Renew the engine placed by hand to the proved build, and take a hand-over from the controller's terminal (hq issue 356)

The file on PATH was the engine a person copied at installation, and every delivered engine ran from the versions directory: the operator typed hand-over and a build from before it answered "unexpected argument". A proved engine now copies itself over that file when the bytes differ (never a link). And the hand-over of a directory used as found (issue 339) gets its channel: the controller's terminal asks this node's engine on mesh.node.<node>.ask.hand-over; the engine judges every value — the ask's size and shape, who asked, the path as it states it, a directory and no link, through no link — and records it or refuses and records nothing. Every text the engine addresses to the operator names the nox line (ADR 0272); the engine's own subcommands are install-time and rescue, and its usage says so.

README.md

@@ -4,11 +4,16 @@ Tier 0 of the Novox Mesh. The one thing ever installed by hand, and the only thi
 a machine.
 
 ```
-scp mesh-host root@machine:/usr/local/bin/
-mesh-host profile
+scp nox-mesh-host root@machine:/usr/bin/
+nox-mesh-host profile
 ```
 
-That is the whole installation. One statically linked binary, nothing else present, no runtime
+That is the whole installation. **It is a daemon**: the launcher runs it, and the operator's command is
+`nox` (`mesh-cli`), on any node, from their own account (`novox/hq` ADR 0272). The subcommands below are
+for installing a machine, joining it, and rescuing one no mesh can reach, run as root at the machine; a
+line the mesh addresses to the operator names a `nox` line, never this binary. Once a delivered engine
+proves itself, it renews the one placed by hand here to its own build, so the command on the machine is
+the engine that runs it (`novox/hq` issue 356). One statically linked binary, nothing else present, no runtime
 to install first ([`novox/hq` ADR 0005](https://git.novox.be/novox/hq)).
 
 ## What it is for
@@ -23,7 +28,7 @@ the host never queries the mesh database. It receives declarations and applies t
 ## Joining a mesh
 
 ```
-mesh-host enrol --token <token> --name <what this machine is called>
+nox-mesh-host enrol --token <token> --name <what this machine is called>
 ```
 
 **The node generates its own identity** — an Ed25519 keypair whose private half never leaves the
@@ -52,20 +57,22 @@ and then stops, having saved nothing — so it can be run again unchanged.
 connects to nothing and listens on nothing — what it applies comes from a file.
 
 ```
-mesh-host profile        what this machine can be asked to do
-mesh-host inventory      what this machine is, and what it holds
-mesh-host apply FILE     make this machine match a declaration from a file
-mesh-host reconcile      make this machine match what the mesh last told it — or, before
-                         any mesh has, the bundle this host carries
-mesh-host bundle         show what this host carries
-mesh-host owned          what this host has applied and still owns
+nox-mesh-host profile        what this machine can be asked to do
+nox-mesh-host inventory      what this machine is, and what it holds
+nox-mesh-host apply FILE     make this machine match a declaration from a file
+nox-mesh-host reconcile      make this machine match what the mesh last told it — or, before
+                             any mesh has, the bundle this host carries
+nox-mesh-host bundle         show what this host carries
+nox-mesh-host owned          what this host has applied and still owns
+nox-mesh-host hand-over DIR  hand a directory used as found to the mesh, on a machine no mesh
+                             can reach; the operator's line is `nox node hand-over <node> DIR`
   --json                 machine-readable
   --state                where this node keeps what it knows
   --dry-run              say what applying would change, and change nothing
 ```
 
 ```
-$ mesh-host profile
+$ nox-mesh-host profile
 linux/amd64
 
   yes  container-runtime  29.7.2
@@ -115,7 +122,7 @@ A host built for a machine carries its declaration **inside the binary**:
 make host BUNDLE=path/to/foundation.lock
 ```
 
-`mesh-host reconcile` then applies it, on a machine the mesh has told nothing yet. That is the
+`nox-mesh-host reconcile` then applies it, on a machine the mesh has told nothing yet. That is the
 first node's path — no mesh present, nothing fetched, nothing else copied onto the machine. Once
 the mesh has spoken, `reconcile` holds the machine to what it last said and never to the bundle,
 which genesis consumed; a bundle or a file is refused when it says the other mode than the node

cmd/mesh-host/found.go

@@ -22,14 +22,20 @@ import (
 // directory it never made to whichever account a declaration named. That is safe and, said nowhere, a module
 // whose data directory it cannot write would simply fail. So each such directory is said three ways: a WARNING
 // line in every apply's log; a resource of its module of kind `directory`, stated unhealthy on every look,
-// which the controller raises as that module's condition naming the directory and its owner; and a way out at
-// the machine's terminal, `mesh-host hand-over <path>`, after which the next apply gives it the declared owner
-// and mode like any directory of the mesh's.
+// which the controller raises as that module's condition naming the directory and its owner; and a way out,
+// which is the operator's line at the controller's terminal — `nox node hand-over <node> <path>` on the
+// control-node (novox/hq ADR 0272, issue 356) — after which the next apply gives it the declared owner and
+// mode like any directory of the mesh's. The controller carries that line to this engine over the bus
+// (link.AskHandOverSubject); the engine's own `hand-over` subcommand is the same act for a machine with no
+// mesh to reach.
 
 // foundState is where the node's state is, for the looks to read the directories used as found from; empty
 // until the engine runs.
 var foundState atomic.Value
 
+// foundNode is this node's name in the mesh, for the way out to name it; empty until the engine is enrolled.
+var foundNode atomic.Value
+
 // handedOverFrom has the apply read the directories handed over from beside the state at statePath.
 func handedOverFrom(statePath string) {
 	apply.HandedOver = func() map[string]bool {
@@ -58,9 +64,21 @@ func foundDirectories(known store.State) []store.Applied {
 	return out
 }
 
