mapped-devices: Ensure 'cryptsetup open' gets a tty.

Fixes <https://issues.guix.gnu.org/54770>.
Regression introduced in 400c9ed3d7.

Previously, for an encrypted /home (say), "cryptsetup open" would be
invoked by shepherd, with /dev/null as its standard input.  It would
thus run in non-interactive mode and, instead of asking for a
passphrase, fail with:

  Nothing to read on input.

This change ensures it runs in interactive mode.

* gnu/build/file-systems.scm (system*/console, system*/tty): New
procedures.
* gnu/system/mapped-devices.scm (open-luks-device): Use 'system*/tty'
instead of 'system*'.

gnu/build/file-systems.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2014, 2015, 2016, 2017, 2018, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014-2018, 2020-2022 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2016, 2017 David Craven <david@craven.ch>
 ;;; Copyright © 2017 Mathieu Othacehe <m.othacehe@gmail.com>
 ;;; Copyright © 2019 Guillaume Le Vaillant <glv@posteo.net>
@@ -54,6 +54,7 @@ (define-module (gnu build file-systems)
 
             bind-mount
 
+            system*/tty
             mount-flags->bit-mask
             check-file-system
             mount-file-system
@@ -67,6 +68,33 @@ (define-module (gnu build file-systems)
 ;;;
 ;;; Code:
 
+(define (system*/console program . args)
+  "Run PROGRAM with ARGS in a tty on top of /dev/console.  The return value is
+as for 'system*'."
+  (match (primitive-fork)
+    (0
+     (dynamic-wind
+       (const #t)
+       (lambda ()
+         (login-tty (open-fdes "/dev/console" O_RDWR))
+         (apply execlp program program args))
+       (lambda ()
+         (primitive-_exit 127))))
+    (pid
+     (cdr (waitpid pid)))))
+
+(define (system*/tty program . args)
+  "Run PROGRAM with ARGS, creating a tty if its standard input isn't one.
+The return value is as for 'system*'.
+
+This is necessary for commands such as 'cryptsetup open' or 'fsck' that may
+need to interact with the user but might be invoked from shepherd, where
+standard input is /dev/null."
+  (apply (if (isatty? (current-input-port))
+             system*
+             system*/console)
+         program args))
+
 (define (bind-mount source target)
   "Bind-mount SOURCE at TARGET."
   (mount source target "" MS_BIND))

gnu/system/mapped-devices.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014-2022 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2016 Andreas Enge <andreas@enge.fr>
 ;;; Copyright © 2017, 2018 Mark H Weaver <mhw@netris.org>
 ;;;
@@ -202,7 +202,8 @@ (define (open-luks-device source targets)
            ;; XXX: 'use-modules' should be at the top level.
            (use-modules (rnrs bytevectors) ;bytevector?
                         ((gnu build file-systems)
-                         #:select (find-partition-by-luks-uuid))
+                         #:select (find-partition-by-luks-uuid
+                                   system*/tty))
                         ((guix build utils) #:select (mkdir-p)))
 
            ;; Create '/run/cryptsetup/' if it does not exist, as device locking
@@ -211,7 +212,11 @@ (define (open-luks-device source targets)
 
            ;; Use 'cryptsetup-static', not 'cryptsetup', to avoid pulling the
            ;; whole world inside the initrd (for when we're in an initrd).
-           (zero? (system* #$(file-append cryptsetup-static "/sbin/cryptsetup")
+           ;; 'cryptsetup open' requires standard input to be a tty to allow
+           ;; for interaction but shepherd sets standard input to /dev/null;
+           ;; thus, explicitly request a tty.
+           (zero? (system*/tty
+                   #$(file-append cryptsetup-static "/sbin/cryptsetup")
                    "open" "--type" "luks"
 
                    ;; Note: We cannot use the "UUID=source" syntax here