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;

test/functional/ex_cmds/packadd_spec.lua

@@ -0,0 +1,74 @@
+local helpers = require('test.functional.helpers')(after_each)
+
+local clear = helpers.clear
+local eq = helpers.eq
+local exec_lua = helpers.exec_lua
+
+describe('packadd', function()
+  before_each(function()
+    -- Primarily taken from test/functional/legacy/packadd_spec.lua
+    clear()
+    exec_lua [[
+      TopDirectory = vim.fn.expand(vim.fn.getcwd() .. '/Xdir_lua')
+      PlugDirectory = TopDirectory .. '/pack/mine/opt/mytest'
+
+      vim.o.packpath = TopDirectory
+
+      function FindPathsContainingDir(dir)
+        return vim.fn.filter(
+          vim.split(package.path, ';'),
+          function(k, v)
+            return string.find(v, 'mytest') ~= nil
+          end
+        )
+      end
+    ]]
+  end)
+
+  after_each(function()
+    exec_lua [[
+      vim.fn.delete(TopDirectory, 'rf')
+    ]]
+  end)
+
+  it('should immediately update package.path in lua', function()
+    local count_of_paths = exec_lua [[
+      vim.fn.mkdir(PlugDirectory .. '/lua/', 'p')
+
+      local num_paths_before = #FindPathsContainingDir('mytest')
+
+      vim.cmd("packadd mytest")
+
+      local num_paths_after = #FindPathsContainingDir('mytest')
+
+      return { num_paths_before, num_paths_after }
+    ]]
+
+    eq({0, 2}, count_of_paths)
+  end)
+
+  it('should immediately update package.path in lua even if lua directory does not exist', function()
+    local count_of_paths = exec_lua [[
+      vim.fn.mkdir(PlugDirectory .. '/plugin/', 'p')
+
+      local num_paths_before = #FindPathsContainingDir('mytest')
+
+      vim.cmd("packadd mytest")
+
+      local num_paths_after = #FindPathsContainingDir('mytest')
+
+      return { num_paths_before, num_paths_after }
+    ]]
+
+    eq({0, 2}, count_of_paths)
+  end)
+
+  it('should error for invalid paths', function()
+    local count_of_paths = exec_lua [[
+      local ok, err = pcall(vim.cmd, "packadd asdf")
+      return ok
+    ]]
+
+    eq(false, count_of_paths)
+  end)
+end)