+// handOverLine is the operator's line that hands a directory to the mesh, with this node's name when it is
+// known (novox/hq ADR 0272): typed on the control-node, from the operator's own account. The directory's
+// path is the resource's target beside the reason, never in it: the controller says a reason on the
+// operator's channel, which withholds a path (ADR 0234 §6), and keeps the target as evidence.
+func handOverLine() string {
+	node, _ := foundNode.Load().(string)
+	if node == "" {
+		node = "<node>"
+	}
+	return fmt.Sprintf("`nox node hand-over %s <directory>` on the control-node", node)
+}
+
 // withFoundDirectories adds to a statement every directory used as found, as its module's resource of kind
-// directory, unhealthy: the reason names its owner and mode as they are now, what was declared, and how to hand
-// it over. The path is the target, which the controller keeps as evidence.
+// directory, unhealthy: the reason names its owner and mode as they are now, what was declared, and the line
+// that hands it over. The path is the target, which the controller keeps as evidence.
 func withFoundDirectories(h *link.Health, found []store.Applied) *link.Health {
 	for _, r := range found {
 		at := strings.LastIndex(r.ID, ".")
@@ -80,9 +98,8 @@ func withFoundDirectories(h *link.Health, found []store.Applied) *link.Health {
 		}
 		h.Resources = append(h.Resources, link.ResourceHealth{Module: r.ID[:at], Resource: r.ID,
 			Kind: link.KindDirectory, Target: r.Target, State: link.StateUnhealthy,
-			Reason: fmt.Sprintf("%s owned by %s, mode %s, as found; %s was declared and not given it — "+
-				"`mesh-host hand-over` at the machine hands it to the mesh", link.ReasonUsedAsFound, owner, mode,
-				declared),
+			Reason: fmt.Sprintf("%s owned by %s, mode %s, as found; %s was declared and not given it — %s, with its "+
+				"path, hands it to the mesh", link.ReasonUsedAsFound, owner, mode, declared, handOverLine()),
 			Since: foundSince(r).UTC()})
 	}
 	return h
@@ -102,34 +119,81 @@ func foundNow() []store.Applied {
 	return foundDirectories(known)
 }
 
-// handOverCommand hands one directory used as found to the mesh: run by a person, as root, at the machine.
-// It changes nothing on disk itself; the next apply gives the directory its declared owner and mode, and says so.
-func handOverCommand(opts options) error {
-	path := filepath.Clean(opts.file)
+// handOverFound hands one directory used as found to the mesh, on the operator's word carried by the
+// controller (link.HandOverFunc) or typed at this machine. It changes nothing on disk itself; the next apply
+// gives the directory its declared owner and mode, and says so.
+//
+// **Every value is judged, and a refusal records nothing.** The path is the directory's absolute path exactly
+// as the engine states it — clean, so `..` and a doubled separator never name a directory; one this machine's
+// state records as used as found; on disk a directory and not a link, reached through no link. The engine
+// never follows a path it did not state: a hand-over is root giving a directory to an account at the next
+// apply, and a path that resolves elsewhere would give that elsewhere.
+func handOverFound(statePath, path, by string) (string, error) {
 	if !filepath.IsAbs(path) {
-		return fmt.Errorf("hand-over takes the directory's absolute path, and %q is not one", opts.file)
+		return "", fmt.Errorf("hand-over takes the directory's absolute path, and %q is not one; nothing was handed over", path)
 	}
-	known, err := store.Load(opts.state)
+	if filepath.Clean(path) != path {
+		return "", fmt.Errorf("%q is not the directory's path as the engine states it (no `..`, no doubled or trailing "+
+			"separator); nothing was handed over", path)
+	}
+	known, err := store.Load(statePath)
+	if err != nil {
+		return "", err
+	}
+	var found *store.Applied
+	for _, r := range foundDirectories(known) {
+		if filepath.Clean(r.Target) == path {
+			r := r
+			found = &r
+			break
+		}
+	}
+	if found == nil {
+		return "", errors.New(path + " is not a directory this machine uses as found (the module's condition names " +
+			"the ones that are); nothing was handed over")
+	}
+	info, err := os.Lstat(path)
+	switch {
+	case err != nil:
+		return "", fmt.Errorf("%s cannot be read on this machine: %v; nothing was handed over", path, err)
+	case info.Mode()&os.ModeSymlink != 0:
+		return "", fmt.Errorf("%s is a link, and the mesh hands over no link (novox/hq ADR 0012); nothing was handed over", path)
+	case !info.IsDir():
+		return "", fmt.Errorf("%s is not a directory on this machine; nothing was handed over", path)
+	}
+	resolved, err := filepath.EvalSymlinks(path)
+	if err != nil {
+		return "", fmt.Errorf("%s cannot be resolved on this machine: %v; nothing was handed over", path, err)
+	}
+	if resolved != path {
+		return "", fmt.Errorf("%s is reached through a link (it is %s), and the mesh hands over nothing through one "+
+			"(novox/hq issue 339); nothing was handed over", path, resolved)
+	}
+	if err := store.AddHandedOver(statePath, store.HandOver{ID: found.ID, Path: path, At: time.Now().UTC(), By: by}); err != nil {
+		return "", err
+	}
+	return fmt.Sprintf("%s (%s) is handed to the mesh by %s: the next apply gives it %s, and it is the mesh's from "+
+		"then on — removed when empty and no longer declared", path, found.ID, by, orUnsaid(found.Declared)), nil
+}
+
+// handOverFrom is the engine's hand-over for the controller's ask: the state beside statePath, the path as asked.
+func handOverFrom(statePath string) link.HandOverFunc {
+	return func(path, by string) (string, error) { return handOverFound(statePath, path, by) }
+}
+
+// handOverCommand is `nox-mesh-host hand-over <dir>` at this machine, as root: the same act, for a machine
+// with no mesh to reach. The operator's line is `nox node hand-over <node> <dir>` on the control-node.
+func handOverCommand(opts options) error {
+	by := "a person at the machine"
+	if u, err := user.Current(); err == nil {
+		by = u.Username + " at the machine"
+	}
+	said, err := handOverFound(opts.state, filepath.Clean(opts.file), by)
 	if err != nil {
 		return err
 	}
-	for _, r := range foundDirectories(known) {
-		if filepath.Clean(r.Target) != path {
-			continue
-		}
-		by := "a person at the machine"
-		if u, err := user.Current(); err == nil {
-			by = u.Username
-		}
-		if err := store.AddHandedOver(opts.state, store.HandOver{ID: r.ID, Path: path, At: time.Now().UTC(), By: by}); err != nil {
-			return err
-		}
-		fmt.Fprintf(opts.out, "%s (%s) is handed to the mesh: the next apply gives it %s, and it is the mesh's from "+
-			"then on — removed when empty and no longer declared\n", path, r.ID, orUnsaid(r.Declared))
-		return nil
-	}
-	return errors.New(path + " is not a directory this machine uses as found (`mesh-host owned` lists what the " +
-		"engine applied); nothing was handed over")
+	fmt.Fprintln(opts.out, said)
+	return nil
 }
 
