git: return better error message when packfile cannot be downloaded

plumbing/format/packfile/common.go

@@ -51,7 +51,13 @@ func WritePackfileToObjectStorage(
 	}
 
 	defer ioutil.CheckClose(w, &err)
-	_, err = io.Copy(w, packfile)
+
+	var n int64
+	n, err = io.Copy(w, packfile)
+	if err == nil && n == 0 {
+		return ErrEmptyPackfile
+	}
+
 	return err
 }
 

plumbing/format/packfile/common_test.go

@@ -1,15 +1,29 @@
 package packfile
 
 import (
+	"bytes"
 	"testing"
 
 	"gopkg.in/src-d/go-git.v4/plumbing"
+	"gopkg.in/src-d/go-git.v4/storage/memory"
 
 	. "gopkg.in/check.v1"
 )
 
 func Test(t *testing.T) { TestingT(t) }
 
+type CommonSuite struct{}
+
+var _ = Suite(&CommonSuite{})
+
+func (s *CommonSuite) TestEmptyUpdateObjectStorage(c *C) {
+	var buf bytes.Buffer
+	sto := memory.NewStorage()
+
+	err := UpdateObjectStorage(sto, &buf)
+	c.Assert(err, Equals, ErrEmptyPackfile)
+}
+
 func newObject(t plumbing.ObjectType, cont []byte) plumbing.EncodedObject {
 	o := plumbing.MemoryObject{}
 	o.SetType(t)

repository.go

@@ -41,6 +41,8 @@ var (
 	ErrTagExists = errors.New("tag already exists")
 	// ErrTagNotFound an error stating the specified tag does not exist
 	ErrTagNotFound = errors.New("tag not found")
+	// ErrFetching is returned when the packfile could not be downloaded
+	ErrFetching = errors.New("unable to fetch packfile")
 
 	ErrInvalidReference          = errors.New("invalid reference, should be a tag or a branch")
 	ErrRepositoryNotExists       = errors.New("repository does not exist")
@@ -858,6 +860,8 @@ func (r *Repository) fetchAndUpdateReferences(
 	remoteRefs, err := remote.fetch(ctx, o)
 	if err == NoErrAlreadyUpToDate {
 		objsUpdated = false
+	} else if err == packfile.ErrEmptyPackfile {
+		return nil, ErrFetching
 	} else if err != nil {
 		return nil, err
 	}