lnd.xprv/contractcourt/chain_watcher.go

package contractcourt

import (
	"fmt"
	"sync"

	"github.com/davecgh/go-spew/spew"
	"github.com/lightningnetwork/lnd/chainntnfs"
	"github.com/lightningnetwork/lnd/channeldb"
	"github.com/lightningnetwork/lnd/lnwallet"
	"github.com/roasbeef/btcd/wire"
)

// ChainEventSubscription is a struct that houses a subscription to be notified
// for any on-chain events related to a channel. There are three types of
// possible on-chain events: a cooperative channel closure, a unilateral
// channel closure, and a channel breach. The fourth type: a force close is
// locally initiated, so we don't provide any event stream for said event.
type ChainEventSubscription struct {
	// ChanPoint is that channel that chain events will be dispatched fo.
	ChanPoint wire.OutPoint

	// UnilateralClosure is a channel that will be sent upon in the event that
	// the remote party broadcasts their latest version of the commitment
	// transaction.
	UnilateralClosure chan *lnwallet.UnilateralCloseSummary

	// CooperativeClosure is a signal that will be sent upon once a cooperative
	// channel closure has been detected.
	//
	// TODO(roasbeef): or something else
	CooperativeClosure chan struct{}

	// ContractBreach is a channel that will be sent upon if we detect a
	// contract breach. The struct sent across the channel contains all the
	// material required to bring the cheating channel peer to justice.
	ContractBreach chan *lnwallet.BreachRetribution

	// ProcessACK is a channel that'll be used by the chainWatcher to
	// synchronize dispatch and processing of the notification with the act
	// of updating the state of the channel on disk. This ensures that the
	// event can be reliably handed off.
	ProcessACK chan struct{}

	// Cancel cancels the subscription to the event stream for a particular
	// channel. This method should be called once the caller no longer needs to
	// be notified of any on-chain events for a particular channel.
	Cancel func()
}

// chainWatcher is a system that's assigned to every active channel. The duty
// of this system is to watch the chain for spends of the channels chan point.
// If a spend is detected then with chain watcher will notify all subscribers
// that the channel has been closed, and also give them the materials necessary
// to sweep the funds of the channel on chain eventually.
type chainWatcher struct {
	quit chan struct{}
	wg   sync.WaitGroup

	// chanState is a snapshot of the persistent state of the channel that
	// we're watching. In the event of an on-chain event, we'll query the
	// database to ensure that we act using the most up to date state.
	chanState *channeldb.OpenChannel

	// stateHintObfuscator is a 48-bit state hint that's used to obfsucate
	// the current state number on the commitment transactions.
	stateHintObfuscator [lnwallet.StateHintSize]byte

	// notifier is a reference to the channel notifier that we'll use to be
	// notified of output spends and when transactions are confirmed.
	notifier chainntnfs.ChainNotifier

	// pCache is a reference to the shared preimage cache. We'll use this
	// to see if we can settle any incoming HTLC's during a remote
	// commitment close event.
	pCache WitnessBeacon

	// signer is the main signer instances that will be responsible for
	// signing any HTLC and commitment transaction generated by the state
	// machine.
	signer lnwallet.Signer

	// All the fields below are protected by this mutex.
	sync.RWMutex

	// clientID is an ephemeral counter used to keep track of each
	// individual client subscription.
	clientID uint64

	// clientSubscriptions is a map that keeps track of all the active
	// client subscriptions for events related to this channel.
	clientSubscriptions map[uint64]*ChainEventSubscription
}

// newChainWatcher returns a new instance of a chainWatcher for a channel given
// the chan point to watch, and also a notifier instance that will allow us to
// detect on chain events.
func newChainWatcher(chanState *channeldb.OpenChannel,
	notifier chainntnfs.ChainNotifier, pCache WitnessBeacon,
	signer lnwallet.Signer) (*chainWatcher, error) {

