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+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2016 Ludovic Courtès <ludo@gnu.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (gnu tests)
+  #:use-module (guix gexp)
+  #:use-module (gnu system)
+  #:use-module (gnu services)
+  #:use-module (gnu services shepherd)
+  #:export (backdoor-service-type
+            marionette-operating-system))
+
+;;; Commentary:
+;;;
+;;; This module provides the infrastructure to run operating system tests.
+;;; The most important part of that is tools to instrument the OS under test,
+;;; essentially allowing to run in a virtual machine controlled by the host
+;;; system--hence the name "marionette".
+;;;
+;;; Code:
+
+(define (marionette-shepherd-service imported-modules)
+  "Return the Shepherd service for the marionette REPL"
+  (define device
+    "/dev/hvc0")
+
+  (list (shepherd-service
+         (provision '(marionette))
+         (requirement '(udev))                    ;so that DEVICE is available
+         (modules '((ice-9 match)
+                    (srfi srfi-9 gnu)
+                    (guix build syscalls)
+                    (rnrs bytevectors)))
+         (imported-modules `((guix build syscalls)
+                             ,@imported-modules))
+         (start
+          #~(lambda ()
+              (define (clear-echo termios)
+                (set-field termios (termios-local-flags)
+                           (logand (lognot (local-flags ECHO))
+                                   (termios-local-flags termios))))
+
+              (define (self-quoting? x)
+                (letrec-syntax ((one-of (syntax-rules ()
+                                          ((_) #f)
+                                          ((_ pred rest ...)
+                                           (or (pred x)
+                                               (one-of rest ...))))))
+                  (one-of symbol? string? pair? null? vector?
+                          bytevector? number? boolean?)))
+
+              (match (primitive-fork)
+                (0
+                 (dynamic-wind
+                   (const #t)
+                   (lambda ()
+                     (let* ((repl    (open-file #$device "r+0"))
+                            (termios (tcgetattr (fileno repl)))
+                            (console (open-file "/dev/console" "r+0")))
+                       ;; Don't echo input back.
+                       (tcsetattr (fileno repl) (tcsetattr-action TCSANOW)
+                                  (clear-echo termios))
+
+                       ;; Redirect output to the console.
+                       (close-fdes 1)
+                       (close-fdes 2)
+                       (dup2 (fileno console) 1)
+                       (dup2 (fileno console) 2)
+                       (close-port console)
+
+                       (display 'ready repl)
+                       (let loop ()
+                         (newline repl)
+
+                         (match (read repl)
+                           ((? eof-object?)
+                            (primitive-exit 0))
+                           (expr
+                            (catch #t
+                              (lambda ()
+                                (let ((result (primitive-eval expr)))
+                                  (write (if (self-quoting? result)
+                                             result
+                                             (object->string result))
+                                         repl)))
+                              (lambda (key . args)
+                                (print-exception (current-error-port)
+                                                 (stack-ref (make-stack #t) 1)
+                                                 key args)
+                                (write #f repl)))))
+                         (loop))))
+                   (lambda ()
+                     (primitive-exit 1))))
+                (pid
+                 pid))))
+         (stop #~(make-kill-destructor)))))
+
+(define marionette-service-type
+  ;; This is the type of the "marionette" service, allowing a guest system to
+  ;; be manipulated from the host.  This marionette REPL is essentially a
+  ;; universal marionette.
+  (service-type (name 'marionette-repl)
+                (extensions
+                 (list (service-extension shepherd-root-service-type
+                                          marionette-shepherd-service)))))
+
+(define* (marionette-operating-system os
+                                      #:key (imported-modules '()))
+  "Return a marionetteed variant of OS such that OS can be used as a marionette
+in a virtual machine--i.e., controlled from the host system."
+  (operating-system
+    (inherit os)
+    (services (cons (service marionette-service-type imported-modules)
+                    (operating-system-user-services os)))))
+
+;;; tests.scm ends here