Let plans wait for a plan to move and answer it as data (issue 229)

An agent following a rollout had plans answer once, in prose, and polled the
console with a curl loop of its own to be told a plan moved. plans now takes
wait (a plan's id), until (moved, tier or done) and bound, and answers when the
plan moves or the bound passes, saying which, with the plan as data and its
line as the output.

A call's one answer has to reach the console inside its thirty seconds, so
link.AnswersWithin lets a handler that waits on purpose hold its answer up to
LongestAnswer (25s) instead of AnswerWithin; the bound is at most 20s, and a
caller following a plan further asks again.

internal/link/calls.go

@@ -601,6 +601,24 @@ func Acknowledge(ctx context.Context) {
 	}
 }
 
+// LongestAnswer is the longest a handler that waits on purpose may hold its caller's one answer
+// (AnswersWithin, novox/hq issue 229): inside the console's thirty seconds with room for the trip back,
+// and inside the bus's window for an answer (broker.ResponseTTL). A variable so a test need not wait.
+var LongestAnswer = 25 * time.Second
+
+type answerWithinKey struct{}
+
+// AnswersWithin asks that the call's caller be answered in full if the handler ends within d of the
+// call's start, rather than within AnswerWithin: for a handler that waits on purpose, with a bound —
+// `plans` waiting for a plan to move (novox/hq issue 229). Past d, or past LongestAnswer whichever is
+// shorter, its caller is told it is running, as for any call. Never shortens the window; without effect
+// outside a served call, and once the caller was answered.
+func AnswersWithin(ctx context.Context, d time.Duration) {
+	if f, ok := ctx.Value(answerWithinKey{}).(func(time.Duration)); ok {
+		f(d)
+	}
+}
+
 // running is the interim answer: what a caller reads when the call has not finished.
 func running(c *Call, within time.Duration, acknowledged bool, follow string) []byte {
 	why := fmt.Sprintf("it has not finished in %s", within)
@@ -645,6 +663,13 @@ func (l *CallLog) serveCallWithin(seat, verb string, args json.RawMessage, reply
 	acknowledged := make(chan struct{})
 	var once sync.Once
 	ctx = context.WithValue(ctx, answerNowKey{}, func() { once.Do(func() { close(acknowledged) }) })
+	longer := make(chan time.Duration, 1)
+	ctx = context.WithValue(ctx, answerWithinKey{}, func(d time.Duration) {
+		select {
+		case longer <- d:
+		default:
+		}
+	})
 
 	type outcome struct {
 		body   []byte
@@ -703,17 +728,29 @@ func (l *CallLog) serveCallWithin(seat, verb string, args json.RawMessage, reply
 			}
 		}
 	}
-	timer := time.NewTimer(AnswerWithin)
+	within, start := AnswerWithin, time.Now()
+	timer := time.NewTimer(within)
 	defer timer.Stop()
-	select {
-	case o := <-done:
-		l.finish(c, o.body, o.failed, false)
-		say(o.body)
-		return
-	case <-acknowledged:
-		say(running(c, AnswerWithin, true, l.Follow))
-	case <-timer.C:
-		say(running(c, AnswerWithin, false, l.Follow))
+waiting:
+	for {
+		select {
+		case o := <-done:
+			l.finish(c, o.body, o.failed, false)
+			say(o.body)
+			return
+		case <-acknowledged:
+			say(running(c, within, true, l.Follow))
+			break waiting
+		case d := <-longer:
+			// A handler waiting on purpose (AnswersWithin): its window is stretched, never past LongestAnswer.
+			if d = min(d, LongestAnswer); d > within {
+				within = d
+				timer.Reset(time.Until(start.Add(d)))
+			}
+		case <-timer.C:
+			say(running(c, within, false, l.Follow))
+			break waiting
+		}
 	}
 	o := <-done
 	l.finish(c, o.body, o.failed, true)