common/BitArray: reduce fragility with methods

Fixes https://github.com/tendermint/tmlibs/issues/145
Fixes https://github.com/tendermint/tmlibs/issues/146

The code in here has been fragile when it comes to nil
but these edge cases were never tested, although they've
showed up in the wild and were only noticed because
the reporter actually read the logs otherwise
we'd have never known.

This changes covers some of these cases and adds some tests.

common/bit_array.go

@@ -99,8 +99,14 @@ func (bA *BitArray) copyBits(bits int) *BitArray {
 
 // Returns a BitArray of larger bits size.
 func (bA *BitArray) Or(o *BitArray) *BitArray {
-	if bA == nil {
-		o.Copy()
+	if bA == nil && o == nil {
+		return nil
+	}
+	if bA == nil && o != nil {
+		return o.Copy()
+	}
+	if o == nil {
+		return bA.Copy()
 	}
 	bA.mtx.Lock()
 	defer bA.mtx.Unlock()
@@ -113,7 +119,7 @@ func (bA *BitArray) Or(o *BitArray) *BitArray {
 
 // Returns a BitArray of smaller bit size.
 func (bA *BitArray) And(o *BitArray) *BitArray {
-	if bA == nil {
+	if bA == nil || o == nil {
 		return nil
 	}
 	bA.mtx.Lock()
@@ -143,7 +149,8 @@ func (bA *BitArray) Not() *BitArray {
 }
 
 func (bA *BitArray) Sub(o *BitArray) *BitArray {
-	if bA == nil {
+	if bA == nil || o == nil {
+		// TODO: Decide if we should do 1's complement here?
 		return nil
 	}
 	bA.mtx.Lock()

common/bit_array_test.go

@@ -3,6 +3,8 @@ package common
 import (
 	"bytes"
 	"testing"
+
+	"github.com/stretchr/testify/require"
 )
 
 func randBitArray(bits int) (*BitArray, []byte) {
@@ -26,6 +28,11 @@ func TestAnd(t *testing.T) {
 	bA2, _ := randBitArray(31)
 	bA3 := bA1.And(bA2)
 
+	var bNil *BitArray
+	require.Equal(t, bNil.And(bA1), (*BitArray)(nil))
+	require.Equal(t, bA1.And(nil), (*BitArray)(nil))
+	require.Equal(t, bNil.And(nil), (*BitArray)(nil))
+
 	if bA3.Bits != 31 {
 		t.Error("Expected min bits", bA3.Bits)
 	}
@@ -46,6 +53,11 @@ func TestOr(t *testing.T) {
 	bA2, _ := randBitArray(31)
 	bA3 := bA1.Or(bA2)
 
+	bNil := (*BitArray)(nil)
+	require.Equal(t, bNil.Or(bA1), bA1)
+	require.Equal(t, bA1.Or(nil), bA1)
+	require.Equal(t, bNil.Or(nil), (*BitArray)(nil))
+
 	if bA3.Bits != 51 {
 		t.Error("Expected max bits")
 	}
@@ -66,6 +78,11 @@ func TestSub1(t *testing.T) {
 	bA2, _ := randBitArray(51)
 	bA3 := bA1.Sub(bA2)
 
+	bNil := (*BitArray)(nil)
+	require.Equal(t, bNil.Sub(bA1), (*BitArray)(nil))
+	require.Equal(t, bA1.Sub(nil), (*BitArray)(nil))
+	require.Equal(t, bNil.Sub(nil), (*BitArray)(nil))
+
 	if bA3.Bits != bA1.Bits {
 		t.Error("Expected bA1 bits")
 	}
@@ -89,6 +106,11 @@ func TestSub2(t *testing.T) {
 	bA2, _ := randBitArray(31)
 	bA3 := bA1.Sub(bA2)
 
+	bNil := (*BitArray)(nil)
+	require.Equal(t, bNil.Sub(bA1), (*BitArray)(nil))
+	require.Equal(t, bA1.Sub(nil), (*BitArray)(nil))
+	require.Equal(t, bNil.Sub(nil), (*BitArray)(nil))
+
 	if bA3.Bits != bA1.Bits {
 		t.Error("Expected bA1 bits")
 	}