drop external dep and update copy function

Author: kjzz

daghelpers.go

@@ -95,22 +95,15 @@ func Copy(ctx context.Context, from, to DAGService, root *cid.Cid) error {
 		return err
 	}
 	links := node.Links()
-	if len(links) == 0 {
-		err := to.Add(ctx, node)
-		if err != nil {
-			return err
-		}
-		return nil
-	}
 	for _, link := range links {
 		err := Copy(ctx, from, to, link.Cid)
 		if err != nil {
 			return err
 		}
-		err = to.Add(ctx, node)
-		if err != nil {
-			return err
-		}
+	}
+	err = to.Add(ctx, node)
+	if err != nil {
+		return err
 	}
 	return nil
 }

daghelpers_test.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"github.com/ipfs/go-cid"
 	mh "github.com/multiformats/go-multihash"
-	"github.com/stretchr/testify/assert"
 	"testing"
 )
 
@@ -132,13 +131,11 @@ func TestCopy(t *testing.T) {
 	}
 
 	r, err := to.Get(ctx, root.Cid())
-	if err != nil {
-		t.Error(err)
+	if err != nil || len(r.Links()) != 2 {
+		t.Error("fail to copy dag")
 	}
 	l1, err := to.Get(ctx, l11.Cid())
-	if err != nil {
-		t.Error(err)
+	if err != nil || len(l1.Links()) != 3 {
+		t.Error("fail to copy dag")
 	}
-	assert.Equal(t, len(r.Links()), 2)
-	assert.Equal(t, len(l1.Links()), 3)
 }