multi: refactor NewAnchorResolutions to return fixed values

This commit adds a new struct AnchorResolutions which wraps the anchor
resolutions for local/remote/pending remote commitment transactions. It
is then returned from NewAnchorResolutions. Thus the caller knows how to
retrieve a certain anchor resolution.

contractcourt/chain_arbitrator.go

@@ -241,7 +241,7 @@ type arbChannel struct {
 // commitment transactions.
 //
 // NOTE: Part of the ArbChannel interface.
-func (a *arbChannel) NewAnchorResolutions() ([]*lnwallet.AnchorResolution,
+func (a *arbChannel) NewAnchorResolutions() (*lnwallet.AnchorResolutions,
 	error) {
 
 	// Get a fresh copy of the database state to base the anchor resolutions

contractcourt/channel_arbitrator.go

@@ -91,7 +91,7 @@ type ArbChannel interface {
 
 	// NewAnchorResolutions returns the anchor resolutions for currently
 	// valid commitment transactions.
-	NewAnchorResolutions() ([]*lnwallet.AnchorResolution, error)
+	NewAnchorResolutions() (*lnwallet.AnchorResolutions, error)
 }
 
 // ChannelArbitratorConfig contains all the functionality that the
@@ -1087,14 +1087,18 @@ func (c *ChannelArbitrator) stateStep(
 // sweepAnchors offers all given anchor resolutions to the sweeper. It requests
 // sweeping at the minimum fee rate. This fee rate can be upped manually by the
 // user via the BumpFee rpc.
-func (c *ChannelArbitrator) sweepAnchors(anchors []*lnwallet.AnchorResolution,
+func (c *ChannelArbitrator) sweepAnchors(anchors *lnwallet.AnchorResolutions,
 	heightHint uint32) error {
 
 	// Use the chan id as the exclusive group. This prevents any of the
 	// anchors from being batched together.
 	exclusiveGroup := c.cfg.ShortChanID.ToUint64()
 
-	for _, anchor := range anchors {
+	// TODO: refactor this function in next commit.
+	for _, anchor := range []*lnwallet.AnchorResolution{
+		anchors.Local, anchors.Remote, anchors.RemotePending,
+	} {
+
 		log.Debugf("ChannelArbitrator(%v): pre-confirmation sweep of "+
 			"anchor of tx %v", c.cfg.ChanPoint, anchor.CommitAnchor)
 

contractcourt/channel_arbitrator_test.go

@@ -2119,9 +2119,7 @@ func TestChannelArbitratorAnchors(t *testing.T) {
 
 	// Setup two pre-confirmation anchor resolutions on the mock channel.
 	chanArb.cfg.Channel.(*mockChannel).anchorResolutions =
-		[]*lnwallet.AnchorResolution{
-			{}, {},
-		}
+		&lnwallet.AnchorResolutions{}
 
 	if err := chanArb.Start(nil); err != nil {
 		t.Fatalf("unable to start ChannelArbitrator: %v", err)
@@ -2286,15 +2284,18 @@ func assertResolverReport(t *testing.T, reports chan *channeldb.ResolverReport,
 }
 
 type mockChannel struct {
-	anchorResolutions []*lnwallet.AnchorResolution
+	anchorResolutions *lnwallet.AnchorResolutions
 }
 
-func (m *mockChannel) NewAnchorResolutions() ([]*lnwallet.AnchorResolution,
+func (m *mockChannel) NewAnchorResolutions() (*lnwallet.AnchorResolutions,
 	error) {
-
+	if m.anchorResolutions != nil {
 		return m.anchorResolutions, nil
 	}
 
+	return &lnwallet.AnchorResolutions{}, nil
+}
+
 func (m *mockChannel) ForceCloseChan() (*lnwallet.LocalForceCloseSummary, error) {
 	summary := &lnwallet.LocalForceCloseSummary{
 		CloseTx:         &wire.MsgTx{},

lnwallet/channel.go

@@ -6404,16 +6404,33 @@ func (lc *LightningChannel) CompleteCooperativeClose(
 	return closeTx, ourBalance, nil
 }
 
