watchtower/client: immediately negotiate session with first candidate tower

docs/release-notes/release-notes-0.16.0.md

@@ -130,6 +130,8 @@ crash](https://github.com/lightningnetwork/lnd/pull/7019).
 * [test: fix loop variables being accessed in
   closures](https://github.com/lightningnetwork/lnd/pull/7032).
 
+* [Do not delay session negotiation with first candidate tower]()
+
 ### Tooling and documentation
 
 * [The `golangci-lint` tool was updated to

watchtower/wtclient/candidate_iterator.go

@@ -25,6 +25,10 @@ type TowerCandidateIterator interface {
 	// iterator.
 	IsActive(wtdb.TowerID) bool
 
+	// IsEmpty determines whether the iterator has any candidate towers
+	// with which we could potentially establish a session.
+	IsEmpty() bool
+
 	// Reset clears any internal iterator state, making previously taken
 	// candidates available as long as they remain in the set.
 	Reset() error
@@ -162,4 +166,13 @@ func (t *towerListIterator) IsActive(tower wtdb.TowerID) bool {
 	return ok
 }
 
+// IsEmpty indicates whether the iterator has any candidate towers
+// with which we could potentially establish a session.
+func (t *towerListIterator) IsEmpty() bool {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	return len(t.candidates) == 0
+}
+
 // TODO(conner): implement graph-backed candidate iterator for public towers.

watchtower/wtclient/candidate_iterator_test.go

@@ -100,6 +100,12 @@ func TestTowerCandidateIterator(t *testing.T) {
 	}
 	towerIterator := newTowerListIterator(towerCopies...)
 
+	// The iterator has towers and should report as non-empty.
+	empty := towerIterator.IsEmpty()
+	if empty {
+		t.Fatal("iterator with towers incorrectly reports as empty")
+	}
+
 	// We should expect to see all of our candidates in the order that they
 	// were added.
 	for _, expTower := range towers {
@@ -152,4 +158,11 @@ func TestTowerCandidateIterator(t *testing.T) {
 	towerIterator.AddCandidate(secondTower)
 	assertActiveCandidate(t, towerIterator, secondTower, true)
 	assertNextCandidate(t, towerIterator, secondTower)
+
+	// An empty iterator should correctly report so.
+	emptyIterator := newTowerListIterator()
+	empty = emptyIterator.IsEmpty()
+	if !empty {
+		t.Fatal("empty iterator incorrectly reports as non-empty")
+	}
 }

watchtower/wtclient/client.go

@@ -760,8 +760,52 @@ func (c *TowerClient) backupDispatcher() {
 	for {
 		switch {
 
-		// No active session queue and no additional sessions.
+		// No active session queue and no additional sessions,
+		// so we'll need to request a new one.
 		case c.sessionQueue == nil && len(c.candidateSessions) == 0:
+
+			// Wait until we have at least one candidate tower.
+			// with which to request a session.
+			if c.candidateTowers.IsEmpty() {
+				c.log.Debug("No candidate towers. Waiting " +
+					"for tower before requesting session.")
+
+				var towerID []byte
+
+			awaitTower:
+				select {
+				// A candidate tower has been added.
+				case msg := <-c.newTowers:
+					msg.errChan <- c.handleNewTower(msg)
+
+					towerID = msg.addr.IdentityKey.
+						SerializeCompressed()[:10]
+
+				case msg := <-c.staleTowers:
+					msg.errChan <- errors.New("there are " +
+						"no towers to remove")
+					goto awaitTower
+
+				case <-c.statTicker.C:
+					c.log.Infof("Client stats: %s", c.stats)
+					goto awaitTower
+
+				case <-c.forceQuit:
+					return
+				}
+
+				// If we have any sessions from previous
+				// negotiations with this tower then
+				// we'll use those.
+				if len(c.candidateSessions) != 0 {
+					c.log.Debugf("Using previously "+
+						"negotiated session(s) for "+
+						"tower: %x", towerID)
+
+					continue
+				}
+			}
+
 			c.log.Infof("Requesting new session.")
 
 			// Immediately request a new session.