Adopt GC test for local-overlay store

Doesn't yet pass. Fixes are needed.

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

@@ -0,0 +1,57 @@
+#!/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"
+outPath=$(nix-build ../hermetic.nix --no-out-link --arg busybox "$busybox" --arg seed 2)
+
+# Set a GC root.
+mkdir -p "$stateB"
+rm -f "$stateB"/gcroots/foo
+ln -sf $outPath "$stateB"/gcroots/foo
+
+[ "$(nix-store -q --roots $outPath)" = "$stateB/gcroots/foo -> $outPath" ]
+
+nix-store --gc --print-roots | grep $outPath
+nix-store --gc --print-live | grep $outPath
+if nix-store --gc --print-dead | grep -E $outPath$; then false; fi
+
+nix-store --gc --print-dead
+
+expect 1 nix-store --delete $outPath
+test -e "$storeBRoot/$outPath"
+
+shopt -s nullglob
+for i in $storeBRoot/*; do
+    if [[ $i =~ /trash ]]; then continue; fi # compat with old daemon
+    touch $i.lock
+    touch $i.chroot
+done
+
+nix-collect-garbage
+
+# Check that the root and its dependencies haven't been deleted.
+cat "$storeBRoot/$outPath"
+
+rm "$stateB"/gcroots/foo
+
+nix-collect-garbage
+
+# Check that the output has been GC'd.
+test ! -e $outPath
+
+# Check that the store is empty.
+[ "$(ls -1 "$storeBTop" | wc -l)" = "0" ]

tests/overlay-local-store/gc.sh

@@ -0,0 +1,5 @@
+source common.sh
+
+requireEnvironment
+setupConfig
+execUnshare ./gc-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)/gc.sh \
   $(d)/verify.sh \
   $(d)/optimise.sh