Use array instead of hash for backend.ID

Since backend.ID is always a slice of constant length, use an array
instead of a slice. Mostly, arrays behave as slices, except that an
array cannot be nil, so use `*backend.ID` insteaf of `backend.ID` in
places where the absence of an ID is possible (e.g. for the Subtree of a
Node, which may not present when the node is a file node).

This change allows to directly use backend.ID as the the key for a map,
so that arbitrary data structures (e.g. a Set implemented as a
map[backend.ID]struct{}) can easily be formed.

pack/pack.go

@@ -165,8 +165,8 @@ func (p *Packer) writeHeader(wr io.Writer) (bytesWritten uint, err error) {
 		entry := headerEntry{
 			Type:   b.Type,
 			Length: b.Length,
+			ID:     b.ID,
 		}
-		copy(entry.ID[:], b.ID)
 
 		err := binary.Write(wr, binary.LittleEndian, entry)
 		if err != nil {
@@ -184,7 +184,11 @@ func (p *Packer) ID() backend.ID {
 	p.m.Lock()
 	defer p.m.Unlock()
 
-	return p.hw.Sum(nil)
+	hash := p.hw.Sum(nil)
+	id := backend.ID{}
+	copy(id[:], hash)
+
+	return id
 }
 
 // Size returns the number of bytes written so far.
@@ -273,7 +277,7 @@ func NewUnpacker(k *crypto.Key, entries []Blob, rd io.ReadSeeker) (*Unpacker, er
 			entries = append(entries, Blob{
 				Type:   e.Type,
 				Length: e.Length,
-				ID:     e.ID[:],
+				ID:     e.ID,
 				Offset: pos,
 			})
 

pack/pack_test.go

@@ -31,7 +31,7 @@ func TestCreatePack(t *testing.T) {
 		_, err := io.ReadFull(rand.Reader, b)
 		OK(t, err)
 		h := sha256.Sum256(b)
-		bufs = append(bufs, Buf{data: b, id: h[:]})
+		bufs = append(bufs, Buf{data: b, id: h})
 	}
 
 	file := bytes.NewBuffer(nil)