* Add a test to check that when we cannot realise a closure

expression, we should invalidate it and go back to the derivation
  for which it is a successor.

tests/fall-back.sh

@@ -0,0 +1,14 @@
+storeExpr=$($TOP/src/nix-instantiate/nix-instantiate fall-back.nix)
+
+echo "store expr is $storeExpr"
+
+# Register a non-existant successor.
+suc=$NIX_STORE_DIR/deadbeafdeadbeafdeadbeafdeadbeaf-s.store
+$TOP/src/nix-store/nix-store --successor $storeExpr $suc
+
+outPath=$($TOP/src/nix-store/nix-store -qnfvvvvv "$storeExpr")
+
+echo "output path is $outPath"
+
+text=$(cat "$outPath"/hello)
+if test "$text" != "Hello World!"; then exit 1; fi