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)}
+}

backend/interface.go

@@ -18,7 +18,7 @@ const (
 	Version = 1
 )
 
-// A Backend manages blobs of data.
+// A Backend manages data stored somewhere.
 type Backend interface {
 	// Location returns a string that specifies the location of the repository,
 	// like a URL.
@@ -31,6 +31,10 @@ type Backend interface {
 	// Get returns an io.ReadCloser for the Blob with the given name of type t.
 	Get(t Type, name string) (io.ReadCloser, error)
 
+	// GetReader returns an io.ReadCloser for the Blob with the given name of
+	// type t at offset and length.
+	GetReader(t Type, name string, offset, length uint) (io.ReadCloser, error)
+
 	// Test a boolean value whether a Blob with the name and type exists.
 	Test(t Type, name string) (bool, error)
 

backend/local/local.go

@@ -302,6 +302,26 @@ func (b *Local) Get(t backend.Type, name string) (io.ReadCloser, error) {
 	return os.Open(filename(b.p, t, name))
 }
 
+// GetReader returns an io.ReadCloser for the Blob with the given name of
+// type t at offset and length. If length is 0, the reader reads until EOF.
+func (b *Local) GetReader(t backend.Type, name string, offset, length uint) (io.ReadCloser, error) {
+	f, err := os.Open(filename(b.p, t, name))
+	if err != nil {
+		return nil, err
+	}
+
+	_, err = f.Seek(int64(offset), 0)
+	if err != nil {
+		return nil, err
+	}
+
+	if length == 0 {
+		return f, nil
+	}
+
+	return backend.LimitReader(f, int64(length)), nil
+}
+
 // Test returns true if a blob of the given type and name exists in the backend.
 func (b *Local) Test(t backend.Type, name string) (bool, error) {
 	_, err := os.Stat(filename(b.p, t, name))

backend/sftp/sftp.go

@@ -436,6 +436,26 @@ func (r *SFTP) Get(t backend.Type, name string) (io.ReadCloser, error) {
 	return file, nil
 }
 
+// GetReader returns an io.ReadCloser for the Blob with the given name of
+// type t at offset and length. If length is 0, the reader reads until EOF.
+func (r *SFTP) GetReader(t backend.Type, name string, offset, length uint) (io.ReadCloser, error) {
+	f, err := r.c.Open(r.filename(t, name))
+	if err != nil {
+		return nil, err
+	}
+
+	_, err = f.Seek(int64(offset), 0)
+	if err != nil {
+		return nil, err
+	}
+
+	if length == 0 {
+		return f, nil
+	}
+
+	return backend.LimitReader(f, int64(length)), nil
+}
+
 // Test returns true if a blob of the given type and name exists in the backend.
 func (r *SFTP) Test(t backend.Type, name string) (bool, error) {
 	_, err := r.c.Lstat(r.filename(t, name))