 // foundSince is when a directory was first found as it is: kept across applies, so the controller can tell one

cmd/mesh-host/found_test.go

@@ -14,13 +14,16 @@ import (
 )
 
 // Every directory used as found is its module's resource of kind directory, unhealthy, naming its owner, on
-// every statement (novox/hq issue 339): the controller raises that as the module's condition.
+// every statement (novox/hq issue 339): the controller raises that as the module's condition. The way out it
+// names is the operator's line, with this node's name (novox/hq ADR 0272, issue 356).
 func TestAFoundDirectoryIsStatedUnhealthy(t *testing.T) {
 	dir := t.TempDir()
 	known := store.State{Resources: []store.Applied{
 		{ID: "notes.data", Type: "directory", Target: dir, AsFound: true, Declared: "root, mode 755", AppliedAt: time.Now()},
 		{ID: "notes.cache", Type: "directory", Target: dir},
 	}}
+	foundNode.Store("laptop")
+	t.Cleanup(func() { foundNode.Store("") })
 	h := withFoundDirectories(&link.Health{}, foundDirectories(known))
 	if len(h.Resources) != 1 {
 		t.Fatalf("stated: %+v", h.Resources)
@@ -28,16 +31,45 @@ func TestAFoundDirectoryIsStatedUnhealthy(t *testing.T) {
 	r := h.Resources[0]
 	if r.Module != "notes" || r.Kind != link.KindDirectory || r.State != link.StateUnhealthy || r.Target != dir ||
 		!strings.HasPrefix(r.Reason, link.ReasonUsedAsFound) || !strings.Contains(r.Reason, "owned by ") ||
-		!strings.Contains(r.Reason, "root, mode 755") || !strings.Contains(r.Reason, "hand-over") {
+		!strings.Contains(r.Reason, "root, mode 755") {
 		t.Fatalf("stated as: %+v", r)
 	}
+	// The path is the target, kept as evidence, and never in the reason: the operator's channel withholds a path
+	// (novox/hq ADR 0234 §6), and the controller says the reason there.
+	if !strings.Contains(r.Reason, "`nox node hand-over laptop <directory>` on the control-node") ||
+		strings.Contains(r.Reason, "mesh-host") || strings.Contains(r.Reason, dir) {
+		t.Fatalf("the way out is not the operator's line, or carries the path: %q", r.Reason)
+	}
+	foundNode.Store("")
+	if r := withFoundDirectories(&link.Health{}, foundDirectories(known)).Resources[0]; !strings.Contains(r.Reason,
+		"`nox node hand-over <node> <directory>`") {
+		t.Fatalf("before enrolment the line names a placeholder: %q", r.Reason)
+	}
 }
 
-// `mesh-host hand-over` records a directory used as found, and refuses one that is not.
+// realDir is a temporary directory with every link in its path resolved, so a judgement of links is about the
+// links the test makes.
+func realDir(t *testing.T) string {
+	t.Helper()
+	dir, err := filepath.EvalSymlinks(t.TempDir())
+	if err != nil {
+		t.Fatal(err)
+	}
+	return dir
+}
+
+// A hand-over records a directory used as found, and refuses one that is not — through the engine's own
+// subcommand and through the controller's ask alike, which are one judgement.
 func TestHandOverRecordsOnlyAFoundDirectory(t *testing.T) {
-	dir := t.TempDir()
+	dir := realDir(t)
 	statePath := filepath.Join(dir, "state.json")
 	found := filepath.Join(dir, "found")
+	if err := os.Mkdir(found, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Mkdir(filepath.Join(dir, "mine"), 0o755); err != nil {
+		t.Fatal(err)
+	}
 	if err := store.Save(statePath, store.State{Resources: []store.Applied{
 		{ID: "notes.data", Type: "directory", Target: found, AsFound: true, Declared: "root, mode 755"},
 		{ID: "notes.mine", Type: "directory", Target: filepath.Join(dir, "mine")},
@@ -52,7 +84,7 @@ func TestHandOverRecordsOnlyAFoundDirectory(t *testing.T) {
 		t.Fatal(err)
 	}
 	all, err := store.LoadHandedOver(statePath)
-	if err != nil || len(all) != 1 || all[0].Path != found {
+	if err != nil || len(all) != 1 || all[0].Path != found || !strings.HasSuffix(all[0].By, " at the machine") {
 		t.Fatalf("recorded: %+v %v", all, err)
 	}
 	handedOverFrom(statePath)
@@ -62,5 +94,83 @@ func TestHandOverRecordsOnlyAFoundDirectory(t *testing.T) {
 	if !strings.Contains(out.String(), "root, mode 755") {
 		t.Fatalf("said: %s", out.String())
 	}
-	_ = os.Remove(statePath)
+
+	// The controller's ask is the same judgement, recording who asked in the controller's words.
+	_ = os.Remove(store.HandedOverPath(statePath))
+	said, err := handOverFrom(statePath)(found, "jo through mesh-cli on anchor")
+	if err != nil || !strings.Contains(said, "by jo through mesh-cli on anchor") || !strings.Contains(said, "root, mode 755") {
+		t.Fatalf("the ask answered %q, %v", said, err)
+	}
+	all, _ = store.LoadHandedOver(statePath)
+	if len(all) != 1 || all[0].By != "jo through mesh-cli on anchor" || all[0].ID != "notes.data" {
+		t.Fatalf("recorded: %+v", all)
+	}
+}
+
+// Every value of a hand-over is judged before anything is recorded, and each refusal records nothing: a relative
+// path, a path that is not the engine's statement of the directory (`..`, a doubled or trailing separator), one
+// not used as found, a link at the path, a link in the path, a file, a directory that is gone.
+func TestAHandOverIsRefusedForEveryPathThatIsNotTheFoundDirectory(t *testing.T) {
+	dir := realDir(t)
+	statePath := filepath.Join(dir, "state.json")
+	found := filepath.Join(dir, "found")
+	if err := os.Mkdir(found, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	// A link at the path of a second found directory, and a found directory below a linked directory.
+	linked := filepath.Join(dir, "linked")
+	if err := os.Symlink(found, linked); err != nil {
+		t.Fatal(err)
+	}
+	through := filepath.Join(dir, "via", "data")
+	if err := os.Symlink(dir, filepath.Join(dir, "via")); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Mkdir(filepath.Join(dir, "data"), 0o700); err != nil {
+		t.Fatal(err)
+	}
+	file := filepath.Join(dir, "file")
+	if err := os.WriteFile(file, nil, 0o600); err != nil {
+		t.Fatal(err)
+	}
+	gone := filepath.Join(dir, "gone")
+	if err := store.Save(statePath, store.State{Resources: []store.Applied{
+		{ID: "notes.data", Type: "directory", Target: found, AsFound: true},
+		{ID: "notes.linked", Type: "directory", Target: linked, AsFound: true},
+		{ID: "notes.through", Type: "directory", Target: through, AsFound: true},
+		{ID: "notes.file", Type: "directory", Target: file, AsFound: true},
+		{ID: "notes.gone", Type: "directory", Target: gone, AsFound: true},
+	}}); err != nil {
+		t.Fatal(err)
+	}
+	refused := map[string]string{
+		"a relative path":               "found",
+		"a path with ..":                dir + "/x/../found",
+		"a doubled separator":           dir + "//found",
+		"a trailing separator":          found + "/",
+		"a directory not used as found": filepath.Join(dir, "other"),
+		"a link at the path":            linked,
+		"a link in the path":            through,
+		"a file":                        file,
+		"a directory that is gone":      gone,
+	}
+	for name, path := range refused {
+		said, err := handOverFound(statePath, path, "jo")
+		if err == nil || said != "" {
+			t.Fatalf("%s (%s) was handed over: %q", name, path, said)
+		}
+		if !strings.Contains(err.Error(), "nothing was handed over") {
+			t.Fatalf("%s's refusal does not say nothing was handed over: %v", name, err)
+		}
+	}
+	if all, err := store.LoadHandedOver(statePath); err != nil || len(all) != 0 {
+		t.Fatalf("a refusal recorded something: %+v %v", all, err)
+	}
+	// The found directory itself, stated exactly, is taken.
+	if _, err := handOverFound(statePath, found, "jo"); err != nil {
+		t.Fatal(err)
+	}
+	if all, _ := store.LoadHandedOver(statePath); len(all) != 1 || all[0].Path != found {
+		t.Fatalf("recorded: %+v", all)
+	}
 }

