Stop hearing mesh.assignment: a process follows its membership from the grants part's state alone

ADR 0571 clause 10: the grants part publishes memberships on mesh.assignment until the lease write and stops
then. FollowMembership now reads and watches its own key of the state memberships and nothing else, and refuses
to follow, naming the state, when it cannot read it (there is nothing else to follow, and a silent follow of
nothing is the fault). A module's principal and the runtime lose their subscribe on mesh.assignment; the grants
part's publish grant stays until the write (S21 stops the publishing).

Tests: TestFollowMembershipHearsWhatIsIssuedToTheAccount (integration) edited: no state is a refusal, a
membership put in the state is applied, one published on the subject is not; run locally against its
embedded bus. Rows added to TestTheCopiedComposerComposesAsTheControllers: nobody subscribes mesh.assignment,
and the controller's golden grant of it is the one row the copy now lacks. No new test function.

(issue 774, 774/S5)

Claude-Session: https://claude.ai/code/session_01NcbgfJ9Gum47zkFKWRaovL

go/process/follow.go

@@ -5,7 +5,6 @@ import (
 	"fmt"
 	"time"
 
-	"github.com/nats-io/nats.go"
 	"github.com/nats-io/nats.go/jetstream"
 
 	"git.novox.be/novox/mesh-sdk/go/membership"
@@ -17,10 +16,10 @@ import (
 // not a membership is handed to onBad (nil to ignore it) and skipped.
 //
 // It reads the membership from the grants part's state memberships, its own key (novox/hq ADR 0571 clause 7),
-// watched so the value there now comes first and each change after it; and it hears the subject the grants part
-// publishes on until the lease write. The assignments stream is no longer read: the bus module's tools delete it
-// at the lease write (research 072 row 33). A state not there yet, or not readable, is told to onBad and the
-// subject is followed alone. The returned error is a refusal to follow at all.
+// watched so the value there now comes first and each change after it. Nothing else: the subject the grants part
+// published on is no longer heard (ADR 0571 clause 10, issue 774), since it stops publishing at the lease write, and
+// the assignments stream is no longer read (research 072 row 33). A state not there or not readable is a refusal to
+// follow, returned: there is nothing else to follow.
 func (c *Conn) FollowMembership(apply func(membership.Membership), onBad func(error)) error {
 	if c.account.Node == "" || c.account.Module == "" {
 		return fmt.Errorf("the account names no node or module, so it has no membership")
@@ -38,27 +37,20 @@ func (c *Conn) FollowMembership(apply func(membership.Membership), onBad func(er
 		}
 		apply(issued)
 	}
-	subject := membership.Subject(c.account.Node, c.account.Module)
-	if _, err := c.nc.Subscribe(subject, func(m *nats.Msg) { take(m.Data) }); err != nil {
-		return fmt.Errorf("cannot follow %s: %w", subject, err)
-	}
 	key := membership.GrantsKey(c.account.Node, c.account.Module)
 	js, err := jetstream.New(c.nc)
 	if err != nil {
-		bad(fmt.Errorf("cannot read %s of %s (%v); following %s alone", key, membership.GrantsBucket, err, subject))
-		return nil
+		return fmt.Errorf("cannot read %s of %s: %w", key, membership.GrantsBucket, err)
 	}
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
 	kv, err := js.KeyValue(ctx, membership.GrantsBucket)
 	if err != nil {
-		bad(fmt.Errorf("cannot read %s of %s yet (%v); following %s alone", key, membership.GrantsBucket, err, subject))
-		return nil
+		return fmt.Errorf("cannot read %s of %s: %w", key, membership.GrantsBucket, err)
 	}
 	w, err := kv.Watch(context.Background(), key)
 	if err != nil {
-		bad(fmt.Errorf("cannot watch %s of %s (%v); following %s alone", key, membership.GrantsBucket, err, subject))
-		return nil
+		return fmt.Errorf("cannot watch %s of %s: %w", key, membership.GrantsBucket, err)
 	}
 	go func() {
 		for e := range w.Updates() {

go/process/follow_integration_test.go

@@ -3,14 +3,20 @@
 package process
 
 import (
+	"context"
+	"strings"
 	"testing"
 	"time"
 
