Return nil as Index reader when reading indexless CARv2

Fix an issue where if a  `v2.Reader` is given an indexless CARv2, an
invalid section reader is returned.

Add tests to assert fix using CARv1 and indexless CARv2 sample files.


This commit was moved from ipld/go-car@1bac13d

ipld/car/v2/reader.go

@@ -113,10 +113,10 @@ func (r *Reader) DataReader() SectionReader {
 // present. Otherwise, returns nil.
 // Note, this function will always return nil if the backing payload represents a CARv1.
 func (r *Reader) IndexReader() io.Reader {
-	if r.Version == 2 {
-		return internalio.NewOffsetReadSeeker(r.r, int64(r.Header.IndexOffset))
+	if r.Version == 1 || !r.Header.HasIndex() {
+		return nil
 	}
-	return nil
+	return internalio.NewOffsetReadSeeker(r.r, int64(r.Header.IndexOffset))
 }
 
 // Close closes the underlying reader if it was opened by OpenReader.

ipld/car/v2/reader_test.go

@@ -212,6 +212,30 @@ func TestOpenReader_DoesNotPanicForReadersCreatedAfterClosure(t *testing.T) {
 	require.NotPanics(t, func() { panicTest(iReaderAfterClosure) })
 }
 
+func TestReader_ReturnsNilWhenThereIsNoIndex(t *testing.T) {
+	tests := []struct {
+		name string
+		path string
+	}{
+		{
+			name: "IndexlessCarV2",
+			path: "testdata/sample-v2-indexless.car",
+		},
+		{
+			name: "CarV1",
+			path: "testdata/sample-v1.car",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			subject, err := carv2.OpenReader(tt.path)
+			require.NoError(t, err)
+			t.Cleanup(func() { require.NoError(t, subject.Close()) })
+			require.Nil(t, subject.IndexReader())
+		})
+	}
+}
+
 func requireNewCarV1ReaderFromV2File(t *testing.T, carV12Path string, zerLenAsEOF bool) *carv1.CarReader {
 	f, err := os.Open(carV12Path)
 	require.NoError(t, err)