object_db.go: introduce Object()

Historically, callers have been able to access different objects stored
in the object database by (1) knowing the type of the object that they
desire to open and (2) calling the function associated with that type
(e.g., `Blob()`, `Commit()`, and so on) based on that information.

Sometimes, however, callers would like to open an object of known
existence but unknown type. Let's introduce `Object()` to do just that,
and construct a concrete implementation based on the type of object in
the header.

Author: ttaylorr

object_db.go

@@ -66,6 +66,38 @@ func (o *ObjectDatabase) Close() error {
 	return nil
 }
 
+// Object returns an Object (of unknown implementation) satisfying the type
+// associated with the object named "sha".
+//
+// If the object could not be opened, is of unknown type, or could not be
+// decoded, than an appropriate error is returned instead.
+func (o *ObjectDatabase) Object(sha []byte) (Object, error) {
+	r, err := o.open(sha)
+	if err != nil {
+		return nil, err
+	}
+
+	typ, _, err := r.Header()
+	if err != nil {
+		return nil, err
+	}
+
+	var into Object
+	switch typ {
+	case BlobObjectType:
+		into = new(Blob)
+	case TreeObjectType:
+		into = new(Tree)
+	case CommitObjectType:
+		into = new(Commit)
+	case TagObjectType:
+		into = new(Tag)
+	default:
+		return nil, fmt.Errorf("gitobj: unknown object type: %s", typ)
+	}
+	return into, o.decode(r, into)
+}
+
 // Blob returns a *Blob as identified by the SHA given, or an error if one was
 // encountered.
 func (o *ObjectDatabase) Blob(sha []byte) (*Blob, error) {

object_db_test.go

@@ -15,6 +15,32 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func TestDecodeObject(t *testing.T) {
+	sha := "af5626b4a114abcb82d63db7c8082c3c4756e51b"
+	contents := "Hello, world!\n"
+
+	var buf bytes.Buffer
+
+	zw := zlib.NewWriter(&buf)
+	fmt.Fprintf(zw, "blob 14\x00%s", contents)
+	zw.Close()
+
+	odb := &ObjectDatabase{s: newMemoryStorer(map[string]io.ReadWriter{
+		sha: &buf,
+	})}
+
+	shaHex, _ := hex.DecodeString(sha)
+	obj, err := odb.Object(shaHex)
+	blob, ok := obj.(*Blob)
+
+	require.NoError(t, err)
+	require.True(t, ok)
+
+	got, err := ioutil.ReadAll(blob.Contents)
+	assert.Nil(t, err)
+	assert.Equal(t, contents, string(got))
+}
+
 func TestDecodeBlob(t *testing.T) {
 	sha := "af5626b4a114abcb82d63db7c8082c3c4756e51b"
 	contents := "Hello, world!\n"