cmd/mesh-host/main.go

@@ -94,7 +94,12 @@ func versionAt(self, stamped string) string {
 	return stamped
 }
 
-const usage = `mesh-host — the node host
+const usage = `nox-mesh-host — the node-engine
+
+A daemon: its launcher runs it, and it applies what the mesh declares for this machine. The
+operator's command is nox (mesh-cli), on any node, from their own account (novox/hq ADR 0272);
+the subcommands here are for installing a machine, joining it, and rescuing one no mesh can
+reach, run as root at the machine.
 
   profile     what this machine can be asked to do
   inventory   what this machine is, and what it holds
@@ -108,7 +113,9 @@ const usage = `mesh-host — the node host
   overlay take  take over the tunnel found here (novox/hq ADR 0105): its key becomes this
               node's overlay key and the mesh is told, signed; --tunnel <iface> when several are up
   owned       what this host has applied and still owns
-  hand-over <dir>  hand a directory used as found to the mesh: the next apply gives it its declared owner and mode
+  hand-over <dir>  hand a directory used as found to the mesh: the next apply gives it its declared
+              owner and mode. The operator's line is nox node hand-over <node> <dir> on the
+              control-node; this is the same act for a machine with no mesh to reach
   version
 
   --json      machine-readable output
@@ -221,7 +228,7 @@ func parseArgs(args []string) (string, options, error) {
 	// Anything left over was neither the command nor a flag. Refused rather than ignored: a
 	// mistyped argument that changes nothing and reports success is worse than an error.
 	if len(positionals) > 0 {
-		return "", opts, fmt.Errorf("unexpected argument %q — try `mesh-host help`", positionals[0])
+		return "", opts, fmt.Errorf("unexpected argument %q — try `nox-mesh-host help`", positionals[0])
 	}
 	return command, opts, nil
 }
@@ -327,7 +334,7 @@ func run(ctx context.Context, command string, opts options) error {
 		return nil
 
 	default:
-		return fmt.Errorf("unknown command %q — try `mesh-host help`", command)
+		return fmt.Errorf("unknown command %q — try `nox-mesh-host help`", command)
 	}
 }
 
@@ -1077,6 +1084,8 @@ func runLink(ctx context.Context, opts options) error {
 	// password, say — and restarting the host (design 28, task 5.2). Same file, same check.
 	say := func(line string) { fmt.Println(line) }
 	adoptDeliveredMembership(identity.Path(opts.state), &mine, say)
+	// So every statement about a directory used as found names the operator's line for this node.
+	foundNode.Store(mine.Node)
 
 	fmt.Printf("node %s, linking to %s\n", mine.Node, mine.Membership.Broker)
 
@@ -1181,8 +1190,11 @@ func runLink(ctx context.Context, opts options) error {
 		},
 		Unsaid:  unsaidBeside(opts.state),
 		Numbers: numbersBeside(opts.state),
-		Say:     say,
-		Timeout: opts.timeout,
+		// And the operator's hand-over of a directory used as found, carried by the controller from the
+		// terminal (novox/hq issue 356): judged and recorded here, applied by the next apply.
+		HandOver: handOverFrom(opts.state),
+		Say:      say,
+		Timeout:  opts.timeout,
 	}
 	worker := make(chan struct{})
 	go func() {

cmd/mesh-host/witnessing.go

@@ -70,10 +70,32 @@ func (p *proving) heard(r link.Report) {
 		if len(retired) > 0 {
 			p.say(fmt.Sprintf("host %s is proved; retired the host version(s) %s", p.version, strings.Join(retired, ", ")))
 		}
+		renewFallback(p.version, p.say)
 		renewLauncher(p.say)
 	})
 }
 
