Ask a node's engine which of its directories the walk would refuse, read-only (issue 358)

Before the node-engine gives a mesh directory its mode and owner only through
a walk that follows no link another account could make, the operator is to
see which directories that walk would refuse on every node, the control-node
included, where no tool reads a path. `node judge-directories <node>` signs
the ask as a hand-over is, under its own context, and prints the engine's
one line per directory; it changes nothing, so it is a seat verb an agent may
call. The node is granted the subject, the controller alone its publish.

cmd/mesh-controller/nodes.go

@@ -127,9 +127,14 @@ func nodeCommand(ctx context.Context, args []string) error {
 		// and whoever may call a verb includes agents.
 		return nodeSetuidSearch(ctx, open, args[1:])
 
+	case "judge-directories":
+		// Which of a node's declared directories the walk that follows no link another account could make would
+		// refuse (novox/hq issue 358). Read-only on the node and here, so a verb may run it too.
+		return nodeJudgeDirectories(ctx, open, args[1:])
+
 	default:
-		return fmt.Errorf("node has no %q; it has add, list, show, public-domain, account, agent-account, hand-over "+
-			"and setuid-search", args[0])
+		return fmt.Errorf("node has no %q; it has add, list, show, public-domain, account, agent-account, hand-over, "+
+			"setuid-search and judge-directories", args[0])
 	}
 }
 
@@ -937,3 +942,57 @@ func printJSONTo(w io.Writer, v any) error {
 	fmt.Fprintln(w, string(body))
 	return nil
 }
+
+const judgeDirectoriesUsage = "node judge-directories <node> — read-only: which of the directories <node> is " +
+	"declared the node-engine's walk that follows no link another account could make would refuse, one line each " +
+	"(novox/hq issue 358). Nothing is made, chmodded or chowned"
+
+// nodeJudgeDirectories asks the node's engine to judge its declared directories, signed with the mesh's key as a
+// hand-over is. Read-only: the engine changes nothing and keeps nothing.
+func nodeJudgeDirectories(ctx context.Context, open *stores, args []string) error {
+	known := func(node string) error {
+		_, err := open.inventory.NodeByName(ctx, node)
+		return err
+	}
+	ask := func(node, by string) (link.HandOverAnswer, error) {
+		ident, err := open.Identity(ctx)
+		if err != nil {
+			return link.HandOverAnswer{}, fmt.Errorf("the mesh's signing key cannot be read, so nothing was asked of %s: %w",
+				node, err)
+		}
+		address, err := broker.BusAddress()
+		if err != nil {
+			return link.HandOverAnswer{}, err
+		}
+		js, err := broker.Dial(address)
+		if err != nil {
+			return link.HandOverAnswer{}, fmt.Errorf("cannot reach the bus, so nothing was asked of %s: %w", node, err)
+		}
+		defer js.Close()
+		return link.AskJudgeDirectories(ctx, js.Conn(), ident, node, by, link.HandOverWithin)
+	}
+	return judgeDirectoriesAsked(args, known, ask, os.Stdout)
+}
+
+// judgeDirectoriesAsked is the line with its two acts given: whether the mesh knows the node, and the ask. The
+// engine's refusal is this command's failure.
+func judgeDirectoriesAsked(args []string, known func(node string) error,
+	ask func(node, by string) (link.HandOverAnswer, error), out io.Writer) error {
+	if len(args) != 1 || args[0] == "" || strings.HasPrefix(args[0], "-") {
+		return errors.New(judgeDirectoriesUsage)
+	}
+	node := args[0]
+	if err := known(node); err != nil {
+		return fmt.Errorf("nothing was asked: %w", err)
+	}
+	answer, err := ask(node, handOverBy())
+	if err != nil {
+		return err
+	}
+	if answer.Refused != "" {
+		return fmt.Errorf("%s refused: %s", node, answer.Refused)
+	}
+	fmt.Fprintf(out, "%s (read-only: nothing on %s was changed)\n", node, node)
+	fmt.Fprintln(out, answer.Said)
+	return nil
+}