Ask the plans again when the moved path starts before its runtime answers (issue 943)

postgres and keycloak start their moved path beside Serve, and a watch
asked before Serve runs is refused by the runtime. Run returned on that
refusal and main only logged it, so a start that lost the race left the
provider answering nobody and retiring nothing until its next restart:
postgres on novox at 16:52, 18:29 and 18:48, on ace at 16:22, which kept
issue 937's retirement condition on novox from clearing. Run now asks
again, the wait doubling from a second to a minute, until the runtime
answers or its context ends.

modules/postgres/cmd/postgres-provider/moved.go

@@ -38,6 +38,9 @@ const servedPort = 5432
 type movedRun struct {
 	h *Harness
 
+	// FirstRetry is the first wait before a watch asked too early is asked again (issue 943); a second when zero.
+	FirstRetry time.Duration
+
 	mu          sync.Mutex // guards assignments and loaded
 	assignments map[string][]byte
 	loaded      bool
@@ -252,9 +255,29 @@ func (m *movedRun) watch(ctx context.Context, bus provider.Bus) error {
 }
 
 // Run answers the consumers through the helper and retires through the harness's rules, until ctx ends.
+//
+// The bus is reached through the runtime that launched this bundle, which answers only once Serve runs, and Run starts
+// beside Serve: a watch asked too early is asked again, after a wait that doubles from FirstRetry up to a minute,
+// until it is answered or ctx ends, so a start that loses the race never leaves the consumers unanswered (issue 943).
 func (m *movedRun) Run(ctx context.Context, p *provider.Provider, bus provider.Bus) error {
-	if err := m.watch(ctx, bus); err != nil {
-		return err
+	wait := m.FirstRetry
+	if wait <= 0 {
+		wait = time.Second
+	}
+	for tries := 1; ; tries++ {
+		err := m.watch(ctx, bus)
+		if err == nil {
+			break
+		}
+		if tries == 4 || tries%20 == 0 {
+			m.h.say("the plans cannot be watched yet (%d tries), asking again: %v", tries, err)
+		}
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		case <-time.After(wait):
+		}
+		wait = min(wait*2, time.Minute)
 	}
 	go func() {
 		for {