channeldb: add new WitnessCache structure

In this commit, we add the WitnessCache sub-storage system of the
greater database. The WitnessCache is a persistent cache of all
witnesses we’ve encountered on the network. We’ll use this cache to
share any on-chain discoveries between active channels. Eventually
we’ll also use this to enforce the variant that a preimage is only to
be used ONCE on the network.

channeldb/witness_cache.go

@@ -0,0 +1,185 @@
+package channeldb
+
+import (
+	"crypto/sha256"
+	"fmt"
+
+	"github.com/boltdb/bolt"
+)
+
+var (
+	// ErrNoWitnesses is an error that's returned when no new witnesses have
+	// been added to the WitnessCache.
+	ErrNoWitnesses = fmt.Errorf("no witnesses")
+
+	// ErrUnknownWitnessType is returned if a caller attempts to
+	ErrUnknownWitnessType = fmt.Errorf("unknown witness type")
+)
+
+// WitnessType is enum that denotes what "type" of witness is being
+// stored/retrieved. As the WitnessCache itself is agnostic and doesn't enforce
+// any structure on added witnesses, we use this type to partition the
+// witnesses on disk, and also to know how to map a witness to its look up key.
+type WitnessType uint8
+
+var (
+	// Sha256HashWitness is a witness that is simply the pre image to a
+	// hash image. In order to map to its key, we'll use sha256.
+	Sha256HashWitness WitnessType = 1
+)
+
+// toDBKey is a helper method that maps a witness type to the key that we'll
+// use to store it within the database.
+func (w WitnessType) toDBKey() ([]byte, error) {
+	switch w {
+
+	case Sha256HashWitness:
+		return []byte{byte(w)}, nil
+
+	default:
+		return nil, ErrUnknownWitnessType
+	}
+}
+
+var (
+	// witnessBucketKey is the name of the bucket that we use to store all
+	// witnesses encountered. Within this bucket, we'll create a sub-bucket for
+	// each witness type.
+	witnessBucketKey = []byte("byte")
+)
+
+// WitnessCache is a persistent cache of all witnesses we've encountered on the
+// network. In the case of multi-hop, multi-step contracts, a cache of all
+// witnesses can be useful in the case of partial contract resolution. If
+// negotiations break down, we may be forced to locate the witness for a
+// portion of the contract on-chain. In this case, we'll then add that witness
+// to the cache so the incoming contract can fully resolve witness.
+// Additionally, as one MUST always use a unique witness on the network, we may
+// use this cache to detect duplicate witnesses.
+//
+// TODO(roasbeef): need expiry policy?
+//  * encrypt?
+type WitnessCache struct {
+	db *DB
+}
+
+// NewWitnessCache returns a new instance of the witness cache.
+func (d *DB) NewWitnessCache() *WitnessCache {
+	return &WitnessCache{
+		db: d,
+	}
+}
+
+// AddWitness adds a new witness of wType to the witness cache. The type of the
+// witness will be used to map the witness to the key that will be used to look
+// it up.
+//
+// TODO(roasbeef): fake closure to map instead a constructor?
+func (w *WitnessCache) AddWitness(wType WitnessType, witness []byte) error {
+	return w.db.Batch(func(tx *bolt.Tx) error {
+		witnessBucket, err := tx.CreateBucketIfNotExists(witnessBucketKey)
+		if err != nil {
+			return err
+		}
+
+		witnessTypeBucketKey, err := wType.toDBKey()
+		if err != nil {
+			return err
+		}
+		witnessTypeBucket, err := witnessBucket.CreateBucketIfNotExists(
+			witnessTypeBucketKey,
+		)
+		if err != nil {
+			return err
+		}
+
+		// Now that we have the proper bucket for this witness, we'll map the
+		// witness type to the proper key.
+		var witnessKey []byte
+		switch wType {
+		case Sha256HashWitness:
+			key := sha256.Sum256(witness)
+			witnessKey = key[:]
+		}
+
+		return witnessTypeBucket.Put(witnessKey, witness)
+	})
+}
+
+// LookupWitness attempts to lookup a witness according to its type and also
+// its witness key. In the case that the witness isn't found, ErrNoWitnesses
+// will be returned.
+func (w *WitnessCache) LookupWitness(wType WitnessType, witnessKey []byte) ([]byte, error) {
+	var witness []byte
+	err := w.db.View(func(tx *bolt.Tx) error {
+		witnessBucket := tx.Bucket(witnessBucketKey)
+		if witnessBucket == nil {
+			return ErrNoWitnesses
+		}
+
+		witnessTypeBucketKey, err := wType.toDBKey()
+		if err != nil {
+			return err
+		}
+		witnessTypeBucket := witnessBucket.Bucket(witnessTypeBucketKey)
+		if witnessTypeBucket == nil {
+			return ErrNoWitnesses
+		}
+
+		dbWitness := witnessTypeBucket.Get(witnessKey)
+		if dbWitness == nil {
+			return ErrNoWitnesses
+		}
+
+		witness = make([]byte, len(dbWitness))
+		copy(witness[:], dbWitness)
+
+		return nil
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	return witness, nil
+}
+
+// DeleteWitness attempts to delete a particular witness from the database.
+func (w *WitnessCache) DeleteWitness(wType WitnessType, witnessKey []byte) error {
+	return w.db.Batch(func(tx *bolt.Tx) error {
+		witnessBucket, err := tx.CreateBucketIfNotExists(witnessBucketKey)
+		if err != nil {
+			return err
+		}
+
+		witnessTypeBucketKey, err := wType.toDBKey()
+		if err != nil {
+			return err
+		}
+		witnessTypeBucket, err := witnessBucket.CreateBucketIfNotExists(
+			witnessTypeBucketKey,
+		)
+		if err != nil {
+			return err
+		}
+
+		return witnessTypeBucket.Delete(witnessKey)
+	})
+}
+
+// DeleteWitnessClass attempts to delete an *entire* class of witnesses. After
+// this function return with a non-nil error,
+func (w *WitnessCache) DeleteWitnessClass(wType WitnessType) error {
+	return w.db.Batch(func(tx *bolt.Tx) error {
+		witnessBucket, err := tx.CreateBucketIfNotExists(witnessBucketKey)
+		if err != nil {
+			return err
+		}
+
+		witnessTypeBucketKey, err := wType.toDBKey()
+		if err != nil {
+			return err
+		}
+
+		return witnessBucket.DeleteBucket(witnessTypeBucketKey)
+	})
+}