Major restructure, bundle blobs

This commit also breaks the repository format.

backend/generic.go

@@ -3,6 +3,7 @@ package backend
 import (
 	"crypto/sha256"
 	"errors"
+	"io"
 )
 
 const (
@@ -96,3 +97,33 @@ outer:
 
 	return IDSize, nil
 }
+
+// wrap around io.LimitedReader that implements io.ReadCloser
+type blobReader struct {
+	f      io.Closer
+	rd     io.Reader
+	closed bool
+}
+
+func (l *blobReader) Read(p []byte) (int, error) {
+	n, err := l.rd.Read(p)
+	if err == io.EOF {
+		l.Close()
+	}
+
+	return n, err
+}
+
+func (l *blobReader) Close() error {
+	if !l.closed {
+		err := l.f.Close()
+		l.closed = true
+		return err
+	}
+
+	return nil
+}
+
+func LimitReader(f io.ReadCloser, n int64) *blobReader {
+	return &blobReader{f: f, rd: io.LimitReader(f, n)}
+}