vm: Distinguish between success and failure of the guest code.

Fixes <https://bugs.gnu.org/34276>.
Reported by Tobias Geerinckx-Rice <me@tobias.gr>.

* gnu/system/vm.scm (expression->derivation-in-linux-vm)[loader]:
Produce '/xchg/.exit-status' file upon success.
* gnu/build/vm.scm (load-in-linux-vm): Check for 'xchg/.exit-status'
once QEMU has completed and respond accordingly.

1 files changed, 10 insertions, 1 deletions

diff --git a/gnu/build/vm.scm b/gnu/build/vm.scm
index 9a9e5bd001..79eed48c1f 100644
--- a/gnu/build/vm.scm
+++ b/gnu/build/vm.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013, 2014, 2015, 2016, 2017, 2018, 2019 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2016 Christopher Allan Webber <cwebber@dustycloud.org>
 ;;; Copyright © 2016, 2017 Leo Famulari <leo@famulari.name>
 ;;; Copyright © 2017 Mathieu Othacehe <m.othacehe@gmail.com>
@@ -183,6 +183,15 @@ the #:references-graphs parameter of 'derivation'."
               '())
           arch-specific-flags))
 
+  (unless (file-exists? "xchg/.exit-status")
+    (error "VM did not produce an exit code"))
+
+  (match (call-with-input-file "xchg/.exit-status" read)
+    (0 #t)
+    (status (error "guest VM code exited with a non-zero status" status)))
+
+  (delete-file "xchg/.exit-status")
+
   ;; When MAKE-DISK-IMAGE? is true, the image is in OUTPUT already.
   (unless make-disk-image?
     (if single-file-output?