channeldb/graph: don't nil curve of returned PubKey

Nilling the key would cause signature verification to crash if the
returned key was used.

A test to exercise the behavior is added.

channeldb/graph.go

@@ -1982,7 +1982,6 @@ func (l *LightningNode) PubKey() (*btcec.PublicKey, error) {
 		return nil, err
 	}
 	l.pubKey = key
-	l.pubKey.Curve = nil
 
 	return key, nil
 }

channeldb/graph_test.go

@@ -41,14 +41,9 @@ var (
 	testFeatures = lnwire.NewFeatureVector(nil, lnwire.GlobalFeatures)
 )
 
-func createTestVertex(db *DB) (*LightningNode, error) {
+func createLightningNode(db *DB, priv *btcec.PrivateKey) (*LightningNode, error) {
 	updateTime := prand.Int63()
 
-	priv, err := btcec.NewPrivateKey(btcec.S256())
-	if err != nil {
-		return nil, err
-	}
-
 	pub := priv.PubKey().SerializeCompressed()
 	n := &LightningNode{
 		HaveNodeAnnouncement: true,
@@ -65,6 +60,15 @@ func createTestVertex(db *DB) (*LightningNode, error) {
 	return n, nil
 }
 
+func createTestVertex(db *DB) (*LightningNode, error) {
+	priv, err := btcec.NewPrivateKey(btcec.S256())
+	if err != nil {
+		return nil, err
+	}
+
+	return createLightningNode(db, priv)
+}
+
 func TestNodeInsertionAndDeletion(t *testing.T) {
 	t.Parallel()
 
@@ -3005,3 +3009,54 @@ func compareEdgePolicies(a, b *ChannelEdgePolicy) error {
 	}
 	return nil
 }
+
+// TestLightningNodeSigVerifcation checks that we can use the LightningNode's
+// pubkey to verify signatures.
+func TestLightningNodeSigVerification(t *testing.T) {
+	t.Parallel()
+
+	// Create some dummy data to sign.
+	var data [32]byte
+	if _, err := prand.Read(data[:]); err != nil {
+		t.Fatalf("unable to read prand: %v", err)
+	}
+
+	// Create private key and sign the data with it.
+	priv, err := btcec.NewPrivateKey(btcec.S256())
+	if err != nil {
+		t.Fatalf("unable to crete priv key: %v", err)
+	}
+
+	sign, err := priv.Sign(data[:])
+	if err != nil {
+		t.Fatalf("unable to sign: %v", err)
+	}
+
+	// Sanity check that the signature checks out.
+	if !sign.Verify(data[:], priv.PubKey()) {
+		t.Fatalf("signature doesn't check out")
+	}
+
+	// Create a LightningNode from the same private key.
+	db, cleanUp, err := makeTestDB()
+	if err != nil {
+		t.Fatalf("unable to make test database: %v", err)
+	}
+	defer cleanUp()
+
+	node, err := createLightningNode(db, priv)
+	if err != nil {
+		t.Fatalf("unable to create node: %v", err)
+	}
+
+	// And finally check that we can verify the same signature from the
+	// pubkey returned from the lightning node.
+	nodePub, err := node.PubKey()
+	if err != nil {
+		t.Fatalf("unable to get pubkey: %v", err)
+	}
+
+	if !sign.Verify(data[:], nodePub) {
+		t.Fatalf("unable to verify sig")
+	}
+}