+// renewFallback makes the engine placed by hand — the launcher's fallback, and the `nox-mesh-host` on this
+// machine's PATH — a copy of this proved build when it is not one (novox/hq issue 356). On the same
+// evidence known-good is written on, and said when it happens or cannot: a stale command that answers
+// for a build that is not running would otherwise stay until somebody typed it.
+func renewFallback(version string, say func(string)) {
+	self, err := os.Executable()
+	if err != nil {
+		say("the engine placed by hand could not be renewed: this build does not know its own path: " + err.Error())
+		return
+	}
+	fallback := upgrade.FallbackPath()
+	renewed, err := upgrade.RenewFallback(self, fallback)
+	switch {
+	case err != nil:
+		say(fmt.Sprintf("the engine placed by hand at %s is not this build, and could not be renewed: %v", fallback, err))
+	case renewed:
+		say(fmt.Sprintf("renewed the engine placed by hand at %s to this build, %s: the command on this machine is the "+
+			"engine that runs it", fallback, version))
+	}
+}
+
 // The launcher, as the service manager runs it.
 const (
 	launcherPath = upgrade.DefaultLibexec + "/launch"

internal/apply/apply.go

@@ -781,7 +781,7 @@ func ApplyMindingWindows(
 		report.Outcomes = append(report.Outcomes, outcome)
 		if outcome.asFound {
 			// Never quiet (novox/hq issue 339): said on every apply, and stated unhealthy on every look until a
-			// person hands it over (FoundDirectories, `mesh-host hand-over`).
+			// person hands it over (FoundDirectories, `nox node hand-over`, novox/hq issue 356).
 			log(fmt.Sprintf("  WARNING %s (%s): %s", outcome.ID, outcome.Target, outcome.Detail))
 		}
 		if handed {
@@ -1047,7 +1047,7 @@ func applyDirectory(r *declaration.Directory, previous store.Applied, wasBefore
 	}
 	if existed && wasBefore {
 		ours := previous.Target != "" && filepath.Clean(previous.Target) == filepath.Clean(r.Path) && !previous.AsFound
-		// Or handed to the mesh by a person at the machine (`mesh-host hand-over`): then it is given the
+		// Or handed to the mesh by the operator (`nox node hand-over`, or `hand-over` at the machine): then it is given the
 		// declared owner and mode below, as any directory of the mesh's.
 		if !ours && HandedOver()[handOverKey(r.ID, r.Path)] {
 			ours = true
@@ -1069,8 +1069,8 @@ func applyDirectory(r *declaration.Directory, previous store.Applied, wasBefore
 					out.asFoundAt = previous.FoundAt
 				}
 				out.Detail = fmt.Sprintf("found here before the mesh and used as found: its owner %s and mode %o are "+
-					"left, though %s were declared (novox/hq issue 339); `mesh-host hand-over %s` at the machine "+
-					"hands it to the mesh", ownerText(before), before.Mode().Perm(), out.asFoundDeclared, r.Path)
+					"left, though %s were declared (novox/hq issue 339); `nox node hand-over <node> %s` on the "+
+					"control-node hands it to the mesh", ownerText(before), before.Mode().Perm(), out.asFoundDeclared, r.Path)
 				return out, nil
 			}
 		}
@@ -3417,7 +3417,7 @@ func removeFile(a store.Applied, now time.Time) (string, string, error) {
 		"remove it by hand once you know what it is", aside), nil
 }
 
-// HandedOver is every directory a person handed to the mesh at the machine (`mesh-host hand-over`), by
+// HandedOver is every directory the operator handed to the mesh (`nox node hand-over`; `hand-over` at the machine), by
 // handOverKey: a found directory there is the mesh's, given its declared owner and mode at the next apply
 // (novox/hq issue 339). Set by the engine to read the file beside its state; a test sets its own.
 var HandedOver = func() map[string]bool { return nil }

internal/link/handover.go

@@ -0,0 +1,79 @@
+package link
+
+import (
+	"encoding/json"
+	"fmt"
+	"strings"
+)
+
+// A hand-over asked over the bus (novox/hq issue 356, issue 339).
+//
+// A directory used as found is handed to the mesh by its operator, at the controller's terminal:
+// `nox node hand-over <node> <path>` on the control-node (novox/hq ADR 0272). The controller asks this
+// node's engine on AskHandOverSubject — a request on core NATS, its own machine's subject, which only the
+// controller may publish — and the engine answers once, on the reply the request carried. The engine
+// judges every value it is sent and records the hand-over, or refuses and records nothing; the next
+// apply gives the directory its declared owner and mode. The controller holds the same two shapes in its
+// own link code, with a test on each side holding the field names.
+
+// AskHandOverSubject is where the controller asks this node's engine to take a directory used as found as
+// the mesh's.
+func AskHandOverSubject(node string) string { return "mesh.node." + node + ".ask.hand-over" }
+
+// HandOverAsk is one hand-over asked: the directory's absolute path as the engine states it, and who asked,
+// in the controller's words.
+type HandOverAsk struct {
+	Path string `json:"path"`
+	By   string `json:"by"`
+}
+
+// HandOverAnswer is the engine's answer: what it recorded, or why it refused — never both, never neither.
+type HandOverAnswer struct {
+	Said    string `json:"said,omitempty"`
+	Refused string `json:"refused,omitempty"`
+}
+
+// HandOverFunc judges and records one hand-over: the words to answer with, or the refusal.
+type HandOverFunc func(path, by string) (string, error)
+
+// MaxHandOverAsk bounds a request: a path and a name are hundreds of bytes.
+const MaxHandOverAsk = 8 << 10
+
+// MaxHandOverBy bounds who asked.
+const MaxHandOverBy = 200
+
+// AnswerHandOver is the engine's judgement of one ask's bytes: every value is checked before anything is
+// recorded, and a refusal records nothing. The handler is given only what passed.
+func AnswerHandOver(handle HandOverFunc, body []byte) HandOverAnswer {
+	if handle == nil {
+		return HandOverAnswer{Refused: "this engine takes no hand-over"}
+	}
+	if len(body) > MaxHandOverAsk {
+		return HandOverAnswer{Refused: fmt.Sprintf("the ask is %d bytes, more than a path and a name; nothing was handed over",
+			len(body))}
+	}
+	var ask HandOverAsk
+	if err := json.Unmarshal(body, &ask); err != nil {
+		return HandOverAnswer{Refused: "the ask is not a hand-over: " + err.Error() + "; nothing was handed over"}
+	}
+	by := strings.TrimSpace(ask.By)
+	switch {
+	case by == "":
+		return HandOverAnswer{Refused: "the ask names nobody as asking; nothing was handed over"}
+	case len(by) > MaxHandOverBy || strings.ContainsAny(by, "\r\n\x00"):
+		return HandOverAnswer{Refused: "who asked is not one short line; nothing was handed over"}
+	case ask.Path == "":
+		return HandOverAnswer{Refused: "the ask names no directory; nothing was handed over"}
+	}
+	said, err := handle(ask.Path, by)
+	if err != nil {
+		return HandOverAnswer{Refused: err.Error()}
+	}
+	return HandOverAnswer{Said: said}
+}
+
+// HandOvers is a link that hears the controller asking this node to hand a directory over. Optional, as
+// Asked is.
+type HandOvers interface {
+	HearHandOvers(HandOverFunc)
+}