+	"github.com/nats-io/nats.go/jetstream"
+
 	"git.novox.be/novox/mesh-sdk/go/membership"
 )
 
-// A process connected pinned to a test bus hears the membership published for its node and module, and
-// ignores what is not one (novox/hq issue 1633, AC2).
+// A process connected pinned to a test bus follows the membership the grants part writes for its node and module in
+// its state memberships, and reports what is not one (novox/hq issue 1633, AC2); it no longer hears mesh.assignment,
+// where the grants part stops publishing at the lease write (ADR 0571 clause 10; issue 774, 774/S5/AC1). Without the
+// state it refuses to follow, naming it.
 func TestFollowMembershipHearsWhatIsIssuedToTheAccount(t *testing.T) {
 	url, fingerprint := selfSignedServer(t)
 	account := mailSenderAccount(t, url, fingerprint)
@@ -20,26 +26,52 @@ func TestFollowMembershipHearsWhatIsIssuedToTheAccount(t *testing.T) {
 	}
 	defer conn.Close()
 
-	got := make(chan membership.Membership, 1)
+	got := make(chan membership.Membership, 4)
 	bad := make(chan error, 4)
-	if err := conn.FollowMembership(func(m membership.Membership) { got <- m }, func(e error) { bad <- e }); err != nil {
+	follow := func() error {
+		return conn.FollowMembership(func(m membership.Membership) { got <- m }, func(e error) { bad <- e })
+	}
+	if err := follow(); err == nil || !strings.Contains(err.Error(), membership.GrantsBucket) {
+		t.Fatalf("followed with no state memberships: %v", err)
+	}
+	js, err := jetstream.New(conn.nc)
+	if err != nil {
 		t.Fatal(err)
 	}
-	if err := conn.nc.Publish(membership.Subject("novox", "mesh-mail"), []byte("not json")); err != nil {
+	ctx := context.Background()
+	kv, err := js.CreateKeyValue(ctx, jetstream.KeyValueConfig{Bucket: membership.GrantsBucket})
+	if err != nil {
 		t.Fatal(err)
 	}
-	if err := conn.nc.Publish(membership.Subject("novox", "mesh-mail"), []byte(`{"node":"novox","module":"mesh-mail","mesh":["10.0.0.1"]}`)); err != nil {
+	key := membership.GrantsKey("novox", "mesh-mail")
+	if _, err := kv.Put(ctx, key, []byte("not json")); err != nil {
+		t.Fatal(err)
+	}
+	if err := follow(); err != nil {
+		t.Fatal(err)
+	}
+	// Published on the subject, as the grants part did until the write: not heard.
+	if err := conn.nc.Publish(membership.Subject("novox", "mesh-mail"), []byte(`{"node":"novox","module":"mesh-mail","mesh":["10.0.0.9"]}`)); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := kv.Put(ctx, key, membership.EncodeGrantsMembership(membership.Membership{Node: "novox", Module: "mesh-mail",
+		Mesh: []string{"10.0.0.1"}})); err != nil {
 		t.Fatal(err)
 	}
 	select {
 	case m := <-got:
-		if m.Module != "mesh-mail" || len(m.Mesh) != 1 {
+		if m.Module != "mesh-mail" || len(m.Mesh) != 1 || m.Mesh[0] != "10.0.0.1" {
 			t.Fatalf("membership: %+v", m)
 		}
 	case <-time.After(5 * time.Second):
-		t.Fatal("no membership heard")
+		t.Fatal("no membership followed")
+	}
+	select {
+	case m := <-got:
+		t.Errorf("a second membership was applied, from the subject: %+v", m)
+	case <-time.After(300 * time.Millisecond):
 	}
 	if len(bad) == 0 {
-		t.Error("the body that is not a membership was not reported")
+		t.Error("the value that is not a membership was not reported")
 	}
 }