The host applies the newest declaration, a file may be created once, the foundation filters first

031: a window of unacknowledged declarations is drained to the newest; the
rest are set aside and reported as superseded. 035: a file resource may say
create-once — written when absent, kept untouched when present (ADR 0087).
054: the bundle installs nftables and loads a base ruleset before the store
and broker, in the table the filter module later replaces (ADR 0088).

internal/link/newest_test.go

@@ -0,0 +1,50 @@
+package link
+
+import (
+	"testing"
+	"time"
+
+	amqp "github.com/rabbitmq/amqp091-go"
+)
+
+// A machine asked to be five things becomes the last one: what is already waiting supersedes what
+// arrived first, and everything set aside is named so it can be reported.
+func TestWhatIsAlreadyWaitingSupersedesWhatArrivedFirst(t *testing.T) {
+	deliveries := make(chan amqp.Delivery, 8)
+	for _, id := range []string{"two", "three", "four"} {
+		deliveries <- amqp.Delivery{Body: []byte(id)}
+	}
+	apply, superseded := newest(deliveries, amqp.Delivery{Body: []byte("one")}, 50*time.Millisecond)
+	if string(apply.Body) != "four" {
+		t.Fatalf("applied %q, not the newest", apply.Body)
+	}
+	if len(superseded) != 3 || string(superseded[0].Body) != "one" || string(superseded[2].Body) != "three" {
+		t.Fatalf("set aside %d: %v", len(superseded), superseded)
+	}
+}
+
+// One declaration with nothing behind it is applied as it always was, after the window.
+func TestALoneDeclarationIsAppliedAfterTheWindow(t *testing.T) {
+	deliveries := make(chan amqp.Delivery, 1)
+	began := time.Now()
+	apply, superseded := newest(deliveries, amqp.Delivery{Body: []byte("only")}, 30*time.Millisecond)
+	if string(apply.Body) != "only" || len(superseded) != 0 {
+		t.Fatalf("got %q with %d set aside", apply.Body, len(superseded))
+	}
+	if time.Since(began) < 30*time.Millisecond {
+		t.Fatal("did not wait the window for a straggler")
+	}
+}
+
+// A straggler within the window is taken; one after it is the next push.
+func TestAStragglerWithinTheWindowIsTaken(t *testing.T) {
+	deliveries := make(chan amqp.Delivery, 2)
+	go func() {
+		time.Sleep(20 * time.Millisecond)
+		deliveries <- amqp.Delivery{Body: []byte("late")}
+	}()
+	apply, superseded := newest(deliveries, amqp.Delivery{Body: []byte("first")}, 100*time.Millisecond)
+	if string(apply.Body) != "late" || len(superseded) != 1 {
+		t.Fatalf("got %q with %d set aside", apply.Body, len(superseded))
+	}
+}