pack: add a storage wrapper for packs

Add a wrapper around a pack set that provides a storage interface as
defined by the fs module.  This allows us to emulate object storage from
packs in an abstract way.

Author: bk2204

pack/delayed_object.go

@@ -0,0 +1,41 @@
+package pack
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"strings"
+)
+
+// delayedObjectReader provides an interface for reading from an Object while
+// loading object data into memory only on demand.  It implements io.ReadCloser.
+type delayedObjectReader struct {
+	obj *Object
+	mr  io.Reader
+}
+
+// Read implements the io.Reader method by instantiating a new underlying reader
+// only on demand.
+func (d *delayedObjectReader) Read(b []byte) (int, error) {
+	if d.mr == nil {
+		data, err := d.obj.Unpack()
+		if err != nil {
+			return 0, err
+		}
+		d.mr = io.MultiReader(
+			// Git object header:
+			strings.NewReader(fmt.Sprintf("%s %d\x00",
+				d.obj.Type(), len(data),
+			)),
+
+			// Git object (uncompressed) contents:
+			bytes.NewReader(data),
+		)
+	}
+	return d.mr.Read(b)
+}
+
+// Close implements the io.Closer interface.
+func (d *delayedObjectReader) Close() error {
+	return nil
+}

pack/storage.go

@@ -0,0 +1,38 @@
+package pack
+
+import (
+	"io"
+)
+
+// Storage implements the storage.Storage interface.
+type Storage struct {
+	packs *Set
+}
+
+// NewStorage returns a new storage object based on a pack set.
+func NewStorage(root string) (*Storage, error) {
+	packs, err := NewSet(root)
+	if err != nil {
+		return nil, err
+	}
+	return &Storage{packs: packs}, nil
+}
+
+// Open implements the storage.Storage.Open interface.
+func (f *Storage) Open(oid []byte) (r io.ReadCloser, err error) {
+	obj, err := f.packs.Object(oid)
+	if err != nil {
+		return nil, err
+	}
+	return &delayedObjectReader{obj: obj}, nil
+}
+
+// Open implements the storage.Storage.Open interface.
+func (f *Storage) Close() error {
+	return f.packs.Close()
+}
+
+// IsCompressed returns false, because data returned is already decompressed.
+func (f *Storage) IsCompressed() bool {
+	return false
+}