Let mounts write each share's mount and automount, so no node depends on a hand-written fstab line (hq ADR 0263)

A machine mounts what its assignment names: a node's share from the
provision nfs-share, whose path it reads from nfs-server's state on the
bus, and occasional sources (an SMB device, a disk) whose absence is a
state, not a fault. apply writes and arms the unit pairs as root; watch
tries each source and judges the health; the operator's fstab lines are
taken over only through adopt, on their word, once the replacement is
armed. SMB logins are an outside party's secret, given with secret accept.

modules/mounts/cmd/mounts/machine.go

@@ -42,11 +42,15 @@ type Machine struct {
 	Root     string
 	Run      Runner
 	LookPath func(name string) bool
+	// Escalate runs a command as root: as given where this process is root, else through `sudo -n`, as the
+	// bundle runs as the operator account (novox/hq ADR 0175 §4) and the sudo module lets it.
+	Escalate Runner
 }
 
 // Here is the machine this process runs on.
 func Here() *Machine {
-	return &Machine{Root: "/", Run: ExecRunner, LookPath: func(n string) bool { _, err := exec.LookPath(n); return err == nil }}
+	return &Machine{Root: "/", Run: ExecRunner, Escalate: EscalatedRunner,
+		LookPath: func(n string) bool { _, err := exec.LookPath(n); return err == nil }}
 }
 
 // read answers one file's text under the root. Only regular files the module names are read; a
@@ -81,3 +85,34 @@ func unreadable(path string, err error) string {
 		return fmt.Sprintf("%s: not read (%v)", path, err)
 	}
 }
+
+// EscalatedRunner runs a command as root, through sudo without a prompt where this process is not root.
+// A refusal by sudo is said as one, never read as an empty answer.
+func EscalatedRunner(ctx context.Context, name string, args ...string) (string, error) {
+	ctx, cancel := context.WithTimeout(ctx, 25*time.Second)
+	defer cancel()
+	program, argv := name, args
+	if os.Getuid() != 0 {
+		program, argv = "sudo", append([]string{"-n", name}, args...)
+	}
+	cmd := exec.CommandContext(ctx, program, argv...)
+	var stdout, stderr bytes.Buffer
+	cmd.Stdout, cmd.Stderr = &stdout, &stderr
+	if err := cmd.Run(); err != nil {
+		said := strings.TrimSpace(stderr.String())
+		if program == "sudo" && strings.HasPrefix(said, "sudo:") {
+			return stdout.String(), fmt.Errorf("%s needs root, and the tool runner's account may not run it without a prompt: %s", name, said)
+		}
+		if said != "" {
+			return stdout.String(), fmt.Errorf("%s: %w: %s", name, err, said)
+		}
+		return stdout.String(), fmt.Errorf("%s: %w", name, err)
+	}
+	return stdout.String(), nil
+}
+
+// present says whether a device or a file is there. Never used on a mount point.
+func (m *Machine) present(p string) bool {
+	_, err := os.Stat(filepath.Join(m.Root, p))
+	return err == nil
+}