fix: runtimepath always updates lua package.path

src/nvim/lua/executor.c

@@ -543,6 +543,14 @@ static lua_State *nlua_enter(void)
   return lstate;
 }
 
+/// Force an update of lua's package paths if runtime path has changed.
+bool nlua_update_package_path(void)
+{
+  lua_State *const lstate = nlua_enter();
+
+  return !!lstate;
+}
+
 static void nlua_print_event(void **argv)
 {
   char *str = argv[0];

src/nvim/option.c

@@ -80,6 +80,7 @@
 #ifdef WIN32
 # include "nvim/os/pty_conpty_win.h"
 #endif
+#include "nvim/lua/executor.h"
 #include "nvim/api/private/helpers.h"
 #include "nvim/os/input.h"
 #include "nvim/os/lang.h"
@@ -3344,6 +3345,10 @@ ambw_end:
     if (!parse_winhl_opt(curwin)) {
       errmsg = e_invarg;
     }
+  } else if (varp == &p_rtp) {  // 'runtimepath'
+    if (!nlua_update_package_path()) {
+      errmsg = (char_u *)N_("E970: Failed to initialize lua interpreter");
+    }
   } else {
     // Options that are a list of flags.
     p = NULL;