Announce the build agent's verbs, let a waiting build hear its cancel, and retry a stopped rollout (hq ADR 0219)

The holder's verbs were served and found by nothing; it now answers discovery
with its machine's four, as a runtime announces a seat's verb, so the console
finds <node>/node-build-agent.kill. A cancel publishes no outcome, so a build
waited for also looks at the cancelled set. A plan that stopped at its first
machine is retried by sending that machine the module again, unless a newer
plan holds it.

internal/link/builds_nats.go

@@ -106,7 +106,20 @@ func (b *natsBuilds) Submit(ctx context.Context, request BuildRequest,
 	waiting, cancelWait := context.WithTimeout(ctx, wait)
 	defer cancelWait()
 	for {
-		msg, err := outcomes.NextMsgWithContext(waiting)
+		// **A wait that hears a cancel** (novox/hq ADR 0219). A person cancelling an ask records its
+		// failure without publishing the role's outcome — that subject is the holders', and the
+		// controller may not speak for them — so the waiter also looks, every while, at the cancelled
+		// set the cancel wrote first. A kill needs no look: the holder announces it as any outcome.
+		slice, endSlice := context.WithTimeout(waiting, cancelLook)
+		msg, err := outcomes.NextMsgWithContext(slice)
+		endSlice()
+		if errors.Is(err, context.DeadlineExceeded) && waiting.Err() == nil {
+			if cancelled, _ := IsCancelled(b.js.Conn(), b.role(), request.ID); cancelled {
+				return BuildResult{ID: request.ID, Repository: request.Repository, Path: request.Path,
+					Ref: request.Ref, Source: request.Source, DryRun: request.DryRun, Failed: CancelledByHand}, nil
+			}
+			continue
+		}
 		switch {
 		case errors.Is(err, context.DeadlineExceeded):
 			return BuildResult{}, waitingFor(wait)
@@ -125,6 +138,9 @@ func (b *natsBuilds) Submit(ctx context.Context, request BuildRequest,
 	}
 }
 
+// cancelLook is how often a waiting asker looks whether its ask was cancelled.
+var cancelLook = 2 * time.Second
+
 // --- the machine's side ---------------------------------------------------------------------
 
 type natsMachine struct {

internal/link/queue_test.go

@@ -229,3 +229,39 @@ func TestNatsAHoldersPausedStateIsReadBack(t *testing.T) {
 		t.Fatalf("read back %+v", said)
 	}
 }
+
+// A `build` waiting on its outcome hears a cancel — which publishes no outcome — from the cancelled
+// set, and a kill from the outcome the holder announces (novox/hq ADR 0219).
+func TestNatsAWaitingAskerHearsItsAskCancelledOrKilled(t *testing.T) {
+	js := aBusWithACancelledSet(t)
+	was := cancelLook
+	cancelLook = 200 * time.Millisecond
+	t.Cleanup(func() { cancelLook = was })
+	ask := &natsBuilds{js: js, seat: TheBuildMachine}
+
+	go func() {
+		time.Sleep(500 * time.Millisecond)
+		_ = MarkCancelled(context.Background(), js, TheBuildMachine, "build-waited-cancelled")
+	}()
+	result, err := ask.Submit(context.Background(), BuildRequest{ID: "build-waited-cancelled", Repository: "/r"}, 10*time.Second)
+	if err != nil || result.Failed != CancelledByHand || result.ID != "build-waited-cancelled" {
+		t.Fatalf("the waiter heard %+v (%v)", result, err)
+	}
+
+	// Killed: the holder announces the failure as any outcome, and the waiter has it.
+	ctx, stop := context.WithCancel(context.Background())
+	defer stop()
+	machine := MachineOverNATS(js, "ace")
+	defer machine.Close()
+	go func() {
+		_ = machine.Take(ctx, func(ctx context.Context, work Build) {
+			r := work.Request()
+			_ = work.Announce(ctx, BuildResult{ID: r.ID, Repository: r.Repository, On: "ace", Failed: KilledByHand})
+			_ = work.Done()
+		})
+	}()
+	result, err = ask.Submit(context.Background(), BuildRequest{ID: "build-waited-killed", Repository: "/r"}, 10*time.Second)
+	if err != nil || result.Failed != KilledByHand || result.On != "ace" {
+		t.Fatalf("the waiter heard %+v (%v)", result, err)
+	}
+}