A declaration carries its order, and a host refuses an older one

novox/hq 04-ISSUES/107. A declaration's only identity was the digest of
its bytes: a host could say "not the last" and could not say "older". On
the link, nothing refused an older one at all, and the drain picked the
last to arrive — wrong exactly when it mattered, a backlog drained out of
order or a broker that split a burst.

A declaration may now carry a sequence, one higher per send. A host
refuses one lower than what it kept, whole, and says why. The drain keeps
the highest sequence in a batch rather than the last to arrive.

Only when both sides claim an order. Absent reads as zero — "no ordering
claimed", not "first" — so a controller that sends none is still
understood and a host that kept one before it understood them compares
nothing. That is what lets hosts go first and the controller follow, which
is the order 087 says a new field needs.

cmd/mesh-host/main.go

@@ -457,10 +457,10 @@ func short(digest string) string {
 // them again — and everything the mesh declared read as no longer declared and removed. Even the
 // very declaration the mesh last sent, applied from a file, would plan to remove the foundation.
 // `apply FILE` is for a machine the mesh has not spoken to, and is refused saying so.
-func refuseStale(known store.State, kept store.Declared, keptErr error, digest string, from provenance) error {
+func refuseStale(known store.State, kept store.Declared, keptErr error, digest string, from provenance, sequence int64) error {
 	switch from {
 	case fromDeclared:
-		return nil
+		return refuseOlder(kept, keptErr, sequence)
 	case fromBundle:
 		if known.Genesis == nil || known.Genesis.Digest == digest {
 			return nil
@@ -557,7 +557,7 @@ func runApply(ctx context.Context, opts options, d *declaration.Declaration, raw
 	if err := apply.CheckMode(known, d); err != nil {
 		return err
 	}
-	if err := refuseStale(known, kept, keptErr, digest, from); err != nil {
+	if err := refuseStale(known, kept, keptErr, digest, from, d.Sequence); err != nil {
 		return err
 	}
 
@@ -1594,3 +1594,32 @@ func adoptDeliveredMembership(identityPath string, mine *identity.Identity, say
 	say(fmt.Sprintf("moving to the %s bus at %s — restarting to dial it", next.Transport, next.Broker))
 	os.Exit(0)
 }
+
+// refuseOlder refuses a declaration from the mesh that is older than the one this node holds.
+//
+// **By sequence, not by arrival** (novox/hq 04-ISSUES/107). What the host kept is the last thing
+// the mesh said, signed; a declaration whose sequence is lower was composed before it, whatever
+// order they arrived in — a backlog drained after the node was away, or a broker that split a burst.
+// Applying it would make the machine into something the mesh had already moved past, which is the
+// incident of issue 104 by another door.
+//
+// Only when both sides claim an order. A declaration with no sequence is one an older controller
+// sent, and one kept with no sequence is one this host received before it understood them; in either
+// case there is no order to compare, and refusing on a guess would strand the node the moment the
+// controller is older than the host. Equal is the same declaration again, which reconciling is for.
+func refuseOlder(kept store.Declared, keptErr error, sequence int64) error {
+	if sequence == 0 || keptErr != nil {
+		return nil
+	}
+	last, err := declaration.ParseTrusted(kept.Declaration)
+	if err != nil || last.Sequence == 0 {
+		return nil
+	}
+	if sequence < last.Sequence {
+		return fmt.Errorf("this declaration is older than what the mesh last said to this node: it "+
+			"is sequence %d, and the one kept here is %d. It arrived late — a backlog, or a broker "+
+			"that split a burst — and applying it would make this machine into something the mesh has "+
+			"already moved past. Refused whole; nothing was applied", sequence, last.Sequence)
+	}
+	return nil
+}

cmd/mesh-host/order_test.go

@@ -0,0 +1,58 @@
+package main
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+
+	"github.com/novox/mesh-host/internal/store"
+)
+
+// A declaration carries no order, so a host cannot tell an older one from a newer (novox/hq
+// 04-ISSUES/107). Its only identity was the digest of its bytes: "not the last" could be said,
+// "older" could not.
+
+func keptWith(t *testing.T, sequence int64) store.Declared {
+	t.Helper()
+	body, err := json.Marshal(map[string]any{
+		"declaration": 1, "resources": []any{}, "owns_nothing": true, "sequence": sequence,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	return store.Declared{Declaration: body, Signature: []byte("x")}
+}
+
+func TestAnOlderDeclarationFromTheMeshIsRefused(t *testing.T) {
+	err := refuseOlder(keptWith(t, 7), nil, 5)
+	if err == nil {
+		t.Fatal("sequence 5 was accepted over a kept 7")
+	}
+	if !strings.Contains(err.Error(), "older") || !strings.Contains(err.Error(), "nothing was applied") {
+		t.Fatalf("the refusal does not say what it is: %v", err)
+	}
+}
+
+func TestANewerOrEqualDeclarationIsNot(t *testing.T) {
+	if err := refuseOlder(keptWith(t, 7), nil, 8); err != nil {
+		t.Fatalf("sequence 8 was refused over a kept 7: %v", err)
+	}
+	// Equal is the same declaration again, which reconciling is for.
+	if err := refuseOlder(keptWith(t, 7), nil, 7); err != nil {
+		t.Fatalf("the same sequence was refused: %v", err)
+	}
+}
+
+func TestNoOrderClaimedMeansNoOrderCompared(t *testing.T) {
+	// An older controller sends none; a host that received before it understood them kept none.
+	// Refusing on a guess would strand a node the moment the controller is older than the host.
+	if err := refuseOlder(keptWith(t, 7), nil, 0); err != nil {
+		t.Fatalf("a declaration claiming no order was refused: %v", err)
+	}
+	if err := refuseOlder(keptWith(t, 0), nil, 3); err != nil {
+		t.Fatalf("a declaration was refused against a kept one that claimed no order: %v", err)
+	}
+	if err := refuseOlder(store.Declared{}, store.ErrNothingDeclared, 3); err != nil {
+		t.Fatalf("a first declaration was refused: %v", err)
+	}
+}