 @@
+package s3
+
+import (
+	"fmt"
+	"io"
+)
+
+// ContinuousReader implements an io.Reader on top of an io.ReaderAt, advancing
+// an offset.
+type ContinuousReader struct {
+	R      io.ReaderAt
+	Offset int64
+}
+
+func (c *ContinuousReader) Read(p []byte) (int, error) {
+	fmt.Printf("ContinuousReader %p: ReadAt(offset %v)\n", c, c.Offset)
+	n, err := c.R.ReadAt(p, c.Offset)
+	fmt.Printf("ContinuousReader %p: len(p) = %v, n %v, err %v\n",
+		c, len(p), n, err)
+	fmt.Printf("  %02x\n", p[:n])
+	c.Offset += int64(n)
+	return n, err
+}