Let a node agent hear the creation of the seat's placement and candidates, so it binds them with no timer (ADR 0567 §9, issue 1126)

go/buscompose/buscompose_test.go

@@ -35,7 +35,7 @@ func beyondTheControllers(user string) map[string]bool {
 	if !ok {
 		return out
 	}
-	for _, s := range append(readGrants("node-engine_have", "*.node"), NodeComposeReads(node)...) {
+	for _, s := range append(append(readGrants("node-engine_have", "*.node"), NodeComposeReads(node)...), NodeComposeAdvisories()...) {
 		out[s] = true
 	}
 	return out
@@ -129,6 +129,11 @@ func TestTheCopiedComposerComposesAsTheControllers(t *testing.T) {
 			t.Errorf("a node is granted %s: %v, want %v", bucket, !granted, granted)
 		}
 	}
+	// It hears the creation of those two buckets, and of no other.
+	if got := NodeComposeAdvisories(); len(got) != 2 || got[0] != "$JS.EVENT.ADVISORY.STREAM.CREATED.KV_mesh-delivery-assigner_placement" ||
+		got[1] != "$JS.EVENT.ADVISORY.STREAM.CREATED.KV_mesh-delivery-assigner_candidates" {
+		t.Errorf("a node hears the creation of %v", got)
+	}
 	raw, err := os.ReadFile(filepath.Join("testdata", "controller-user-lists.json"))
 	if err != nil {
 		t.Fatal(err)

go/buscompose/nats.go

@@ -931,6 +931,9 @@ func PermissionsFor(p Principal) (Permissions, error) {
 		// And the backup holder's word that it backed the module up (ADR 0539 clause 16), whichever module holds
 		// node-backup: one event, by its name.
 		sub = append(sub, NodeAgentHears...)
+		// And the creation of the seat mesh-placement's two states it composes from (ADR 0567 §9), so a node agent started
+		// before they exist binds each when the bus says it was made, with no timer; no other bucket's.
+		sub = append(sub, NodeComposeAdvisories()...)
 		// And, while the vault serves it, the vault's move-in in its own name and nobody else's (issue 676): the
 		// values sealed to this machine, which only it opens, handed in sealed to the vault's key for the call.
 		if p.MovesIntoTheVault {

go/buscompose/state.go

@@ -326,6 +326,12 @@ func NodeComposeReads(node string) []string {
 	return append(out, readGrants(ResolvedBucket, node+".>")...)
 }
 
+// NodeComposeAdvisories are the stream-created advisories of the seat mesh-placement's placement and candidates
+// (ADR 0567 §9): what a node agent hears to bind each once it is made. Those two and no wider.
+func NodeComposeAdvisories() []string {
+	return []string{StreamCreatedAdvisory(delivery.PlacementBucket), StreamCreatedAdvisory(delivery.PlacementCandidatesBucket)}
+}
+
 // ServedReadGrants is the read of each key of mesh-config's resolved values named, `<node>.<module>.<key>`
 // (Principal.ServedReads, issue 1126): a get and a watch of that key alone. Each part is checked as every subject token
 // is, so a key cannot widen the grant.