packfile: permit methods on nil *Index

packfile/index.go

@@ -40,7 +40,8 @@ head room.
 */
 
 // Index is an in-memory mapping of object IDs to offsets within a packfile.
-// This maps 1:1 with index files produced by git-index-pack(1).
+// This maps 1:1 with index files produced by git-index-pack(1). (*Index)(nil)
+// is treated the same as the Index for an empty packfile.
 type Index struct {
 	// ObjectIDs is a sorted list of object IDs in the packfile.
 	ObjectIDs []githash.SHA1
@@ -234,6 +235,9 @@ func readFull(r io.Reader, buf []byte) (int, error) {
 
 // EncodeV2 writes idx in Git's packfile index version 2 format.
 func (idx *Index) EncodeV2(w io.Writer) error {
+	if idx == nil {
+		idx = &Index{PackfileSHA1: emptyPackfileSHA1()}
+	}
 	if err := idx.validate(); err != nil {
 		return fmt.Errorf("write packfile index: %w", err)
 	}
@@ -299,6 +303,9 @@ func (idx *Index) EncodeV2(w io.Writer) error {
 // should only be used for compatibility, since the version 1 format does not
 // store PackedChecksums and do not support packfiles larger than 4 GiB.
 func (idx *Index) EncodeV1(w io.Writer) error {
+	if idx == nil {
+		idx = &Index{PackfileSHA1: emptyPackfileSHA1()}
+	}
 	if err := idx.validate(); err != nil {
 		return fmt.Errorf("write packfile index: %w", err)
 	}
@@ -370,6 +377,17 @@ func (idx *Index) encodeFanOut(w io.Writer) error {
 	return nil
 }
 
+func emptyPackfileSHA1() githash.SHA1 {
+	buf := new(bytes.Buffer)
+	w := NewWriter(buf, 0)
+	if err := w.Close(); err != nil {
+		panic(err)
+	}
+	var sum githash.SHA1
+	copy(sum[:], buf.Bytes()[buf.Len()-githash.SHA1Size:])
+	return sum
+}
+
 // MarshalBinary encodes the index in Git's packfile index version 2 format.
 func (idx *Index) MarshalBinary() ([]byte, error) {
 	buf := new(bytes.Buffer)
@@ -383,6 +401,9 @@ func (idx *Index) MarshalBinary() ([]byte, error) {
 // present in the index. The result is undefined if idx.ObjectIDs is not sorted.
 // This search is O(log len(idx.ObjectIDs)).
 func (idx *Index) FindID(id githash.SHA1) int {
+	if idx == nil {
+		return -1
+	}
 	i := sort.Search(len(idx.ObjectIDs), func(i int) bool {
 		return bytes.Compare(idx.ObjectIDs[i][:], id[:]) >= 0
 	})
@@ -394,6 +415,9 @@ func (idx *Index) FindID(id githash.SHA1) int {
 
 // Len returns the number of objects in the index.
 func (idx *Index) Len() int {
+	if idx == nil {
+		return 0
+	}
 	return len(idx.ObjectIDs)
 }
 

packfile/index_test.go

@@ -124,13 +124,27 @@ func TestIndexEncodeV1(t *testing.T) {
 			}
 			got := new(bytes.Buffer)
 			if err := test.wantIndex.EncodeV1(got); err != nil {
-				t.Error("ReadIndex:", err)
+				t.Error("EncodeV1:", err)
 			}
 			if diff := cmp.Diff(want, got.Bytes()); diff != "" {
 				t.Errorf("index (-want +got):\n%s", diff)
 			}
 		})
 	}
+
+	t.Run("Nil", func(t *testing.T) {
+		want, err := ioutil.ReadFile(filepath.Join("testdata", "Empty.idx1"))
+		if err != nil {
+			t.Fatal(err)
+		}
+		got := new(bytes.Buffer)
+		if err := (*Index)(nil).EncodeV1(got); err != nil {
+			t.Error("EncodeV1:", err)
+		}
+		if diff := cmp.Diff(want, got.Bytes()); diff != "" {
+			t.Errorf("index (-want +got):\n%s", diff)
+		}
+	})
 }
 
 func TestIndexEncodeV2(t *testing.T) {
@@ -145,7 +159,7 @@ func TestIndexEncodeV2(t *testing.T) {
 			}
 			got := new(bytes.Buffer)
 			if err := test.wantIndex.EncodeV2(got); err != nil {
-				t.Error("ReadIndex:", err)
+				t.Error("EncodeV2:", err)
 			}
 			if diff := cmp.Diff(want, got.Bytes()); diff != "" {
 				t.Errorf("index (-want +got):\n%s", diff)
@@ -160,7 +174,21 @@ func TestIndexEncodeV2(t *testing.T) {
 		}
 		got := new(bytes.Buffer)
 		if err := bigOffsetIndex.EncodeV2(got); err != nil {
-			t.Error("ReadIndex:", err)
+			t.Error("EncodeV2:", err)
+		}
+		if diff := cmp.Diff(want, got.Bytes()); diff != "" {
+			t.Errorf("index (-want +got):\n%s", diff)
+		}
+	})
+
+	t.Run("Nil", func(t *testing.T) {
+		want, err := ioutil.ReadFile(filepath.Join("testdata", "Empty.idx2"))
+		if err != nil {
+			t.Fatal(err)
+		}
+		got := new(bytes.Buffer)
+		if err := (*Index)(nil).EncodeV2(got); err != nil {
+			t.Error("EncodeV2:", err)
 		}
 		if diff := cmp.Diff(want, got.Bytes()); diff != "" {
 			t.Errorf("index (-want +got):\n%s", diff)