object_db.go: replace decode with openDecode

In a subsequent commit, we will introduce a function to produce an
implementation of Object the concrete type of which is determined by the
type associated with the object named "SHA".

In order to do so, we would like to be able to call "o.decode()" without
having to open the object referred to above more than once (for example,
once to check the type, and the second time to read the object's
contents).

To make this possible, let's teach decode to accept an `*ObjectReader`
instead of an `sha []byte`, and defer the responsibility of calling
`o.open()` (which roughly translates a `[]byte` into an `*ObjectReader,
error)`), to an external caller.

To make calling `decode()` as easy as it was before this change, we'll
introduce `decodeOpen()`, which first calls `o.open()`, and then passes
on either (1) the resulting error, or (2) the result of calling
`o.decode()` with the newly opened object.

Author: ttaylorr

object_db.go

@@ -71,7 +71,7 @@ func (o *ObjectDatabase) Close() error {
 func (o *ObjectDatabase) Blob(sha []byte) (*Blob, error) {
 	var b Blob
 
-	if err := o.decode(sha, &b); err != nil {
+	if err := o.openDecode(sha, &b); err != nil {
 		return nil, err
 	}
 	return &b, nil
@@ -81,7 +81,7 @@ func (o *ObjectDatabase) Blob(sha []byte) (*Blob, error) {
 // encountered.
 func (o *ObjectDatabase) Tree(sha []byte) (*Tree, error) {
 	var t Tree
-	if err := o.decode(sha, &t); err != nil {
+	if err := o.openDecode(sha, &t); err != nil {
 		return nil, err
 	}
 	return &t, nil
@@ -92,7 +92,7 @@ func (o *ObjectDatabase) Tree(sha []byte) (*Tree, error) {
 func (o *ObjectDatabase) Commit(sha []byte) (*Commit, error) {
 	var c Commit
 
-	if err := o.decode(sha, &c); err != nil {
+	if err := o.openDecode(sha, &c); err != nil {
 		return nil, err
 	}
 	return &c, nil
@@ -103,7 +103,7 @@ func (o *ObjectDatabase) Commit(sha []byte) (*Commit, error) {
 func (o *ObjectDatabase) Tag(sha []byte) (*Tag, error) {
 	var t Tag
 
-	if err := o.decode(sha, &t); err != nil {
+	if err := o.openDecode(sha, &t); err != nil {
 		return nil, err
 	}
 	return &t, nil
@@ -272,6 +272,16 @@ func (o *ObjectDatabase) open(sha []byte) (*ObjectReader, error) {
 	return NewObjectReadCloser(f)
 }
 
+// openDecode calls decode (see: below) on the object named "sha" after openin
+// it.
+func (o *ObjectDatabase) openDecode(sha []byte, into Object) error {
+	r, err := o.open(sha)
+	if err != nil {
+		return err
+	}
+	return o.decode(r, into)
+}
+
 // decode decodes an object given by the sha "sha []byte" into the given object
 // "into", or returns an error if one was encountered.
 //
@@ -280,12 +290,7 @@ func (o *ObjectDatabase) open(sha []byte) (*ObjectReader, error) {
 // BlobObjectType. Blob's don't exhaust the buffer completely (they instead
 // maintain a handle on the blob's contents via an io.LimitedReader) and
 // therefore cannot be closed until signaled explicitly by gitobj.Blob.Close().
-func (o *ObjectDatabase) decode(sha []byte, into Object) error {
-	r, err := o.open(sha)
-	if err != nil {
-		return err
-	}
-
+func (o *ObjectDatabase) decode(r *ObjectReader, into Object) error {
 	typ, size, err := r.Header()
 	if err != nil {
 		return err