amp: create amp.ShardTracker

We'll use this AMP-specific ShardTracker for AMP payments.  It will be
used to derive hashes for each HTLC attempt using the underlying AMP
derivation scheme.

amp/shard_tracker.go

@@ -0,0 +1,165 @@
+package amp
+
+import (
+	"crypto/rand"
+	"encoding/binary"
+	"fmt"
+	"sync"
+
+	"github.com/lightningnetwork/lnd/lntypes"
+	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/lightningnetwork/lnd/record"
+	"github.com/lightningnetwork/lnd/routing/shards"
+)
+
+// Shard is an implementation of the shards.PaymentShards interface specific
+// to AMP payments.
+type Shard struct {
+	child *Child
+	mpp   *record.MPP
+	amp   *record.AMP
+}
+
+// A compile time check to ensure Shard implements the shards.PaymentShard
+// interface.
+var _ shards.PaymentShard = (*Shard)(nil)
+
+// Hash returns the hash used for the HTLC representing this AMP shard.
+func (s *Shard) Hash() lntypes.Hash {
+	return s.child.Hash
+}
+
+// MPP returns any extra MPP records that should be set for the final hop on
+// the route used by this shard.
+func (s *Shard) MPP() *record.MPP {
+	return s.mpp
+}
+
+// AMP returns any extra AMP records that should be set for the final hop on
+// the route used by this shard.
+func (s *Shard) AMP() *record.AMP {
+	return s.amp
+}
+
+// ShardTracker is an implementation of the shards.ShardTracker interface
+// that is able to generate payment shards according to the AMP splitting
+// algorithm. It can be used to generate new hashes to use for HTLCs, and also
+// cancel shares used for failed payment shards.
+type ShardTracker struct {
+	setID       [32]byte
+	paymentAddr [32]byte
+	totalAmt    lnwire.MilliSatoshi
+
+	sharer Sharer
+
+	shards map[uint64]*Child
+	sync.Mutex
+}
+
+// A compile time check to ensure ShardTracker implements the
+// shards.ShardTracker interface.
+var _ shards.ShardTracker = (*ShardTracker)(nil)
+
+// NewShardTracker creates a new shard tracker to use for AMP payments. The
+// root shard, setID, payment address and total amount must be correctly set in
+// order for the TLV options to include with each shard to be created
+// correctly.
+func NewShardTracker(root, setID, payAddr [32]byte,
+	totalAmt lnwire.MilliSatoshi) *ShardTracker {
+
+	// Create a new seed sharer from this root.
+	rootShare := Share(root)
+	rootSharer := SeedSharerFromRoot(&rootShare)
+
+	return &ShardTracker{
+		setID:       setID,
+		paymentAddr: payAddr,
+		totalAmt:    totalAmt,
+		sharer:      rootSharer,
+		shards:      make(map[uint64]*Child),
+	}
+}
+
+// NewShard registers a new attempt with the ShardTracker and returns a
+// new shard representing this attempt. This attempt's shard should be canceled
+// if it ends up not being used by the overall payment, i.e. if the attempt
+// fails.
+func (s *ShardTracker) NewShard(pid uint64, last bool) (shards.PaymentShard,
+	error) {
+
+	s.Lock()
+	defer s.Unlock()
+
+	// Use a random child index.
+	var childIndex [4]byte
+	if _, err := rand.Read(childIndex[:]); err != nil {
+		return nil, err
+	}
+	idx := binary.BigEndian.Uint32(childIndex[:])
+
+	// Depending on whether we are requesting the last shard or not, either
+	// split the current share into two, or get a Child directly from the
+	// current sharer.
+	var child *Child
+	if last {
+		child = s.sharer.Child(idx)
+
+		// If this was the last shard, set the current share to the
+		// zero share to indicate we cannot split it further.
+		s.sharer = s.sharer.Zero()
+	} else {
+		left, sharer, err := s.sharer.Split()
+		if err != nil {
+			return nil, err
+		}
+
+		s.sharer = sharer
+		child = left.Child(idx)
+	}
+
+	// Track the new child and return the shard.
+	s.shards[pid] = child
+
+	mpp := record.NewMPP(s.totalAmt, s.paymentAddr)
+	amp := record.NewAMP(
+		child.ChildDesc.Share, s.setID, child.ChildDesc.Index,
+	)
+
+	return &Shard{
+		child: child,
+		mpp:   mpp,
+		amp:   amp,
+	}, nil
+}
+
+// CancelShard cancel's the shard corresponding to the given attempt ID.
+func (s *ShardTracker) CancelShard(pid uint64) error {
+	s.Lock()
+	defer s.Unlock()
+
+	c, ok := s.shards[pid]
+	if !ok {
+		return fmt.Errorf("pid not found")
+	}
+	delete(s.shards, pid)
+
+	// Now that we are canceling this shard, we XOR the share back into our
+	// current share.
+	s.sharer = s.sharer.Merge(c)
+	return nil
+}
+
+// GetHash retrieves the hash used by the shard of the given attempt ID. This
+// will return an error if the attempt ID is unknown.
+func (s *ShardTracker) GetHash(pid uint64) (lntypes.Hash, error) {
+	s.Lock()
+	defer s.Unlock()
+
+	c, ok := s.shards[pid]
+	if !ok {
+		return lntypes.Hash{}, fmt.Errorf("AMP shard for attempt %v "+
+			"not found", pid)
+	}
+
+	return c.Hash, nil
+}