watchtower: remove CommittedUpdates from ClientSession

In this commit, the new ListClientSession functional options and new
FetchSessionCommittedUpdates function are utilised in order to allow us
to completely remove the CommittedUpdates member from the ClientSession
struct.

watchtower/wtdb/client_db.go

@@ -420,8 +420,17 @@ func (c *ClientDB) RemoveTower(pubKey *btcec.PublicKey, addr net.Addr) error {
 			return ErrUninitializedDB
 		}
 		towerID := TowerIDFromBytes(towerIDBytes)
+
+		committedUpdateCount := make(map[SessionID]uint16)
+		perCommittedUpdate := func(s *ClientSession,
+			_ *CommittedUpdate) {
+
+			committedUpdateCount[s.ID]++
+		}
+
 		towerSessions, err := listTowerSessions(
 			towerID, sessions, towers, towersToSessionsIndex,
+			WithPerCommittedUpdate(perCommittedUpdate),
 		)
 		if err != nil {
 			return err
@@ -447,7 +456,7 @@ func (c *ClientDB) RemoveTower(pubKey *btcec.PublicKey, addr net.Addr) error {
 		// have any pending updates to ensure we don't load them upon
 		// restarts.
 		for _, session := range towerSessions {
-			if len(session.CommittedUpdates) > 0 {
+			if committedUpdateCount[session.ID] > 0 {
 				return ErrTowerUnackedUpdates
 			}
 			err := markSessionStatus(
@@ -1257,12 +1266,14 @@ func getClientSession(sessions, towers kvdb.RBucket, idBytes []byte,
 	if err != nil {
 		return nil, err
 	}
+	session.Tower = tower
 
 	// Can't fail because client session body has already been read.
 	sessionBkt := sessions.NestedReadBucket(idBytes)
 
-	// Fetch the committed updates for this session.
-	commitedUpdates, err := getClientSessionCommits(
+	// Pass the session's committed (un-acked) updates through the call-back
+	// if one is provided.
+	err = filterClientSessionCommits(
 		sessionBkt, session, cfg.PerCommittedUpdate,
 	)
 	if err != nil {
@@ -1276,9 +1287,6 @@ func getClientSession(sessions, towers kvdb.RBucket, idBytes []byte,
 		return nil, err
 	}
 
-	session.Tower = tower
-	session.CommittedUpdates = commitedUpdates
-
 	return session, nil
 }
 
@@ -1354,6 +1362,39 @@ func filterClientSessionAcks(sessionBkt kvdb.RBucket, s *ClientSession,
 	return nil
 }
 
+// filterClientSessionCommits retrieves all committed updates for the session
+// identified by the serialized session id and passes them to the given
+// PerCommittedUpdateCB callback.
+func filterClientSessionCommits(sessionBkt kvdb.RBucket, s *ClientSession,
+	cb PerCommittedUpdateCB) error {
+
+	if cb == nil {
+		return nil
+	}
+
+	sessionCommits := sessionBkt.NestedReadBucket(cSessionCommits)
+	if sessionCommits == nil {
+		return nil
+	}
+
+	err := sessionCommits.ForEach(func(k, v []byte) error {
+		var committedUpdate CommittedUpdate
+		err := committedUpdate.Decode(bytes.NewReader(v))
+		if err != nil {
+			return err
+		}
+		committedUpdate.SeqNum = byteOrder.Uint16(k)
+
+		cb(s, &committedUpdate)
+		return nil
+	})
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
 // putClientSessionBody stores the body of the ClientSession (everything but the
 // CommittedUpdates and AckedUpdates).
 func putClientSessionBody(sessions kvdb.RwBucket,

watchtower/wtdb/client_session.go

@@ -37,16 +37,6 @@ type ClientSession struct {
 
 	ClientSessionBody
 
-	// CommittedUpdates is a sorted list of unacked updates. These updates
-	// can be resent after a restart if the updates failed to send or
-	// receive an acknowledgment.
-	//
-	// NOTE: This list is serialized in it's own bucket, separate from the
-	// body of the ClientSession. The representation on disk is a key value
-	// map from sequence number to CommittedUpdateBody to allow efficient
-	// insertion and retrieval.
-	CommittedUpdates []CommittedUpdate
-
 	// Tower holds the pubkey and address of the watchtower.
 	//
 	// NOTE: This value is not serialized. It is recovered by looking up the