gnu: When building in a VM, share a temporary directory.

* gnu/build/vm.scm (load-in-linux-vm): Make a shared temporary directory
  available in the VM.
* gnu/system/vm.scm (%linux-vm-file-systems): Add a corresponding entry.

gnu/build/vm.scm

@@ -126,6 +126,7 @@ the #:references-graphs parameter of 'derivation'."
              (number->string disk-image-size)))
 
   (mkdir "xchg")
+  (mkdir "tmp")
 
   (match references-graphs
     ((graph-files ...)
@@ -146,6 +147,12 @@ the #:references-graphs parameter of 'derivation'."
          "-virtfs"
          (string-append "local,id=xchg_dev,path=xchg"
                         ",security_model=none,mount_tag=xchg")
+         "-virtfs"
+         ;; Some programs require more space in /tmp than is normally
+         ;; available in the guest.  Accommodate such programs by sharing a
+         ;; temporary directory.
+         (string-append "local,id=tmp_dev,path=tmp"
+                        ",security_model=none,mount_tag=tmp")
          "-kernel" linux
          "-initrd" initrd
          (append

gnu/system/vm.scm

@@ -4,6 +4,7 @@
 ;;; Copyright © 2016, 2017 Leo Famulari <leo@famulari.name>
 ;;; Copyright © 2017 Mathieu Othacehe <m.othacehe@gmail.com>
 ;;; Copyright © 2017 Marius Bakke <mbakke@fastmail.com>
+;;; Copyright © 2018 Chris Marusich <cmmarusich@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -87,8 +88,8 @@
 ;;; Code:
 
 (define %linux-vm-file-systems
-  ;; File systems mounted for 'derivation-in-linux-vm'.  The store and /xchg
-  ;; directory are shared with the host over 9p.
+  ;; File systems mounted for 'derivation-in-linux-vm'.  These are shared with
+  ;; the host over 9p.
   (list (file-system
           (mount-point (%store-prefix))
           (device "store")
@@ -102,6 +103,13 @@
           (type "9p")
           (needed-for-boot? #t)
           (options "trans=virtio")
+          (check? #f))
+        (file-system
+          (mount-point "/tmp")
+          (device "tmp")
+          (type "9p")
+          (needed-for-boot? #t)
+          (options "trans=virtio")
           (check? #f))))
 
 (define* (expression->derivation-in-linux-vm name exp