In LuaTeX, ensure that empty paragraphs are empty

base/changes.txt

@@ -6,6 +6,11 @@ completeness or accuracy and it contains some references to files that
 are not part of the distribution.
 ================================================================================
 
+2021-06-21  Marcel Krüger  <Marcel.Krueger@latex-project.org>
+
+	* ltpara.dtx:
+	Don't print effectivly empty paragraphs in RTL contexts in LuaTeX
+
 2021-06-09  Frank Mittelbach  <Frank.Mittelbach@latex-project.org>
 
 	* ltshipout.dtx:

base/ltpara.dtx

@@ -14,8 +14,8 @@
 %%% From File: ltpara.dtx
 %
 %    \begin{macrocode}
-\def\ltparaversion{v1.0g}
-\def\ltparadate{2021/05/27}
+\def\ltparaversion{v1.0h}
+\def\ltparadate{2021/06/21}
 %    \end{macrocode}
 %<*driver>
 \documentclass{l3doc}
@@ -714,6 +714,37 @@
 %
 %
 %
+%  \begin{macro}{\@@_end_empty_par:}
+%    For most engines, running \cs{tex_par:D} in an empty paragraph will drop
+%    the paragraph, but in Lua\TeX\ this sometimes leads to paragraphs which
+%    contain only direction nodes. Therefore we first make sure that the list
+%    is really empty when using Lua\TeX.
+%  \changes{v1.0h}{2021/06/21}{Ensure that empty paragraphs are really empty in Lua\TeX}
+%    \begin{macrocode}
+\sys_if_engine_luatex:TF
+  {
+    \newluafunction \g_@@_end_empty_par_id_int
+    \exp_args:Nx \everyjob {
+      \exp_not:V \everyjob
+      \exp_not:N \lua_now:n {
+        local~texnest, flush_list, par_token = tex.nest, node.flush_list, token.create'tex_par:D'~
+        lua.get_functions_table()[\int_use:N \g_@@_end_empty_par_id_int] = function()
+          local~nest_level = texnest.top~
+          local~cur_head = nest_level.head~
+          flush_list(cur_head.next)
+          nest_level.tail, cur_head.next = cur_head, nil~
+          token.put_next(par_token)
+        end
+      }
+    }
+    \protected \luadef \@@_end_empty_par: \g_@@_end_empty_par_id_int
+  } {
+    \cs_set_eq:NN \@@_end_empty_par: \tex_par:D
+  }
+%
+%
+%    \end{macrocode}
+%  \end{macro}
 %  \begin{macro}{\g_@@_standard_everypar_tl}
 %    Whenever \TeX{} starts a paragraph it inserts first an
 %    indentation box and then executes the tokens stored in
@@ -746,7 +777,7 @@
 %    settings will not get lost as we need them for later.
 %    \begin{macrocode}
   \group_begin:
-    \tex_par:D
+    \@@_end_empty_par:
   \group_end:
 %    \end{macrocode}
 %    We then change \cs{tex_everypar:D} to generate an error so that