-// NewAnchorResolutions returns the anchor resolutions for all currently valid
-// commitment transactions. Because we have no view on the mempool, we can only
-// blindly anchor all of these txes down.
-func (lc *LightningChannel) NewAnchorResolutions() ([]*AnchorResolution,
+// AnchorResolutions is a set of anchor resolutions that's being used when
+// sweeping anchors during local channel force close.
+type AnchorResolutions struct {
+	// Local is the anchor resolution for the local commitment tx.
+	Local *AnchorResolution
+
+	// Remote is the anchor resolution for the remote commitment tx.
+	Remote *AnchorResolution
+
+	// RemotePending is the anchor resolution for the remote pending
+	// commitment tx. The value will be non-nil iff we've created a new
+	// commitment tx for the remote party which they haven't ACKed yet.
+	RemotePending *AnchorResolution
+}
+
+// NewAnchorResolutions returns a set of anchor resolutions wrapped in the
+// struct AnchorResolutions. Because we have no view on the mempool, we can
+// only blindly anchor all of these txes down. Caller needs to check the
+// returned values against nil to decide whether there exists an anchor
+// resolution for local/remote/pending remote commitment txes.
+func (lc *LightningChannel) NewAnchorResolutions() (*AnchorResolutions,
 	error) {
 
 	lc.Lock()
 	defer lc.Unlock()
 
-	var resolutions []*AnchorResolution
+	resolutions := &AnchorResolutions{}
 
 	// Add anchor for local commitment tx, if any.
 	localRes, err := NewAnchorResolution(
@@ -6422,9 +6439,7 @@ func (lc *LightningChannel) NewAnchorResolutions() ([]*AnchorResolution,
 	if err != nil {
 		return nil, err
 	}
-	if localRes != nil {
-		resolutions = append(resolutions, localRes)
-	}
+	resolutions.Local = localRes
 
 	// Add anchor for remote commitment tx, if any.
 	remoteRes, err := NewAnchorResolution(
@@ -6433,9 +6448,7 @@ func (lc *LightningChannel) NewAnchorResolutions() ([]*AnchorResolution,
 	if err != nil {
 		return nil, err
 	}
-	if remoteRes != nil {
-		resolutions = append(resolutions, remoteRes)
-	}
+	resolutions.Remote = remoteRes
 
 	// Add anchor for remote pending commitment tx, if any.
 	remotePendingCommit, err := lc.channelState.RemoteCommitChainTip()
@@ -6451,10 +6464,7 @@ func (lc *LightningChannel) NewAnchorResolutions() ([]*AnchorResolution,
 		if err != nil {
 			return nil, err
 		}
-
-		if remotePendingRes != nil {
-			resolutions = append(resolutions, remotePendingRes)
-		}
+		resolutions.RemotePending = remotePendingRes
 	}
 
 	return resolutions, nil

lnwallet/channel_test.go

@@ -917,14 +917,19 @@ func testForceClose(t *testing.T, testCase *forceCloseTestCase) {
 		}
 
 		// Check the pre-confirmation resolutions.
-		resList, err := aliceChannel.NewAnchorResolutions()
+		res, err := aliceChannel.NewAnchorResolutions()
 		if err != nil {
 			t.Fatalf("pre-confirmation resolution error: %v", err)
 		}
 
-		if len(resList) != 2 {
-			t.Fatal("expected two resolutions")
-		}
+		// Check we have the expected anchor resolutions.
+		require.NotNil(t, res.Local, "expected local anchor resolution")
+		require.NotNil(t,
+			res.Remote, "expected remote anchor resolution",
+		)
+		require.Nil(t,
+			res.RemotePending, "expected no anchor resolution",
+		)
 	}
 
 	// The SelfOutputSignDesc should be non-nil since the output to-self is