channeldb: let AddrsForNode indicate if the node was found or not

Before this commit, the `(channeldb.DB).AddrsForNode` method treats the
results from the channel db and the graph db slightly differently. It
errors out if the channel db is unaware of the node in question but does
not error out if the graph is unaware of the node. So this commit
changes the logic slightly so that a "node-unknown" error from either
backing source is not seen as an error.

channeldb/addr_source_test.go

@@ -36,11 +36,12 @@ func TestMultiAddrSource(t *testing.T) {
 
 		// Let source 1 know of 2 addresses (addr 1 and 2) for node 1.
 		src1.On("AddrsForNode", pk1).Return(
-			[]net.Addr{addr1, addr2}, nil,
+			true, []net.Addr{addr1, addr2}, nil,
 		).Once()
 
 		// Let source 2 know of 2 addresses (addr 2 and 3) for node 1.
 		src2.On("AddrsForNode", pk1).Return(
+			true, []net.Addr{addr2, addr3}, nil,
 			[]net.Addr{addr2, addr3}, nil,
 		).Once()
 
@@ -50,8 +51,9 @@ func TestMultiAddrSource(t *testing.T) {
 
 		// Query it for the addresses known for node 1. The results
 		// should contain addr 1, 2 and 3.
-		addrs, err := multiSrc.AddrsForNode(pk1)
+		known, addrs, err := multiSrc.AddrsForNode(pk1)
 		require.NoError(t, err)
+		require.True(t, known)
 		require.ElementsMatch(t, addrs, []net.Addr{addr1, addr2, addr3})
 	})
 
@@ -69,9 +71,9 @@ func TestMultiAddrSource(t *testing.T) {
 
 		// Let source 1 know of address 1 for node 1.
 		src1.On("AddrsForNode", pk1).Return(
-			[]net.Addr{addr1}, nil,
+			true, []net.Addr{addr1}, nil,
 		).Once()
-		src2.On("AddrsForNode", pk1).Return(nil, nil).Once()
+		src2.On("AddrsForNode", pk1).Return(false, nil, nil).Once()
 
 		// Create a multi-addr source that consists of both source 1
 		// and 2.
@@ -79,11 +81,39 @@ func TestMultiAddrSource(t *testing.T) {
 
 		// Query it for the addresses known for node 1. The results
 		// should contain addr 1.
-		addrs, err := multiSrc.AddrsForNode(pk1)
+		known, addrs, err := multiSrc.AddrsForNode(pk1)
 		require.NoError(t, err)
+		require.True(t, known)
 		require.ElementsMatch(t, addrs, []net.Addr{addr1})
 	})
 
+	t.Run("unknown address", func(t *testing.T) {
+		t.Parallel()
+
+		var (
+			src1 = newMockAddrSource(t)
+			src2 = newMockAddrSource(t)
+		)
+		t.Cleanup(func() {
+			src1.AssertExpectations(t)
+			src2.AssertExpectations(t)
+		})
+
+		// Create a multi-addr source that consists of both source 1
+		// and 2. Neither source known of node 1.
+		multiSrc := NewMultiAddrSource(src1, src2)
+
+		src1.On("AddrsForNode", pk1).Return(false, nil, nil).Once()
+		src2.On("AddrsForNode", pk1).Return(false, nil, nil).Once()
+
+		// Query it for the addresses known for node 1. It should return
+		// false to indicate that the node is unknown to all backing
+		// sources.
+		known, addrs, err := multiSrc.AddrsForNode(pk1)
+		require.NoError(t, err)
+		require.False(t, known)
+		require.Empty(t, addrs)
+	})
 }
 
 type mockAddrSource struct {
@@ -97,18 +127,18 @@ func newMockAddrSource(t *testing.T) *mockAddrSource {
 	return &mockAddrSource{t: t}
 }
 
-func (m *mockAddrSource) AddrsForNode(pub *btcec.PublicKey) ([]net.Addr,
+func (m *mockAddrSource) AddrsForNode(pub *btcec.PublicKey) (bool, []net.Addr,
 	error) {
 
 	args := m.Called(pub)
-	if args.Get(0) == nil {
-		return nil, args.Error(1)
+	if args.Get(1) == nil {
+		return args.Bool(0), nil, args.Error(2)
 	}
 
-	addrs, ok := args.Get(0).([]net.Addr)
+	addrs, ok := args.Get(1).([]net.Addr)
 	require.True(m.t, ok)
 
-	return addrs, args.Error(1)
+	return args.Bool(0), addrs, args.Error(2)
 }
 
 func newTestPubKey(t *testing.T) *btcec.PublicKey {