[#421] governance: Don't construct alphabet list with empty sidechain

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

pkg/innerring/processors/governance/list.go

@@ -10,14 +10,20 @@ import (
 var (
 	errNotEnoughKeys  = errors.New("alphabet list in mainnet is too short")
 	errNotEqualLen    = errors.New("old and new alphabet lists have different length")
+	errEmptySidechain = errors.New("sidechain list is empty")
 )
 
 // newAlphabetList returns updated list of sidechain keys with no more than 1\3
-// of new keys from mainnet list. Function returns `errNotEnoughKeys` if
+// of new keys from mainnet list. Function returns `errEmptySidechain` if
-// mainnet list contains less keys than sidechain list. Function returns
+// sidechain list is empty. Function returns `errNotEnoughKeys` if mainnet
-// (nil, nil) if mainnet list contains all keys from sidechain list.
+// list contains less keys than sidechain list. Function returns (nil, nil) if
+// mainnet list contains all keys from sidechain list.
 func newAlphabetList(sidechain, mainnet keys.PublicKeys) (keys.PublicKeys, error) {
 	ln := len(sidechain)
+	if ln == 0 {
+		return nil, errEmptySidechain
+	}
+
 	if len(mainnet) < ln {
 		return nil, errors.Wrapf(errNotEnoughKeys, "expecting %d keys", ln)
 	}

pkg/innerring/processors/governance/list_test.go

@@ -14,6 +14,11 @@ func TestNewAlphabetList(t *testing.T) {
 
 	orig := keys.PublicKeys{k[0], k[1], k[2], k[3], k[4], k[5], k[6]}
 
+	t.Run("no sidechain keys", func(t *testing.T) {
+		_, err := newAlphabetList(nil, orig)
+		require.Error(t, err)
+	})
+
 	t.Run("same keys", func(t *testing.T) {
 		list, err := newAlphabetList(orig, orig)
 		require.NoError(t, err)