Fix nix-direnv-reload

No longer assumes location of layout directory relative to code.

direnvrc

@@ -37,16 +37,34 @@ _nix_direnv_preflight () {
   if [[ ! -d "$layout_dir/bin" ]]; then
     mkdir -p "$layout_dir/bin"
   fi
+  # N.B. This script relies on variable expansion in *this* shell.
+  # (i.e. The written out file will have the variables expanded)
+  # If the source path changes, the script becomes broken.
+  # Because direnv_layout_dir is user controlled,
+  # we can't assume to be able to reverse it to get the source dir
+  # So there's little to be done about this.
+  cat > "${layout_dir}/bin/nix-direnv-reload" <<-EOF
+#!/usr/bin/env bash
+set -e
+if [[ ! -d "$PWD" ]]; then
+  echo "Cannot find source directory; Did you move it?"
+  echo "(Looking for "$PWD")"
+  echo 'Cannot force reload with this script - use "direnv reload" manually and then try again'
+  exit 1
+fi
+
+# rebuild the cache forcefully
+_nix_direnv_force_reload=1 direnv exec "$PWD" true
+
+# Update the mtime for .envrc.
+# This will cause direnv to reload again - but without re-building.
+touch "$PWD/.envrc"
+
+# Also update the timestamp of whatever profile_rc we have.
+# This makes sure that we know we are up to date.
+touch -r "$PWD/.envrc" ${layout_dir}/*.rc
+EOF
 
-  # shellcheck disable=SC2016
-  echo > "${layout_dir}/bin/nix-direnv-reload" '#!/usr/bin/env bash
-dir="$(realpath $(dirname ${BASH_SOURCE[0]})/../..)"
-_nix_direnv_force_reload=1 direnv exec "$dir" true
-direnv reload
-# direnv reload updates the mtime of .envrc. Also update the timestamp of the
-# profile_rc file to keep track that we actually are up to date.
-touch $dir/.direnv/{nix,flake}-profile-*.rc
-'
   if [[ ! -x "${layout_dir}/bin/nix-direnv-reload" ]]; then
     chmod +x "${layout_dir}/bin/nix-direnv-reload"
   fi