lnwire: add new RequiresFeature method

In this commit, we add a new RequiresFeature method to the feature
vector struct. This method allows us to check if the set of features
we're examining *require* that the even portion of a bit pair be set.
This can be used to check if new behavior should be allowed (after we
flip new bits to be required) for existing contexts.

lnwire/features_test.go

@@ -5,6 +5,8 @@ import (
 	"reflect"
 	"sort"
 	"testing"
+
+	"github.com/stretchr/testify/require"
 )
 
 var testFeatureNames = map[FeatureBit]string{
@@ -87,6 +89,7 @@ func TestFeatureVectorSetUnset(t *testing.T) {
 				t.Errorf("Expectation failed in case %d, bit %d", i, j)
 				break
 			}
+
 		}
 
 		for _, bit := range test.bits {
@@ -95,6 +98,31 @@ func TestFeatureVectorSetUnset(t *testing.T) {
 	}
 }
 
+// TestFeatureVectorRequiresFeature tests that if a feature vector only
+// includes a required feature bit (it's even), then the RequiresFeature method
+// will return true for both that bit as well as it's optional counter party.
+func TestFeatureVectorRequiresFeature(t *testing.T) {
+	t.Parallel()
+
+	// Create a new feature vector with the features above, and set only
+	// the set of required bits. These will be all the even features
+	// referenced above.
+	fv := NewFeatureVector(nil, testFeatureNames)
+	fv.Set(0)
+	fv.Set(4)
+
+	// Next we'll query for those exact bits, these should show up as being
+	// required.
+	require.True(t, fv.RequiresFeature(0))
+	require.True(t, fv.RequiresFeature(4))
+
+	// If we query for the odd (optional) counter party to each of the
+	// features, the method should still return that the backing feature
+	// vector requires the feature to be set.
+	require.True(t, fv.RequiresFeature(1))
+	require.True(t, fv.RequiresFeature(5))
+}
+
 func TestFeatureVectorEncodeDecode(t *testing.T) {
 	t.Parallel()
 
@@ -277,7 +305,7 @@ func TestIsRequired(t *testing.T) {
 }
 
 // TestFeatures asserts that the Features() method on a FeatureVector properly
-// returns the set of feature bits it stores internallly.
+// returns the set of feature bits it stores internally.
 func TestFeatures(t *testing.T) {
 	tests := []struct {
 		name string