Change Lua to use GUIX_LUA_PATH and GUIX_LUA_CPATH to construct the default
LUA_PATH and LUA_CPATH, instead of using hard-coded paths that Guix doesn't
populate.

These paths don't use Lua's usual '?' path wildcard, and thus are compatible
with Guix's search-paths mechanism.

This patch uses functions defined in lua-5.x-search-path-helpers.patch.

--- a/src/loadlib.c
+++ b/src/loadlib.c
@@ -280,7 +280,9 @@
   if (path == NULL)  /* no versioned environment variable? */
     path = getenv(envname);  /* try unversioned name */
   if (path == NULL || noenv(L))  /* no environment variable? */
-    lua_pushexternalstring(L, dft, strlen(dft), NULL, NULL);  /* use default */
+    /* Copy our constructed default string onto the Lua stack.
+     * Otherwise the location might be reused for another string and case us problems! */
+    lua_pushstring(L, dft);  /* use default */
   else if ((dftmark = strstr(path, LUA_PATH_SEP LUA_PATH_SEP)) == NULL)
     lua_pushstring(L, path);  /* nothing to change */
   else {  /* path contains a ";;": insert default path in its place */
@@ -721,14 +729,25 @@
 }
 

+#include "./guixpaths.c"
+
+
 LUAMOD_API int luaopen_package (lua_State *L) {
   luaL_getsubtable(L, LUA_REGISTRYINDEX, CLIBS);  /* create CLIBS table */
   lua_pop(L, 1);  /* will not use it now */
   luaL_newlib(L, pk_funcs);  /* create 'package' table */
   createsearcherstable(L);
+
   /* set paths */
+  /* Calculate default LUA_PATH and LUA_CPATH values from their
+     corresponding GUIX_ environment variables */
+  const char* default_path = guix_path(L); // push default_path
+  const char* default_cpath = guix_cpath(L); // push default_cpath
+  lua_pushvalue(L, -3); // copy the old head of the stack back to the top
-  setpath(L, "path", LUA_PATH_VAR, LUA_PATH_DEFAULT);
-  setpath(L, "cpath", LUA_CPATH_VAR, LUA_CPATH_DEFAULT);
+  setpath(L, "path", LUA_PATH_VAR, default_path);
+  setpath(L, "cpath", LUA_CPATH_VAR, default_cpath);
+  lua_pop(L, 3); // pop our three working values back off the stack
+
   /* store config information */
   lua_pushliteral(L, LUA_DIRSEP "\n" LUA_PATH_SEP "\n" LUA_PATH_MARK "\n"
                      LUA_EXEC_DIR "\n" LUA_IGMARK "\n");