Keep the derived filter in force when a guard resource failed on the way back to adopted (hq ADR 0103)

internal/apply/opening_test.go

@@ -371,6 +371,9 @@ func TestReturningToAdoptedLoadsTheGuardBeforeRemovingTheFilter(t *testing.T) {
 			if _, ok := state.Find("adoption.guard"); !ok {
 				t.Error("the guard applied before the failure was not recorded")
 			}
+			if _, still := state.Find("nftables.load"); !still {
+				t.Error("the filter that would not stop was forgotten, so nothing would stop it later")
+			}
 		} else if err != nil {
 			t.Fatal(err)
 		}
@@ -419,3 +422,40 @@ func TestARetiredFirewallRetriedStillPutsBackTheForwardPolicy(t *testing.T) {
 		t.Errorf("the retry did not put the forward policy back: %s, %+v", u.forward, state.Firewall)
 	}
 }
+
+func TestAGuardThatFailsLeavesTheDerivedFilterInForce(t *testing.T) {
+	// Returning to adopted: if the guard cannot be raised, the filter it replaces must not be
+	// stopped — its stop deletes the mesh's table, and the node would have neither (novox/hq ADR 0103).
+	dir := t.TempDir()
+	blocked := filepath.Join(dir, "not-a-directory")
+	if err := os.WriteFile(blocked, []byte("x"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	guardFile := `{"id":"adoption.guard","type":"file","path":"` + filepath.Join(blocked, "guard.nft") +
+		`","content":"table inet mesh_guard {}\n"}`
+	stopped := false
+	run := func(_ context.Context, name string, args ...string) (string, error) {
+		if name != "systemctl" {
+			return "", &exec.Error{Name: name, Err: exec.ErrNotFound}
+		}
+		if args[0] == "show" {
+			return "LoadState=loaded\nActiveState=active\nType=oneshot\nRemainAfterExit=yes\n", nil
+		}
+		if args[0] == "stop" {
+			stopped = true
+		}
+		return "", nil
+	}
+	converged := store.State{Resources: []store.Applied{
+		{ID: "nftables.load", Type: "service", Target: "mesh-filter.service", Origin: store.OriginDeclared}}}
+	_, state, err := applyWith(t, adopted(t, `{"taken":[]}`, withConf(dir)+","+guardFile), converged, run)
+	if err == nil {
+		t.Fatal("a guard that could not be written reported success")
+	}
+	if stopped {
+		t.Error("the derived filter was stopped though the guard is not up")
+	}
+	if _, gone := state.Find("nftables.load"); !gone {
+		t.Error("the filter was forgotten, so nothing would ever stop it")
+	}
+}