Begin the shadow composer again with a backoff when an input cannot be watched yet, and pin mesh-sdk's fixed head, so a node answers candidates for the whole link (issue 1126, review of L2b round 1)

internal/selfcompose/selfcompose_test.go

@@ -355,7 +355,12 @@ func TestTheNodeComposesItsWantsFromTheBus(t *testing.T) {
 	sh := NewShadow(senv, func(_ context.Context, a map[string]delivery.CandidateAnswer) { answered <- a })
 	sctx, scancel := context.WithCancel(ctx)
 	defer scancel()
-	go func() { _ = sh.Run(sctx) }()
+	// Review of L2b, round 1, finding 2: the candidates' state is not there when the node starts (a delivery before the
+	// indexer made it): the shadow tries again, and answers once it appears.
+	sb.missing(delivery.PlacementCandidatesBucket, true)
+	go sh.Keep(sctx, func(int) time.Duration { return 5 * time.Millisecond })
+	time.Sleep(30 * time.Millisecond)
+	sb.missing(delivery.PlacementCandidatesBucket, false)
 	sb.made(6)
 	rec, _ := delivery.EncodeHaveNode(delivery.HaveNode{Record: &delivery.NodeRecord{Node: "n1", TunnelKey: "k", SealingKey: "sk", Capabilities: []string{"systemd"}}})
 	tp, _ := delivery.EncodePlacement(trunk)
@@ -433,6 +438,17 @@ func TestTheNodeComposesItsWantsFromTheBus(t *testing.T) {
 type patternBus struct {
 	mu      sync.Mutex
 	watches []patternWatch
+	absent  map[string]bool
+}
+
+// missing makes a bucket's watch fail, as one the bus holds no state for, or work again.
+func (b *patternBus) missing(bucket string, gone bool) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	if b.absent == nil {
+		b.absent = map[string]bool{}
+	}
+	b.absent[bucket] = gone
 }
 
 type patternWatch struct {
@@ -445,6 +461,9 @@ func newPatternBus() *patternBus { return &patternBus{} }
 func (b *patternBus) watch(_ context.Context, bucket, pattern string) (<-chan agent.Update, error) {
 	b.mu.Lock()
 	defer b.mu.Unlock()
+	if b.absent[bucket] {
+		return nil, fmt.Errorf("nats: bucket not found")
+	}
 	w := patternWatch{bucket: bucket, pattern: pattern, ch: make(chan agent.Update, 64)}
 	b.watches = append(b.watches, w)
 	return w.ch, nil