	// In order to be able to detect the nature of a potential channel
	// closure we'll need to reconstruct the state hint bytes used to
	// obfuscate the commitment state number encoded in the lock time and
	// sequence fields.
	var stateHint [lnwallet.StateHintSize]byte
	if chanState.IsInitiator {
		stateHint = lnwallet.DeriveStateHintObfuscator(
			chanState.LocalChanCfg.PaymentBasePoint,
			chanState.RemoteChanCfg.PaymentBasePoint,
		)
	} else {
		stateHint = lnwallet.DeriveStateHintObfuscator(
			chanState.RemoteChanCfg.PaymentBasePoint,
			chanState.LocalChanCfg.PaymentBasePoint,
		)
	}

	return &chainWatcher{
		chanState:           chanState,
		stateHintObfuscator: stateHint,
		notifier:            notifier,
		pCache:              pCache,
		signer:              signer,
		quit:                make(chan struct{}),
		clientSubscriptions: make(map[uint64]*ChainEventSubscription),
	}, nil
}

// Start starts all goroutines that the chainWatcher needs to perform its
// duties.
func (c *chainWatcher) Start() error {
	log.Debugf("Starting chain watcher for ChannelPoint(%v)",
		c.chanState.FundingOutpoint)

	// First, we'll register for a notification to be dispatched if the
	// funding output is spent.
	fundingOut := &c.chanState.FundingOutpoint

	// As a height hint, we'll try to use the opening height, but if the
	// channel isn't yet open, then we'll use the height it was broadcast
	// at.
	heightHint := c.chanState.ShortChanID.BlockHeight
	if heightHint == 0 {
		heightHint = c.chanState.FundingBroadcastHeight
	}

	spendNtfn, err := c.notifier.RegisterSpendNtfn(
		fundingOut, heightHint,
	)
	if err != nil {
		return err
	}

	// With the spend notification obtained, we'll now dispatch the
	// closeObserver which will properly react to any changes.
	c.wg.Add(1)
	go c.closeObserver(spendNtfn)

	return nil
}

// Stop signals the close observer to gracefully exit.
func (c *chainWatcher) Stop() error {
	close(c.quit)

	c.wg.Wait()

	return nil
}

// SubscribeChannelEvents returns a n active subscription to the set of channel
// events for the channel watched by this chain watcher. Once clients no longer
// require the subscription, they should call the Cancel() method to allow the
// watcher to regain those committed resources.
func (c *chainWatcher) SubscribeChannelEvents() *ChainEventSubscription {
	c.Lock()
	defer c.Unlock()

	clientID := c.clientID
	c.clientID++

	log.Debugf("New ChainEventSubscription(id=%v) for ChannelPoint(%v)",
		clientID, c.chanState.FundingOutpoint)

	sub := &ChainEventSubscription{
		ChanPoint:          c.chanState.FundingOutpoint,
		UnilateralClosure:  make(chan *lnwallet.UnilateralCloseSummary, 1),
		CooperativeClosure: make(chan struct{}, 1),
		ContractBreach:     make(chan *lnwallet.BreachRetribution, 1),
		Cancel: func() {
			c.Lock()
			delete(c.clientSubscriptions, clientID)
			c.Unlock()
			return
		},
	}

	c.clientSubscriptions[clientID] = sub

	return sub
}

// closeObserver is a dedicated goroutine that will watch for any closes of the
// channel that it's watching on chain. In the event of an on-chain event, the
// close observer will assembled the proper materials required to claim the
// funds of the channel on-chain (if required), then dispatch these as
// notifications to all subscribers.
func (c *chainWatcher) closeObserver(spendNtfn *chainntnfs.SpendEvent) {
	defer c.wg.Done()

	log.Infof("Close observer for ChannelPoint(%v) active",
		c.chanState.FundingOutpoint)

	for {
		select {
		// We've detected a spend of the channel onchain! Depending on
		// the type of spend, we'll act accordingly , so we'll examine
		// the spending transaction to determine what we should do.
		case commitSpend, ok := <-spendNtfn.Spend:
			// If the channel was closed, then this means that the
			// notifier exited, so we will as well.
			if !ok {
				return
			}

