installer: Run the "guix system init" command in a PTY.

Fixes: <https://issues.guix.gnu.org/55360>

* gnu/installer/utils.scm (run-external-command-with-handler/tty): New
procedure.
(run-external-command-with-line-hooks, run-command): Add a TTY? argument.
* gnu/installer/final.scm (install-system): Call run-command with TTY?
argument set to #true.

gnu/installer/final.scm

@@ -211,7 +211,7 @@ or #f.  Return #t on success and #f on failure."
 
              (setenv "PATH" "/run/current-system/profile/bin/")
 
-             (set! ret (run-command install-command)))
+             (set! ret (run-command install-command #:tty? #t)))
            (lambda ()
              ;; Restart guix-daemon so that it does no keep the MNT namespace
              ;; alive.

gnu/installer/utils.scm

@@ -20,6 +20,7 @@
 (define-module (gnu installer utils)
   #:use-module (gnu services herd)
   #:use-module (guix utils)
+  #:use-module ((guix build syscalls) #:select (openpty login-tty))
   #:use-module (guix build utils)
   #:use-module (guix i18n)
   #:use-module (srfi srfi-1)
@@ -45,6 +46,7 @@
             nearest-exact-integer
             read-percentage
             run-external-command-with-handler
+            run-external-command-with-handler/tty
             run-external-command-with-line-hooks
             run-command
             run-command-in-installer
@@ -124,10 +126,37 @@ the child process as returned by waitpid."
     (close-port input)
     (close-pipe dummy-pipe)))
 
-(define (run-external-command-with-line-hooks line-hooks command)
+(define (run-external-command-with-handler/tty handler command)
+  "Run command specified by the list COMMAND in a child operating in a
+pseudoterminal with output handler HANDLER.  HANDLER is a procedure taking an
+input port, to which the command will write its standard output and error.
+Returns the integer status value of the child process as returned by waitpid."
+  (define-values (controller inferior)
+    (openpty))
+
+  (match (primitive-fork)
+    (0
+     (catch #t
+       (lambda ()
+         (close-fdes controller)
+         (login-tty inferior)
+         (apply execlp (car command) command))
+       (lambda _
+         (primitive-exit 127))))
+    (pid
+     (close-fdes inferior)
+     (let* ((port (fdopen controller "r0"))
+            (result (false-if-exception
+                     (handler port))))
+       (close-port port)
+       (cdr (waitpid pid))))))
+
+(define* (run-external-command-with-line-hooks line-hooks command
+                                               #:key (tty? #false))
   "Run command specified by the list COMMAND in a child, processing each
-output line with the procedures in LINE-HOOKS.  Returns the integer status
-value of the child process as returned by waitpid."
+output line with the procedures in LINE-HOOKS.  If TTY is set to #true, the
+COMMAND will be run in a pseudoterminal.  Returns the integer status value of
+the child process as returned by waitpid."
   (define (handler input)
     (and
      (and=> (get-line input)
@@ -136,14 +165,17 @@ value of the child process as returned by waitpid."
                   #f
                   (begin (for-each (lambda (f) (f line))
                                    (append line-hooks
-                                       %default-installer-line-hooks))
+                                           %default-installer-line-hooks))
                          #t))))
      (handler input)))
-  (run-external-command-with-handler handler command))
+  (if tty?
+      (run-external-command-with-handler/tty handler command)
+      (run-external-command-with-handler handler command)))
 
-(define* (run-command command)
+(define* (run-command command #:key (tty? #f))
   "Run COMMAND, a list of strings.  Return true if COMMAND exited
-successfully, #f otherwise."
+successfully, #f otherwise.  If TTY is set to #true, the COMMAND will be run
+in a pseudoterminal."
   (define (pause)
     (format #t (G_ "Press Enter to continue.~%"))
     (send-to-clients '(pause))
@@ -154,8 +186,8 @@ successfully, #f otherwise."
 
   (installer-log-line "running command ~s" command)
   (define result (run-external-command-with-line-hooks
-                  (list %display-line-hook)
-                  command))
+                  (list %display-line-hook) command
+                  #:tty? tty?))
   (define exit-val (status:exit-val result))
   (define term-sig (status:term-sig result))
   (define stop-sig (status:stop-sig result))