internal/link/handover_test.go

@@ -0,0 +1,89 @@
+package link
+
+import (
+	"encoding/json"
+	"errors"
+	"sort"
+	"strings"
+	"testing"
+)
+
+// The hand-over's ask and answer hold these field names; the controller's side holds the same list
+// (mesh-controller internal/link, TestTheHandOverAskAndAnswerKeepTheirFieldNames).
+func TestTheHandOverAskAndAnswerKeepTheirFieldNames(t *testing.T) {
+	body, _ := json.Marshal(HandOverAsk{Path: "/srv/notes", By: "jo through mesh-cli on anchor"})
+	if got := keysOf(t, body); got != "by path" {
+		t.Fatalf("the ask's fields are %q", got)
+	}
+	body, _ = json.Marshal(HandOverAnswer{Said: "s", Refused: "r"})
+	if got := keysOf(t, body); got != "refused said" {
+		t.Fatalf("the answer's fields are %q", got)
+	}
+	if AskHandOverSubject("laptop") != "mesh.node.laptop.ask.hand-over" {
+		t.Fatalf("the subject is %q", AskHandOverSubject("laptop"))
+	}
+}
+
+func keysOf(t *testing.T, body []byte) string {
+	t.Helper()
+	var m map[string]any
+	if err := json.Unmarshal(body, &m); err != nil {
+		t.Fatal(err)
+	}
+	var keys []string
+	for k := range m {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	return strings.Join(keys, " ")
+}
+
+// Every value of an ask is judged before the handler sees it, and a refusal hands nothing to the handler:
+// an ask too large, not a hand-over, naming nobody, naming somebody over more than a line, or naming no
+// directory. What passes reaches the handler with its path as given and who asked trimmed.
+func TestAHandOverAskIsJudgedBeforeAnythingIsRecorded(t *testing.T) {
+	var given []string
+	handle := func(path, by string) (string, error) {
+		given = append(given, path+" by "+by)
+		if path == "/refused" {
+			return "", errors.New("not a directory this machine uses as found")
+		}
+		return path + " is handed to the mesh", nil
+	}
+	refusals := map[string][]byte{
+		"too large":         []byte(`{"path":"/` + strings.Repeat("a", MaxHandOverAsk) + `","by":"jo"}`),
+		"not a hand-over":   []byte(`not json`),
+		"nobody":            []byte(`{"path":"/srv/notes","by":"  "}`),
+		"two lines":         []byte(`{"path":"/srv/notes","by":"jo\nroot"}`),
+		"a name too long":   []byte(`{"path":"/srv/notes","by":"` + strings.Repeat("x", MaxHandOverBy+1) + `"}`),
+		"no directory":      []byte(`{"path":"","by":"jo"}`),
+		"a NUL in the name": []byte(`{"path":"/srv/notes","by":"jo\u0000"}`),
+	}
+	for name, body := range refusals {
+		a := AnswerHandOver(handle, body)
+		if a.Refused == "" || a.Said != "" {
+			t.Fatalf("%s was answered %+v; want a refusal", name, a)
+		}
+		if !strings.Contains(a.Refused, "nothing was handed over") {
+			t.Fatalf("%s's refusal does not say nothing was handed over: %q", name, a.Refused)
+		}
+	}
+	if len(given) != 0 {
+		t.Fatalf("the handler was given %v although every ask was refused", given)
+	}
+	if a := AnswerHandOver(nil, []byte(`{"path":"/srv/notes","by":"jo"}`)); a.Refused == "" {
+		t.Fatal("an engine with no handler took a hand-over")
+	}
+
+	a := AnswerHandOver(handle, []byte(`{"path":"/refused","by":"jo"}`))
+	if a.Refused != "not a directory this machine uses as found" || a.Said != "" {
+		t.Fatalf("the handler's refusal was answered as %+v", a)
+	}
+	a = AnswerHandOver(handle, []byte(`{"path":"/srv/notes","by":" jo through mesh-cli on anchor "}`))
+	if a.Said != "/srv/notes is handed to the mesh" || a.Refused != "" {
+		t.Fatalf("answered %+v", a)
+	}
+	if strings.Join(given, "; ") != "/refused by jo; /srv/notes by jo through mesh-cli on anchor" {
+		t.Fatalf("the handler was given %v", given)
+	}
+}

internal/link/hearing_nats.go

@@ -2,6 +2,7 @@ package link
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"strings"
@@ -60,6 +61,7 @@ type natsLink struct {
 	sub     *nats.Subscription
 	asking  *nats.Subscription
 	asked   chan struct{}
+	handing *nats.Subscription
 	node    string
 	arrived chan Declaration
 	lost    chan error
@@ -192,6 +194,31 @@ func (l *natsLink) hearAsks() {
 	}
 }
 
+// HearHandOvers listens for the controller asking this node to hand a directory used as found to the mesh
+// (novox/hq issue 356): a request on this machine's own subject, answered once on the reply it carried.
+// The engine judges the ask (AnswerHandOver) and runs nothing it is sent. A bus whose user list is older
+// than the grant refuses the subscription asynchronously, said by the client; the operator's line then
+// says that nothing on this node answers.
+func (l *natsLink) HearHandOvers(handle HandOverFunc) {
+	if handle == nil {
+		return
+	}
+	handing, err := l.conn.Subscribe(AskHandOverSubject(l.node), func(msg *nats.Msg) {
+		if msg.Reply == "" {
+			// Nobody to answer: nothing is handed over on a word nobody waits for.
+			return
+		}
+		body, err := json.Marshal(AnswerHandOver(handle, msg.Data))
+		if err != nil {
+			return
+		}
+		_ = msg.Respond(body)
+	})
+	if err == nil {
+		l.handing = handing
+	}
+}
+
 func (l *natsLink) Declarations() <-chan Declaration { return l.arrived }
 func (l *natsLink) AskedToReport() <-chan struct{}   { return l.asked }
 func (l *natsLink) Lost() <-chan error               { return l.lost }
@@ -203,6 +230,9 @@ func (l *natsLink) Close() {
 	if l.asking != nil {
 		_ = l.asking.Unsubscribe()
 	}
+	if l.handing != nil {
+		_ = l.handing.Unsubscribe()
+	}
 	if l.conn != nil {
 		l.conn.Close()
 	}

internal/link/messages.go

@@ -268,7 +268,7 @@ const KindAccount = "account"
 
