Merge pull request #4872 from Roasbeef/fix-tweakless-downgrade

peer: ensure consistent local+global feature bits when downgrading

peer/brontide.go

@@ -2775,6 +2775,7 @@ func (p *Brontide) RemoteFeatures() *lnwire.FeatureVector {
 // our currently supported local and global features.
 func (p *Brontide) sendInitMsg(legacyChan bool) error {
 	features := p.cfg.Features.Clone()
+	legacyFeatures := p.cfg.LegacyFeatures.Clone()
 
 	// If we have a legacy channel open with a peer, we downgrade static
 	// remote required to optional in case the peer does not understand the
@@ -2786,12 +2787,18 @@ func (p *Brontide) sendInitMsg(legacyChan bool) error {
 			"downgrading static remote required feature bit to "+
 			"optional", p.PubKey())
 
+		// Unset and set in both the local and global features to
+		// ensure both sets are consistent and merge able by old and
+		// new nodes.
 		features.Unset(lnwire.StaticRemoteKeyRequired)
+		legacyFeatures.Unset(lnwire.StaticRemoteKeyRequired)
+
 		features.Set(lnwire.StaticRemoteKeyOptional)
+		legacyFeatures.Set(lnwire.StaticRemoteKeyOptional)
 	}
 
 	msg := lnwire.NewInitMessage(
-		p.cfg.LegacyFeatures.RawFeatureVector,
+		legacyFeatures.RawFeatureVector,
 		features.RawFeatureVector,
 	)
 

peer/brontide_test.go

@@ -875,6 +875,11 @@ func TestStaticRemoteDowngrade(t *testing.T) {
 		)
 		legacy = lnwire.NewFeatureVector(rawLegacy, nil)
 
+		legacyCombinedOptional = lnwire.NewRawFeatureVector(
+			lnwire.UpfrontShutdownScriptOptional,
+			lnwire.StaticRemoteKeyOptional,
+		)
+
 		rawFeatureOptional = lnwire.NewRawFeatureVector(
 			lnwire.StaticRemoteKeyOptional,
 		)
@@ -925,12 +930,17 @@ func TestStaticRemoteDowngrade(t *testing.T) {
 				Features:       rawFeatureRequired,
 			},
 		},
+
+		// In this case we need to flip our required bit to optional,
+		// this should also propagate to the legacy set of feature bits
+		// so we have proper consistency: a bit isn't set to optional
+		// in one field and required in the other.
 		{
 			name:     "legacy channel, static required",
 			legacy:   true,
 			features: featureRequired,
 			expectedInit: &lnwire.Init{
-				GlobalFeatures: rawLegacy,
+				GlobalFeatures: legacyCombinedOptional,
 				Features:       rawFeatureOptional,
 			},
 		},