watchtower: add GetTower to tower iterator

Add a GetTower method to the tower iterator.

watchtower/wtclient/candidate_iterator.go

@@ -29,6 +29,10 @@ type TowerCandidateIterator interface {
 	// candidates available as long as they remain in the set.
 	Reset() error
 
+	// GetTower gets the tower with the given ID from the iterator. If no
+	// such tower is found then ErrTowerNotInIterator is returned.
+	GetTower(id wtdb.TowerID) (*Tower, error)
+
 	// Next returns the next candidate tower. The iterator is not required
 	// to return results in any particular order.  If no more candidates are
 	// available, ErrTowerCandidatesExhausted is returned.
@@ -76,6 +80,20 @@ func (t *towerListIterator) Reset() error {
 	return nil
 }
 
+// GetTower gets the tower with the given ID from the iterator. If no such tower
+// is found then ErrTowerNotInIterator is returned.
+func (t *towerListIterator) GetTower(id wtdb.TowerID) (*Tower, error) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	tower, ok := t.candidates[id]
+	if !ok {
+		return nil, ErrTowerNotInIterator
+	}
+
+	return tower, nil
+}
+
 // Next returns the next candidate tower. This iterator will always return
 // candidates in the order given when the iterator was instantiated.  If no more
 // candidates are available, ErrTowerCandidatesExhausted is returned.

watchtower/wtclient/candidate_iterator_test.go

@@ -83,9 +83,15 @@ func assertNextCandidate(t *testing.T, i TowerCandidateIterator, c *Tower) {
 
 	tower, err := i.Next()
 	require.NoError(t, err)
-	require.True(t, tower.IdentityKey.IsEqual(c.IdentityKey))
-	require.Equal(t, tower.ID, c.ID)
-	require.Equal(t, tower.Addresses.GetAll(), c.Addresses.GetAll())
+	assertTowersEqual(t, c, tower)
+}
+
+func assertTowersEqual(t *testing.T, expected, actual *Tower) {
+	t.Helper()
+
+	require.True(t, expected.IdentityKey.IsEqual(actual.IdentityKey))
+	require.Equal(t, expected.ID, actual.ID)
+	require.Equal(t, expected.Addresses.GetAll(), actual.Addresses.GetAll())
 }
 
 // TestTowerCandidateIterator asserts the internal state of a
@@ -155,4 +161,16 @@ func TestTowerCandidateIterator(t *testing.T) {
 	towerIterator.AddCandidate(secondTower)
 	assertActiveCandidate(t, towerIterator, secondTower, true)
 	assertNextCandidate(t, towerIterator, secondTower)
+
+	// Assert that the GetTower correctly returns the tower too.
+	tower, err := towerIterator.GetTower(secondTower.ID)
+	require.NoError(t, err)
+	assertTowersEqual(t, secondTower, tower)
+
+	// Now remove the tower and assert that GetTower returns expected error.
+	err = towerIterator.RemoveCandidate(secondTower.ID, nil)
+	require.NoError(t, err)
+
+	_, err = towerIterator.GetTower(secondTower.ID)
+	require.ErrorIs(t, err, ErrTowerNotInIterator)
 }

watchtower/wtclient/errors.go

@@ -1,6 +1,8 @@
 package wtclient
 
-import "errors"
+import (
+	"errors"
+)
 
 var (
 	// ErrClientExiting signals that the watchtower client is shutting down.
@@ -11,6 +13,10 @@ var (
 	ErrTowerCandidatesExhausted = errors.New("exhausted all tower " +
 		"candidates")
 
+	// ErrTowerNotInIterator is returned when a requested tower was not
+	// found in the iterator.
+	ErrTowerNotInIterator = errors.New("tower not in iterator")
+
 	// ErrPermanentTowerFailure signals that the tower has reported that it
 	// has permanently failed or the client believes this has happened based
 	// on the tower's behavior.