Write test for deleting objects referenced from below

Currently fails, as expected.

tests/hermetic.nix

@@ -37,7 +37,7 @@ let
     buildCommand = ''
       echo hi-input3
       read x < ${input2}
-      echo $x BAZ > $out
+      echo ${input2} $x BAZ > $out
     '';
   };
 
@@ -51,6 +51,6 @@ in
       ''
         read x < ${input1}
         read y < ${input3}
-        echo "$x $y" > $out
+        echo ${input1} ${input3} "$x $y" > $out
       '';
   }

tests/overlay-local-store/delete-inner.sh

@@ -0,0 +1,39 @@
+#!/usr/bin/env bash
+
+set -eu -o pipefail
+
+source common.sh
+
+# Avoid store dir being inside sandbox build-dir
+unset NIX_STORE_DIR
+unset NIX_STATE_DIR
+
+storeDirs
+
+initLowerStore
+
+mountOverlayfs
+
+export NIX_REMOTE="$storeB"
+stateB="$storeBRoot/nix/var/nix"
+hermetic=$(nix-build ../hermetic.nix --no-out-link --arg busybox "$busybox" --arg seed 2)
+input1=$(nix-build ../hermetic.nix --no-out-link --arg busybox "$busybox" --arg seed 2 -A passthru.input1 -j0)
+input2=$(nix-build ../hermetic.nix --no-out-link --arg busybox "$busybox" --arg seed 2 -A passthru.input2 -j0)
+input3=$(nix-build ../hermetic.nix --no-out-link --arg busybox "$busybox" --arg seed 2 -A passthru.input3 -j0)
+
+# Can't delete because referenced
+expectStderr 1 nix-store --delete $input1 | grepQuiet "Cannot delete path"
+expectStderr 1 nix-store --delete $input2 | grepQuiet "Cannot delete path"
+expectStderr 1 nix-store --delete $input3 | grepQuiet "Cannot delete path"
+
+# These same paths are referenced in the lower layer (by the seed 1
+# build done in `initLowerStore`).
+expectStderr 1 nix-store --store "$storeA" --delete $input2 | grepQuiet "Cannot delete path"
+expectStderr 1 nix-store --store "$storeA" --delete $input3 | grepQuiet "Cannot delete path"
+
+# Can delete
+nix-store --delete $hermetic
+
+# Now unreferenced in upper layer, can delete
+nix-store --delete $input3
+nix-store --delete $input2

tests/overlay-local-store/delete.sh

@@ -0,0 +1,5 @@
+source common.sh
+
+requireEnvironment
+setupConfig
+execUnshare ./delete-inner.sh

tests/overlay-local-store/local.mk

@@ -4,6 +4,7 @@ overlay-local-store-tests := \
   $(d)/build.sh \
   $(d)/bad-uris.sh \
   $(d)/add-lower.sh \
+  $(d)/delete.sh \
   $(d)/gc.sh \
   $(d)/verify.sh \
   $(d)/optimise.sh