 // KindDirectory is a directory a module declares that the engine uses as found (novox/hq issue 339): there before
 // the mesh, with another owner or mode than declared, and left so. Said unhealthy on every look, its reason
-// starting ReasonUsedAsFound, until a person hands it to the mesh at the machine (`mesh-host hand-over`).
+// starting ReasonUsedAsFound, until the operator hands it to the mesh (`nox node hand-over <node> <path>` on the control-node, novox/hq issue 356).
 const KindDirectory = "directory"
 
 // ReasonUsedAsFound starts the reason of a directory used as found.

internal/link/queue.go

@@ -51,8 +51,11 @@ type Queue struct {
 	// Numbers keeps the report sequence and the refusals counted, across restarts and self-updates.
 	// Nil counts in memory, which is only right where nothing outlives the process — a test.
 	Numbers Numbers
-	Say     Announce
-	Timeout time.Duration
+	// HandOver judges and records a directory used as found handed to the mesh by the operator, asked by
+	// the controller over the link (novox/hq issue 356). Nil answers every such ask with a refusal.
+	HandOver HandOverFunc
+	Say      Announce
+	Timeout  time.Duration
 
 	once sync.Once
 	wake chan struct{}

internal/link/run.go

@@ -239,6 +239,11 @@ func serve(ctx context.Context, link Link, m Membership, queue *Queue, say Annou
 	if a, ok := link.(Asked); ok {
 		asks = a.AskedToReport()
 	}
+	// And the controller asking this node to hand a directory used as found to the mesh (novox/hq issue
+	// 356), answered on the link the question came over.
+	if h, ok := link.(HandOvers); ok {
+		h.HearHandOvers(queue.HandOver)
+	}
 
 	for {
 		// Asked to stop — by standing aside, say — nothing further is taken in.

internal/upgrade/fallback.go

@@ -0,0 +1,137 @@
+package upgrade
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+)
+
+// The engine placed by hand, kept current (novox/hq issue 356).
+//
+// The first engine on a machine is copied there by a person (novox/hq ADR 0141): the launcher runs it
+// while nothing is delivered, and it is the one `nox-mesh-host` on any PATH. Every engine after it is
+// delivered under VersionsDirName and run from there, so the file on PATH fell behind: on 2026-10-09 the
+// operator typed its `hand-over` and was answered "unexpected argument" by a build months older than the
+// one running the machine. A command that answers for a build that is not running is the lie this
+// repository exists to refuse.
+//
+// So a proved engine (Proved) writes itself over the one placed by hand when the two differ: a plain
+// copy, renamed into place — never a link, which the mesh creates none of (novox/hq ADR 0012), and never
+// a write through one. The launcher's fallback is then the newest proved build, which is also the best
+// one to fall back on.
+
+// DefaultFallback is where the engine placed by hand lives: the launcher's fallback, and the command.
+const DefaultFallback = "/usr/bin/nox-mesh-host"
+
+// FallbackVar is the environment the launcher reads the fallback's path from; read here too, so the two
+// never name different files.
+const FallbackVar = "MESH_HOST_BIN"
+
+// FallbackPath is the engine placed by hand: FallbackVar, or DefaultFallback.
+func FallbackPath() string {
+	if p := os.Getenv(FallbackVar); p != "" {
+		return p
+	}
+	return DefaultFallback
+}
+
+// RenewFallback makes the file at fallback a copy of the executable at self, when it is not one already.
+//
+// Nothing is written when the two are the same file (an engine running from the fallback itself, while
+// nothing is delivered) or hold the same bytes. A fallback that is a link is refused: the mesh writes
+// through no link, and a link there would be somebody else's arrangement. Reports whether it was renewed.
+func RenewFallback(self, fallback string) (bool, error) {
+	if self == "" || fallback == "" {
+		return false, errors.New("the running engine or the fallback has no path")
+	}
+	mine, err := os.Stat(self)
+	if err != nil {
+		return false, fmt.Errorf("cannot read the running engine at %s: %w", self, err)
+	}
+	if !mine.Mode().IsRegular() {
+		return false, fmt.Errorf("the running engine at %s is not a regular file", self)
+	}
+	placed, err := os.Lstat(fallback)
+	switch {
+	case errors.Is(err, os.ErrNotExist):
+		// Nothing placed by hand: the launcher would have nothing to fall back on. Placed now.
+	case err != nil:
+		return false, fmt.Errorf("cannot read the engine placed by hand at %s: %w", fallback, err)
+	case placed.Mode()&os.ModeSymlink != 0:
+		return false, fmt.Errorf("%s is a link, and the mesh writes through none (novox/hq ADR 0012): "+
+			"it is left as it is", fallback)
+	case !placed.Mode().IsRegular():
+		return false, fmt.Errorf("%s is not a regular file: it is left as it is", fallback)
+	case os.SameFile(mine, placed):
+		return false, nil
+	default:
+		same, err := sameBytes(self, fallback)
+		if err != nil {
+			return false, err
+		}
+		if same {
+			return false, nil
+		}
+	}
+
+	src, err := os.Open(self)
+	if err != nil {
+		return false, err
+	}
+	defer src.Close()
+	tmp, err := os.CreateTemp(filepath.Dir(fallback), "."+filepath.Base(fallback)+"-*")
+	if err != nil {
+		return false, fmt.Errorf("cannot write beside %s: %w", fallback, err)
+	}
+	defer os.Remove(tmp.Name())
+	if _, err := io.Copy(tmp, src); err != nil {
+		tmp.Close()
+		return false, err
+	}
+	if err := tmp.Sync(); err != nil {
+		tmp.Close()
+		return false, err
+	}
+	if err := tmp.Close(); err != nil {
+		return false, err
+	}
+	if err := os.Chmod(tmp.Name(), 0o755); err != nil {
+		return false, err
+	}
+	// Renamed, never written in place: the kernel refuses to truncate an executable that runs, and a
+	// rename leaves whatever runs from the old file untouched.
+	if err := os.Rename(tmp.Name(), fallback); err != nil {
+		return false, err
+	}
+	return true, nil
+}
+
+// sameBytes says whether two files hold the same bytes, by digest.
+func sameBytes(a, b string) (bool, error) {
+	da, err := digestOf(a)
+	if err != nil {
+		return false, err
+	}
+	db, err := digestOf(b)
+	if err != nil {
+		return false, err
+	}
+	return bytes.Equal(da, db), nil
+}
+
+func digestOf(path string) ([]byte, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	h := sha256.New()
+	if _, err := io.Copy(h, f); err != nil {
+		return nil, fmt.Errorf("reading %s: %w", path, err)
+	}
+	return h.Sum(nil), nil
+}

