wtdb: supply commited update count to PostEvaluateFilterFn

In this commit, we adjust the PostEvaluateFilterFn to also take in a
count representing the number of committed updates (ie, persisted
un-acked updates) that the session has. This will be made use of in an
upcoming commit.

watchtower/wtclient/client.go

@@ -67,8 +67,8 @@ func (c *client) genSessionFilter(
 
 // ExhaustedSessionFilter constructs a wtdb.ClientSessionFilterFn filter
 // function that will filter out any sessions that have been exhausted.
-func ExhaustedSessionFilter() wtdb.ClientSessionFilterFn {
+func ExhaustedSessionFilter() wtdb.ClientSessWithNumCommittedUpdatesFilterFn {
-	return func(session *wtdb.ClientSession) bool {
+	return func(session *wtdb.ClientSession, _ uint16) bool {
 		return session.SeqNum < session.Policy.MaxUpdates
 	}
 }

watchtower/wtdb/client_db.go

@@ -2305,6 +2305,12 @@ func getClientSessionBody(sessions kvdb.RBucket,
 // that read sessions from the DB.
 type ClientSessionFilterFn func(*ClientSession) bool
 
+// ClientSessWithNumCommittedUpdatesFilterFn describes the signature of a
+// callback function that can be used to filter out a session based on the
+// contents of ClientSession along with the number of un-acked committed updates
+// that the session has.
+type ClientSessWithNumCommittedUpdatesFilterFn func(*ClientSession, uint16) bool
+
 // PerMaxHeightCB describes the signature of a callback function that can be
 // called for each channel that a session has updates for to communicate the
 // maximum commitment height that the session has backed up for the channel.
@@ -2366,7 +2372,7 @@ type ClientSessionListCfg struct {
 	// functions in ClientSessionListCfg. If a session fails this filter
 	// function then all it means is that it won't be included in the list
 	// of sessions to return.
-	PostEvaluateFilterFn ClientSessionFilterFn
+	PostEvaluateFilterFn ClientSessWithNumCommittedUpdatesFilterFn
 }
 
 // NewClientSessionCfg constructs a new ClientSessionListCfg.
@@ -2427,7 +2433,9 @@ func WithPreEvalFilterFn(fn ClientSessionFilterFn) ClientSessionListOption {
 // run against the other ClientSessionListCfg call-backs) whereas the session
 // will only reach the PostEvalFilterFn call-back once it has already been
 // evaluated by all the other call-backs.
-func WithPostEvalFilterFn(fn ClientSessionFilterFn) ClientSessionListOption {
+func WithPostEvalFilterFn(
+	fn ClientSessWithNumCommittedUpdatesFilterFn) ClientSessionListOption {
+
 	return func(cfg *ClientSessionListCfg) {
 		cfg.PostEvaluateFilterFn = fn
 	}
@@ -2459,7 +2467,7 @@ func (c *ClientDB) getClientSession(sessionsBkt, chanIDIndexBkt kvdb.RBucket,
 
 	// Pass the session's committed (un-acked) updates through the call-back
 	// if one is provided.
-	err = filterClientSessionCommits(
+	numCommittedUpdates, err := filterClientSessionCommits(
 		sessionBkt, session, cfg.PerCommittedUpdate,
 	)
 	if err != nil {
@@ -2477,7 +2485,7 @@ func (c *ClientDB) getClientSession(sessionsBkt, chanIDIndexBkt kvdb.RBucket,
 	}
 
 	if cfg.PostEvaluateFilterFn != nil &&
-		!cfg.PostEvaluateFilterFn(session) {
+		!cfg.PostEvaluateFilterFn(session, numCommittedUpdates) {
 
 		return nil, ErrSessionFailedFilterFn
 	}
@@ -2586,18 +2594,21 @@ func (c *ClientDB) filterClientSessionAcks(sessionBkt,
 // identified by the serialized session id and passes them to the given
 // PerCommittedUpdateCB callback.
 func filterClientSessionCommits(sessionBkt kvdb.RBucket, s *ClientSession,
-	cb PerCommittedUpdateCB) error {
+	cb PerCommittedUpdateCB) (uint16, error) {
-
-	if cb == nil {
-		return nil
-	}
 
 	sessionCommits := sessionBkt.NestedReadBucket(cSessionCommits)
 	if sessionCommits == nil {
-		return nil
+		return 0, nil
 	}
 
+	var numUpdates uint16
 	err := sessionCommits.ForEach(func(k, v []byte) error {
+		numUpdates++
+
+		if cb == nil {
+			return nil
+		}
+
 		var committedUpdate CommittedUpdate
 		err := committedUpdate.Decode(bytes.NewReader(v))
 		if err != nil {
@@ -2606,13 +2617,14 @@ func filterClientSessionCommits(sessionBkt kvdb.RBucket, s *ClientSession,
 		committedUpdate.SeqNum = byteOrder.Uint16(k)
 
 		cb(s, &committedUpdate)
+
 		return nil
 	})
 	if err != nil {
-		return err
+		return 0, err
 	}
 
-	return nil
+	return numUpdates, nil
 }
 
 // putClientSessionBody stores the body of the ClientSession (everything but the