add minimal test case

mutable_tree_test.go

@@ -332,3 +332,69 @@ func TestDeleteVersion_issue261(t *testing.T) {
 	// 9 is about to be created.
 	makeVersions(t, levelDB, 9, 4)
 }
+
+func TestDeleteVersion_issue261_minimal(t *testing.T) {
+	// Use the same on-disk database for all runs.
+	tempdir, err := ioutil.TempDir("", "iavl")
+	require.NoError(t, err)
+	defer os.RemoveAll(tempdir)
+
+	levelDB, err := db.NewGoLevelDB("leveldb", tempdir)
+	require.NoError(t, err)
+
+	// Create a new tree that persists data to disk every 2 versions. KeepRecent: 1 mirrors
+	// SDK behavior.
+	tree, err := NewMutableTreeWithOpts(levelDB, db.NewMemDB(), 0, &Options{
+		KeepEvery:  2,
+		KeepRecent: 1,
+	})
+	require.NoError(t, err)
+	version, err := tree.Load()
+	require.NoError(t, err)
+	require.EqualValues(t, 0, version)
+
+	// Write two versions, where the second is persisted to disk.
+	tree.Set([]byte{1}, []byte{1})
+	tree.Set([]byte{2}, []byte{2})
+	_, _, err = tree.SaveVersion()
+	require.NoError(t, err)
+
+	tree.Set([]byte{3}, []byte{3})
+	tree.Set([]byte{4}, []byte{4})
+	_, version, err = tree.SaveVersion()
+	require.NoError(t, err)
+
+	// Now, make a single version that is only persisted in memory. This will cause the previous
+	// node to become orphaned, and the orphaning incorrectly persisted to disk.
+	tree.Set([]byte{4}, []byte{0})
+	_, _, err = tree.SaveVersion()
+	require.NoError(t, err)
+
+	// Close the tree, and set up a new one which loads the persisted version 2.
+	tree, err = NewMutableTreeWithOpts(levelDB, db.NewMemDB(), 0, &Options{
+		KeepEvery:  2,
+		KeepRecent: 1,
+	})
+	require.NoError(t, err)
+	version, err = tree.Load()
+	require.NoError(t, err)
+	require.EqualValues(t, 2, version)
+
+	// If we now write two other versions, which do not touch the branch of key 4, and then
+	// delete version 2 (following SDK behavior), this will cause key 4 to go missing because the
+	// incorrect orphan written during in-memory version 3 has scheduled it for deletion, even
+	// though the change that caused it to be orphaned now hasn't happened.
+	tree.Set([]byte{1}, []byte{0})
+	_, _, err = tree.SaveVersion()
+	require.NoError(t, err)
+
+	tree.Set([]byte{2}, []byte{0})
+	_, _, err = tree.SaveVersion()
+	require.NoError(t, err)
+
+	err = tree.DeleteVersion(2)
+	require.NoError(t, err)
+
+	v, _ := tree.Get([]byte{4})
+	assert.Equal(t, []byte{4}, v)
+}