			// Otherwise, the remote party might have broadcast a
			// prior revoked state...!!!
			commitTxBroadcast := commitSpend.SpendingTx

			localCommit, remoteCommit, err := c.chanState.LatestCommitments()
			if err != nil {
				log.Errorf("Unable to fetch channel state for "+
					"chan_point=%v", c.chanState.FundingOutpoint)
				return
			}

			// We'll not retrieve the latest sate of the revocation
			// store so we can populate the information within the
			// channel state object that we have.
			//
			// TODO(roasbeef): mutation is bad mkay
			_, err = c.chanState.RemoteRevocationStore()
			if err != nil {
				log.Errorf("Unable to fetch revocation state for "+
					"chan_point=%v", c.chanState.FundingOutpoint)
				return
			}

			// If this is our commitment transaction, then we can
			// exit here as we don't have any further processing we
			// need to do (we can't cheat ourselves :p).
			commitmentHash := localCommit.CommitTx.TxHash()
			isOurCommitment := commitSpend.SpenderTxHash.IsEqual(
				&commitmentHash,
			)
			if isOurCommitment {
				return
			}

			// Next, we'll check to see if this is a cooperative
			// channel closure or not. This is characterized by
			//
			// TODO(roasbeef): check to see if txid amongst those
			// that we know are co-op channel closes

			log.Warnf("Unprompted commitment broadcast for "+
				"ChannelPoint(%v) ", c.chanState.FundingOutpoint)

			// Decode the state hint encoded within the commitment
			// transaction to determine if this is a revoked state
			// or not.
			obfuscator := c.stateHintObfuscator
			broadcastStateNum := lnwallet.GetStateNumHint(
				commitTxBroadcast, obfuscator,
			)
			remoteStateNum := remoteCommit.CommitHeight

			switch {
			// If state number spending transaction matches the
			// current latest state, then they've initiated a
			// unilateral close. So we'll trigger the unilateral
			// close signal so subscribers can clean up the state
			// as necessary.
			//
			// We'll also handle the case of the remote party
			// broadcasting their commitment transaction which is
			// one height above ours. This case can arise when we
			// initiate a state transition, but the remote party
			// has a fail crash _after_ accepting the new state,
			// but _before_ sending their signature to us.
			case broadcastStateNum >= remoteStateNum:
				if err := c.dispatchRemoteClose(
					commitSpend, *remoteCommit,
				); err != nil {
					log.Errorf("unable to handle remote "+
						"close for chan_point=%v",
						c.chanState.FundingOutpoint, err)
				}

			// If the state number broadcast is lower than the
			// remote node's current un-revoked height, then
			// THEY'RE ATTEMPTING TO VIOLATE THE CONTRACT LAID OUT
			// WITHIN THE PAYMENT CHANNEL.  Therefore we close the
			// signal indicating a revoked broadcast to allow
			// subscribers to
			// swiftly dispatch justice!!!
			case broadcastStateNum < remoteStateNum:
				if err := c.dispatchContractBreach(
					commitSpend, remoteCommit,
				); err != nil {
					log.Errorf("unable to handle channel "+
						"breach for chan_point=%v: %v",
						c.chanState.FundingOutpoint, err)
				}
			}

			// Now that a spend has been detected, we've done our
			// job, so we'll exit immediately.
			return

		// The chainWatcher has been signalled to exit, so we'll do so now.
		case <-c.quit:
		}
	}
}

// dispatchRemoteClose processes a detected unilateral channel closure by the
// remote party. This function will prepare a UnilateralCloseSummary which will
// then be sent to any subscribers allowing them to resolve all our funds in
// the channel on chain. Once this close summary is prepared, all registered
// subscribers will receive a notification of this event.
func (c *chainWatcher) dispatchRemoteClose(commitSpend *chainntnfs.SpendDetail,
	remoteCommit channeldb.ChannelCommitment) error {

	log.Infof("Unilateral close of ChannelPoint(%v) "+
		"detected", c.chanState.FundingOutpoint)

	// First, we'll create a closure summary that contains all the
	// materials required to let each subscriber sweep the funds in the
	// channel on-chain.
	uniClose, err := lnwallet.NewUnilateralCloseSummary(c.chanState,
		c.signer, c.pCache, commitSpend, remoteCommit,
	)
	if err != nil {
		return err
	}

