watchtower: add channelID index

In this commit, a new channel-ID index is added to the tower client db
with the help of a migration. This index holds a mapping from a
db-assigned-ID (a uint64 encoded using BigSize encoding) to real channel
ID (32 bytes). This mapping will help us save space in future when
persisting references to channels.

watchtower/wtdb/migration3/client_db.go

@@ -0,0 +1,104 @@
+package migration3
+
+import (
+	"bytes"
+	"encoding/binary"
+	"errors"
+
+	"github.com/lightningnetwork/lnd/kvdb"
+	"github.com/lightningnetwork/lnd/tlv"
+)
+
+var (
+	// cChanDetailsBkt is a top-level bucket storing:
+	//   channel-id => cChannelSummary -> encoded ClientChanSummary.
+	// 		=> cChanDBID -> db-assigned-id
+	cChanDetailsBkt = []byte("client-channel-detail-bucket")
+
+	// cChanDBID is a key used in the cChanDetailsBkt to store the
+	// db-assigned-id of a channel.
+	cChanDBID = []byte("client-channel-db-id")
+
+	// cChanIDIndexBkt is a top-level bucket storing:
+	//    db-assigned-id -> channel-ID
+	cChanIDIndexBkt = []byte("client-channel-id-index")
+
+	// cChannelSummary is a key used in cChanDetailsBkt to store the encoded
+	// body of ClientChanSummary.
+	cChannelSummary = []byte("client-channel-summary")
+
+	// ErrUninitializedDB signals that top-level buckets for the database
+	// have not been initialized.
+	ErrUninitializedDB = errors.New("db not initialized")
+
+	// ErrCorruptChanDetails signals that the clients channel detail's
+	// on-disk structure deviates from what is expected.
+	ErrCorruptChanDetails = errors.New("channel details corrupted")
+
+	byteOrder = binary.BigEndian
+)
+
+// MigrateChannelIDIndex adds a new channel ID index to the tower client db.
+// This index is a mapping from db-assigned ID (a uint64 encoded using BigSize
+// encoding) to real channel ID (32 bytes). This mapping will allow us to
+// persist channel pointers with fewer bytes in the future.
+func MigrateChannelIDIndex(tx kvdb.RwTx) error {
+	log.Infof("Migrating the tower client db to add a new channel ID " +
+		"index which stores a mapping from db-assigned ID to real " +
+		"channel ID")
+
+	// Create a new top-level bucket for the new index.
+	indexBkt, err := tx.CreateTopLevelBucket(cChanIDIndexBkt)
+	if err != nil {
+		return err
+	}
+
+	// Get the top-level channel-details bucket. The keys of this bucket
+	// are the real channel IDs.
+	chanDetailsBkt := tx.ReadWriteBucket(cChanDetailsBkt)
+	if chanDetailsBkt == nil {
+		return ErrUninitializedDB
+	}
+
+	// Iterate over the keys of the channel-details bucket.
+	return chanDetailsBkt.ForEach(func(chanID, _ []byte) error {
+		// Ask the db for a new, unique, ID for the index bucket.
+		nextSeq, err := indexBkt.NextSequence()
+		if err != nil {
+			return err
+		}
+
+		// Encode the sequence number using BigSize encoding.
+		var newIndex bytes.Buffer
+		err = tlv.WriteVarInt(&newIndex, nextSeq, &[8]byte{})
+		if err != nil {
+			return err
+		}
+
+		// Add the mapping from the db-assigned ID to the channel ID
+		// to the new index.
+		newIndexBytes := newIndex.Bytes()
+		err = indexBkt.Put(newIndexBytes, chanID)
+		if err != nil {
+			return err
+		}
+
+		chanDetails := chanDetailsBkt.NestedReadWriteBucket(chanID)
+		if chanDetails == nil {
+			return ErrCorruptChanDetails
+		}
+
+		// Here we ensure that the channel-details bucket includes a
+		// channel summary. The only reason we do this is so that we can
+		// simulate a migration fail in a test to ensure that a
+		// migration fail results in an untouched db.
+		chanSummaryBytes := chanDetails.Get(cChannelSummary)
+		if chanSummaryBytes == nil {
+			return ErrCorruptChanDetails
+		}
+
+		// In the channel-details sub-bucket for this channel, add the
+		// new DB-assigned ID for this channel under the cChanDBID key.
+		return chanDetails.Put(cChanDBID, newIndexBytes)
+	})
+}