containers: Don't descend into mounts on destroy.

This tells the sad tale of @the-kenny who had bind-mounted his home
directory into a container. After doing `nixos-container destroy` he
discovered that his home directory went from "full of precious data" to
"no more data".

We want to avoid having similar sad tales in the future, so this now also
check this in the containers VM test.

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

nixos/modules/virtualisation/nixos-container.pl

@@ -201,15 +201,32 @@ sub runInContainer {
     die "cannot run ‘nsenter’: $!\n";
 }
 
+# Remove a directory while recursively unmounting all mounted filesystems within
+# that directory and unmounting/removing that directory afterwards as well.
+#
+# NOTE: If the specified path is a mountpoint, its contents will be removed,
+#       only mountpoints underneath that path will be unmounted properly.
+sub safeRemoveTree {
+    my ($path) = @_;
+    system("find", $path, "-mindepth", "1", "-xdev",
+           "(", "-type", "d", "-exec", "mountpoint", "-q", "{}", ";", ")",
+           "-exec", "umount", "-fR", "{}", "+");
+    system("rm", "--one-file-system", "-rf", $path);
+    if (-e $path) {
+        system("umount", "-fR", $path);
+        system("rm", "--one-file-system", "-rf", $path);
+    }
+}
+
 if ($action eq "destroy") {
     die "$0: cannot destroy declarative container (remove it from your configuration.nix instead)\n"
         unless POSIX::access($confFile, &POSIX::W_OK);
 
     stopContainer if isContainerRunning;
 
-    rmtree($profileDir) if -e $profileDir;
-    rmtree($gcRootsDir) if -e $gcRootsDir;
-    rmtree($root) if -e $root;
+    safeRemoveTree($profileDir) if -e $profileDir;
+    safeRemoveTree($gcRootsDir) if -e $gcRootsDir;
+    safeRemoveTree($root) if -e $root;
     unlink($confFile) or die;
 }