	// As we've detected that the channel has been closed, immediately
	// delete the state from disk, creating a close summary for future
	// usage by related sub-systems.
	err = c.chanState.CloseChannel(&uniClose.ChannelCloseSummary)
	if err != nil {
		return fmt.Errorf("unable to delete channel state: %v", err)
	}

	// With the event processed, we'll now notify all subscribers of the
	// event.
	c.Lock()
	for _, sub := range c.clientSubscriptions {
		// TODO(roasbeef): send msg before writing to disk
		//  * need to ensure proper fault tolerance in all cases
		//  * get ACK from the consumer of the ntfn before writing to disk?
		//  * no harm in repeated ntfns: at least once semantics
		select {
		case sub.UnilateralClosure <- uniClose:
		case <-c.quit:
			return fmt.Errorf("exiting")
		}
	}
	c.Unlock()

	return nil
}

// dispatchContractBreach processes a detected contract breached by the remote
// party. This method is to be called once we detect that the remote party has
// broadcast a prior revoked commitment state. This method well prepare all the
// materials required to bring the cheater to justice, then notify all
// registered subscribers of this event.
func (c *chainWatcher) dispatchContractBreach(spendEvent *chainntnfs.SpendDetail,
	remoteCommit *channeldb.ChannelCommitment) error {

	log.Warnf("Remote peer has breached the channel contract for "+
		"ChannelPoint(%v). Revoked state #%v was broadcast!!!",
		c.chanState.FundingOutpoint, remoteCommit.CommitHeight)

	if err := c.chanState.MarkBorked(); err != nil {
		return fmt.Errorf("unable to mark channel as borked: %v", err)
	}

	var (
		broadcastStateNum = remoteCommit.CommitHeight
		commitTxBroadcast = spendEvent.SpendingTx
		spendHeight       = uint32(spendEvent.SpendingHeight)
	)

	// Create a new reach retribution struct which contains all the data
	// needed to swiftly bring the cheating peer to justice.
	//
	// TODO(roasbeef): move to same package
	retribution, err := lnwallet.NewBreachRetribution(
		c.chanState, broadcastStateNum, commitTxBroadcast,
		spendHeight,
	)
	if err != nil {
		return fmt.Errorf("unable to create breach retribution: %v", err)
	}

	log.Debugf("Punishment breach retribution created: %v",
		spew.Sdump(retribution))

	// With the event processed, we'll now notify all subscribers of the
	// event.
	c.Lock()
	for _, sub := range c.clientSubscriptions {
		select {
		case sub.ContractBreach <- retribution:
		case <-c.quit:
			return fmt.Errorf("quitting")
		}

		// Wait for the breach arbiter to ACK the handoff before
		// marking the channel as pending force closed in channeldb.
		select {
		case <-sub.ProcessACK:
			// Bail if the handoff failed.
			if err != nil {
				return fmt.Errorf("unable to handoff "+
					"retribution info: %v", err)
			}

		case <-c.quit:
			return fmt.Errorf("quitting")
		}
	}
	c.Unlock()

	// At this point, we've successfully received an ack for the breach
	// close. We now construct and persist  the close summary, marking the
	// channel as pending force closed.
	//
	// TODO(roasbeef): instead mark we got all the monies?
	settledBalance := remoteCommit.LocalBalance.ToSatoshis()
	closeSummary := channeldb.ChannelCloseSummary{
		ChanPoint:      c.chanState.FundingOutpoint,
		ChainHash:      c.chanState.ChainHash,
		ClosingTXID:    *spendEvent.SpenderTxHash,
		CloseHeight:    spendHeight,
		RemotePub:      c.chanState.IdentityPub,
		Capacity:       c.chanState.Capacity,
		SettledBalance: settledBalance,
		CloseType:      channeldb.BreachClose,
		IsPending:      true,
		ShortChanID:    c.chanState.ShortChanID,
	}

	log.Infof("Breached channel=%v marked pending-closed",
		c.chanState.FundingOutpoint)

	return c.chanState.CloseChannel(&closeSummary)
}