vm/windows: Avoid shadowing the Nix store.

This is done by putting the non-initrd /nix/store into a subdirectory,
which we then chroot to and pass along the SSH command.

Also, we now collect the exit code after the chroot command and power
off the VM thereafter, because the store is no longer shadowed and we
have still access to the busybox inside the initrd.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

pkgs/build-support/vm/windows/controller/default.nix

@@ -8,8 +8,7 @@
 
 let
   inherit (import <nixpkgs> {}) lib stdenv writeScript vmTools makeInitrd;
-  inherit (import <nixpkgs> {}) samba vde2 busybox openssh;
-  inherit (import <nixpkgs> {}) socat netcat coreutils gzip;
+  inherit (import <nixpkgs> {}) samba vde2 openssh socat netcat coreutils gzip;
 
   preInitScript = writeScript "preinit.sh" ''
     #!${vmTools.initrdUtils}/bin/ash -e
@@ -26,7 +25,9 @@ let
       insmod $i
     done
 
-    mkdir -p /tmp /dev
+    mkdir -p /dev /fs
+
+    mount -t tmpfs none /dev
     mknod /dev/null    c 1 3
     mknod /dev/zero    c 1 5
     mknod /dev/random  c 1 8
@@ -36,22 +37,31 @@ let
     ifconfig lo up
     ifconfig eth0 up 192.168.0.2
 
-    mkdir -p /xchg /nix/store /etc /var/run /var/log
+    mount -t tmpfs none /fs
+    mkdir -p /fs/nix/store /fs/xchg /fs/dev /fs/sys /fs/proc /fs/etc /fs/tmp
 
-    cat > /etc/passwd <<PASSWD
+    mount -o bind /dev /fs/dev
+    mount -t sysfs none /fs/sys
+    mount -t proc none /fs/proc
+
+    mount -t 9p \
+      -o trans=virtio,version=9p2000.L,msize=262144,cache=loose \
+      store /fs/nix/store
+
+    mount -t 9p \
+      -o trans=virtio,version=9p2000.L,msize=262144,cache=loose \
+      xchg /fs/xchg
+
+    cat > /fs/etc/passwd <<PASSWD
     root:x:0:0::/root:/bin/false
     nobody:x:65534:65534::/var/empty:/bin/false
     PASSWD
 
-    mount -t 9p \
-      -o trans=virtio,version=9p2000.L,msize=262144,cache=loose \
-      xchg /xchg
+    set +e
+    chroot /fs $command $out
+    echo $? > /fs/xchg/in-vm-exit
 
-    mount -t 9p \
-      -o trans=virtio,version=9p2000.L,msize=262144,cache=loose \
-      store /nix/store
-
-    exec "$command"
+    poweroff -f
   '';
 
   initrd = makeInitrd {
@@ -80,7 +90,8 @@ let
     # Loop forever, because this VM is going to be killed.
     ${loopForever}
   '' else ''
-    ${coreutils}/bin/mkdir -p /etc/samba /etc/samba/private /var/lib/samba
+    ${coreutils}/bin/mkdir -p /etc/samba /etc/samba/private \
+                              /var/lib/samba /var/log /var/run
     ${coreutils}/bin/cat > /etc/samba/smb.conf <<CONFIG
     [global]
     security = user
@@ -105,6 +116,7 @@ let
 
     ${samba}/sbin/nmbd -D
     ${samba}/sbin/smbd -D
+
     echo -n "Waiting for Windows VM to become available..."
     while ! ${netcat}/bin/netcat -z 192.168.0.1 22; do
       echo -n .
@@ -123,7 +135,6 @@ let
     ${coreutils}/bin/touch /xchg/suspend_now
     ${loopForever}
     ''}
-    ${busybox}/sbin/poweroff -f
   ''));
 
   kernelAppend = lib.concatStringsSep " " [