sane-private-do: run a shell by default, and leave the mount in its original state on exit

pkgs/additional/sane-scripts/default.nix

@@ -106,7 +106,7 @@ let
     private-do = static-nix-shell.mkBash {
       pname = "sane-private-do";
       src = ./src;
-      pkgs = [ "sane-scripts.private-unlock" ];
+      pkgs = [ "util-linux" ];
     };
     private-init = static-nix-shell.mkBash {
       pname = "sane-private-init";

pkgs/additional/sane-scripts/src/sane-private-do

@@ -1,12 +1,36 @@
 #!/usr/bin/env nix-shell
-#!nix-shell -i bash -p sane-scripts.private-unlock
+#!nix-shell -i bash -p util-linux
 
-# unlock the ~/private store, run some command, and then re-lock the store
+# unlock the private store, run some command, and then re-lock the store
+# N.B.: `mount`ing requires elevated privileges (cap_sys_admin).
+#       for this reason, it's expected that this script is run from within a mount namespace, not directly.
+#       this also has the side effect that parent/sibling processes don't see our mount in their namespace.
+#       i.e. the real /mnt/persist/private path is never modified.
 
-set -x
+mntpoint=/mnt/persist/private
+if mountpoint -q "$mntpoint"; then
+  # don't mount/unmount if it already exists (re-entrancy)
+  mntpoint=
+fi
 
-external_cmd=$@
+external_cmd=("$@")
+if [ -z "$external_cmd" ]; then
+  if [ -n "$SHELL" ]; then
+    external_cmd=("$SHELL" "-i")
+  else
+    external_cmd=("/bin/sh" "-i")
+  fi
+fi
 
-sane-private-unlock
+if [ -n "$mntpoint" ]; then
-$external_cmd
+  mount "$mntpoint"
-exec sane-private-lock
+fi
+
+"${external_cmd[@]}"
+RC=$?
+
+if [ -n "$mntpoint" ]; then
+  umount "$mntpoint"
+fi
+
+exit "$RC"