The node agent begins again composing its own when the catalogue places its node, with no restart (issue 1126, ADR 0549 §4)

cmd/mesh-host/nodeagent.go

@@ -496,12 +496,18 @@ func nodeAgent(ctx context.Context, opts options, signer ed25519.PublicKey, sess
 		}
 		// The node composes its own modules once the catalogue places it (novox/hq ADR 0547 §2, ADR 0549, issue 1126): its
 		// placement key is read at the agent's start; until then it applies the controller's compositions as before.
+		run, stopRun := context.WithCancel(linked)
 		if composer := selfComposer(linked, session, node, say); composer != nil {
 			env.Composed, env.SelfComposed, env.Trusted, env.Policy = composer, true, nil, composer.Policy
+		} else {
+			// And once the catalogue places it, the agent begins again composing its own: the placement key appearing ends
+			// this run, so a node does not wait for a restart to follow the seed of its placement (ADR 0549 §4).
+			go endWhenPlaced(run, session, node, stopRun, say)
 		}
 		ag = agent.New(env)
 		current.set(ag)
-		err = ag.Run(linked)
+		err = ag.Run(run)
+		stopRun()
 		cancel()
 		current.set(nil)
 		if ctx.Err() != nil {
@@ -818,3 +824,25 @@ func selfComposer(ctx context.Context, session *link.Session, node string, say f
 		// And the vault's word that a secret changed, on which it composes again (ADR 0547 §2).
 		Hear: session.Hear})
 }
+
+// endWhenPlaced calls stop once this node's placement key appears in the indexer's placement (a value after the watch
+// caught up, or one already there), so the agent begins again and composes its own. A watch that cannot be made is said;
+// the node then switches at its next start.
+func endWhenPlaced(ctx context.Context, session *link.Session, node string, stop func(), say link.Announce) {
+	key, err := delivery.PlacementKey(node)
+	if err != nil {
+		return
+	}
+	updates, err := bucketWatch{session, delivery.PlacementBucket}.Watch(ctx, key)
+	if err != nil {
+		say("whether the catalogue places this node is not followed (" + err.Error() + "): it composes its own from its next start")
+		return
+	}
+	for u := range updates {
+		if !u.Caught && !u.Deleted && len(u.Value) > 0 {
+			say("the catalogue places this node now (" + key + "): the node agent begins again, composing its own modules")
+			stop()
+			return
+		}
+	}
+}

cmd/mesh-host/nodeagent_integration_test.go

@@ -313,8 +313,8 @@ func TestNatsTheHandPathAppliesACompositionTheOperatorSigned(t *testing.T) {
 	})
 }
 
-// Issue 1126 (ADR 0547 §2, the milestone's H8): the node agent started whole against a real bus, on a node the catalogue
-// places, composes a module from its want, the indexer's manifest, its record and its mesh-config value, and applies it,
+// Issue 1126 (ADR 0547 §2, the milestone's H8): the node agent started whole against a real bus, once the catalogue
+// places its node (the placement written after it started), composes a module from its want, the indexer's manifest, its record and its mesh-config value, and applies it,
 // with no key in the controller's compositions; a changed config value is composed again on its event and applied as a
 // revision of what runs.
 func TestNatsANodeThePlacementNamesComposesAndAppliesItsOwn(t *testing.T) {
@@ -345,7 +345,7 @@ func TestNatsANodeThePlacementNamesComposesAndAppliesItsOwn(t *testing.T) {
 		kv    nats.KeyValue
 		key   string
 		value []byte
-	}{{manifests, mkey, mraw}, {modules, "web", modv}, {placement, pkey, place}, {b.haves, hkey, rec},
+	}{{manifests, mkey, mraw}, {modules, "web", modv}, {b.haves, hkey, rec},
 		{resolved, itNode + ".web.greeting", []byte(`{"value":"bonjour"}`)}} {
 		if _, err := w.kv.Put(w.key, w.value); err != nil {
 			t.Fatal(err)
@@ -359,7 +359,16 @@ func TestNatsANodeThePlacementNamesComposesAndAppliesItsOwn(t *testing.T) {
 
 	stop := b.start(false)
 	defer stop()
-	within(t, 3*time.Second, "the node did not compose and apply its own module", func() bool { return b.file("web") == "greeting=bonjour\n" })
+	// Not placed yet: the agent applies the controller's compositions, of which there are none. The catalogue's seed then
+	// places the node, and the agent begins again composing its own (ADR 0549 §4), with no restart of the engine.
+	time.Sleep(300 * time.Millisecond)
+	if b.file("web") != "" {
+		t.Fatal("a node the catalogue does not place composed its own")
+	}
+	if _, err := placement.Put(pkey, place); err != nil {
+		t.Fatal(err)
+	}
+	within(t, 5*time.Second, "the node did not compose and apply its own module once placed", func() bool { return b.file("web") == "greeting=bonjour\n" })
 	within(t, 2*time.Second, "the have does not name the build", func() bool {
 		h := b.have("web")
 		return h.FP == fp && h.Composed != "" && h.Phase == delivery.PhaseRunning