internal/upgrade/fallback_test.go

@@ -0,0 +1,154 @@
+package upgrade
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// The engine placed by hand becomes a copy of the proved one when the two differ (novox/hq issue 356): the
+// command on PATH is the build that runs the machine.
+func TestTheFallbackBecomesACopyOfTheProvedEngine(t *testing.T) {
+	dir := t.TempDir()
+	self := filepath.Join(dir, VersionsDirName, "v2", BinaryName)
+	if err := os.MkdirAll(filepath.Dir(self), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(self, []byte("the proved engine"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	fallback := filepath.Join(dir, "bin", BinaryName)
+	if err := os.MkdirAll(filepath.Dir(fallback), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(fallback, []byte("the stale engine placed by hand"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	renewed, err := RenewFallback(self, fallback)
+	if err != nil || !renewed {
+		t.Fatalf("renewed %v, %v", renewed, err)
+	}
+	got, err := os.ReadFile(fallback)
+	if err != nil || string(got) != "the proved engine" {
+		t.Fatalf("the fallback holds %q, %v", got, err)
+	}
+	info, err := os.Stat(fallback)
+	if err != nil || info.Mode().Perm() != 0o755 {
+		t.Fatalf("the fallback's mode is %v, %v; want 0755 so it can be run", info.Mode(), err)
+	}
+	if _, err := os.Stat(self); err != nil {
+		t.Fatalf("the running engine was touched: %v", err)
+	}
+
+	// Already a copy: left alone, so a proof every start does not rewrite it every start.
+	before, _ := os.Stat(fallback)
+	renewed, err = RenewFallback(self, fallback)
+	if err != nil || renewed {
+		t.Fatalf("a fallback that is already a copy was renewed again: %v, %v", renewed, err)
+	}
+	after, _ := os.Stat(fallback)
+	if !os.SameFile(before, after) {
+		t.Fatal("a fallback that is already a copy was replaced")
+	}
+	if left, _ := filepath.Glob(filepath.Join(filepath.Dir(fallback), "."+BinaryName+"-*")); len(left) > 0 {
+		t.Fatalf("temporary files left beside the fallback: %v", left)
+	}
+}
+
+// An engine running from the fallback itself — nothing delivered yet — renews nothing: it would copy a file
+// over itself.
+func TestAnEngineRunningFromTheFallbackRenewsNothing(t *testing.T) {
+	dir := t.TempDir()
+	fallback := filepath.Join(dir, BinaryName)
+	if err := os.WriteFile(fallback, []byte("placed by hand and running"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	renewed, err := RenewFallback(fallback, fallback)
+	if err != nil || renewed {
+		t.Fatalf("renewed %v, %v", renewed, err)
+	}
+}
+
+// Nothing placed by hand: the proved engine is placed, so the launcher has a fallback and the machine a command.
+func TestAMissingFallbackIsPlaced(t *testing.T) {
+	dir := t.TempDir()
+	self := filepath.Join(dir, "v1", BinaryName)
+	if err := os.MkdirAll(filepath.Dir(self), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(self, []byte("proved"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	fallback := filepath.Join(dir, BinaryName)
+	renewed, err := RenewFallback(self, fallback)
+	if err != nil || !renewed {
+		t.Fatalf("renewed %v, %v", renewed, err)
+	}
+	if got, _ := os.ReadFile(fallback); string(got) != "proved" {
+		t.Fatalf("placed %q", got)
+	}
+}
+
+// A link at the fallback's path is refused and left as it is: the mesh creates no link and writes through none
+// (novox/hq ADR 0012). The link's target is untouched too.
+func TestALinkAtTheFallbackIsRefusedAndLeft(t *testing.T) {
+	dir := t.TempDir()
+	self := filepath.Join(dir, "v1", BinaryName)
+	if err := os.MkdirAll(filepath.Dir(self), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(self, []byte("proved"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	target := filepath.Join(dir, "somebody-elses")
+	if err := os.WriteFile(target, []byte("theirs"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	fallback := filepath.Join(dir, BinaryName)
+	if err := os.Symlink(target, fallback); err != nil {
+		t.Fatal(err)
+	}
+	renewed, err := RenewFallback(self, fallback)
+	if err == nil || renewed || !strings.Contains(err.Error(), "link") {
+		t.Fatalf("a link was taken: %v, %v", renewed, err)
+	}
+	if got, _ := os.ReadFile(target); string(got) != "theirs" {
+		t.Fatalf("the link's target was written: %q", got)
+	}
+	if info, err := os.Lstat(fallback); err != nil || info.Mode()&os.ModeSymlink == 0 {
+		t.Fatalf("the link was replaced: %v %v", info, err)
+	}
+}
+
+// A running engine that cannot be read is said, not copied as nothing.
+func TestAnUnreadableRunningEngineIsSaid(t *testing.T) {
+	dir := t.TempDir()
+	fallback := filepath.Join(dir, BinaryName)
+	if err := os.WriteFile(fallback, []byte("placed"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	renewed, err := RenewFallback(filepath.Join(dir, "gone"), fallback)
+	if err == nil || renewed {
+		t.Fatalf("renewed %v, %v", renewed, err)
+	}
+	if got, _ := os.ReadFile(fallback); string(got) != "placed" {
+		t.Fatalf("the fallback was written from nothing: %q", got)
+	}
+	if _, err := RenewFallback("", fallback); err == nil {
+		t.Fatal("an empty path was taken")
+	}
+}
+
+// The fallback's path is the launcher's: its environment variable, or the default.
+func TestTheFallbackPathIsTheLaunchers(t *testing.T) {
+	t.Setenv(FallbackVar, "")
+	if FallbackPath() != DefaultFallback {
+		t.Fatalf("the default is %q", FallbackPath())
+	}
+	t.Setenv(FallbackVar, "/elsewhere/nox-mesh-host")
+	if FallbackPath() != "/elsewhere/nox-mesh-host" {
+		t.Fatalf("the override is %q", FallbackPath())
+	}
+}