fix(ui): textlock still causes issues for UI callbacks #37513

Problem:  There are still ways to run into textlock errors with
          vim.ui_attach callbacks trying to display a UI event.
Solution: Disregard textlock again during vim.ui_attach() callbacks
          (also when scheduled). Partially revert 3277dc3b; avoiding
          to flush while textlock is set is still helpful.

src/nvim/lua/executor.c

@@ -382,10 +382,18 @@ static void nlua_schedule_event(void **argv)
   lua_State *const lstate = global_lstate;
   nlua_pushref(lstate, cb);
   nlua_unref_global(lstate, cb);
+
+  // Don't impose textlock restrictions upon UI event handlers.
+  int save_expr_map_lock = expr_map_lock;
+  int save_textlock = textlock;
+  expr_map_lock = ns_id > 0 ? 0 : expr_map_lock;
+  textlock = ns_id > 0 ? 0 : textlock;
   if (nlua_pcall(lstate, 0, 0)) {
     nlua_error(lstate, _("vim.schedule callback: %.*s"));
     ui_remove_cb(ns_id, true);
   }
+  expr_map_lock = save_expr_map_lock;
+  textlock = save_textlock;
 }
 
 /// Schedule Lua callback on main loop's event queue

src/nvim/ui.c

@@ -550,7 +550,7 @@ void ui_flush(void)
 
   win_ui_flush(false);
   // Avoid flushing callbacks expected to change text during textlock.
-  if (textlock == 0) {
+  if (textlock == 0 && expr_map_lock == 0) {
     cmdline_ui_flush();
     msg_ext_ui_flush();
   }
@@ -761,9 +761,14 @@ void ui_call_event(char *name, Array args)
     fast = !strequal(not_fast[i], args.items[0].data.string.data);
   }
 
+  // Don't impose textlock restrictions upon UI event handlers.
+  int save_expr_map_lock = expr_map_lock;
+  int save_textlock = textlock;
+  expr_map_lock = 0;
+  textlock = 0;
+
   bool handled = false;
   UIEventCallback *event_cb;
-
   map_foreach(&ui_event_cbs, ui_event_ns_id, event_cb, {
     Error err = ERROR_INIT;
     uint32_t ns_id = ui_event_ns_id;
@@ -778,6 +783,8 @@ void ui_call_event(char *name, Array args)
     }
     api_clear_error(&err);
   })
+  expr_map_lock = save_expr_map_lock;
+  textlock = save_textlock;
 
   if (!handled) {
     UI_CALL(true, event, ui, name, args);

test/functional/ui/messages2_spec.lua

@@ -429,8 +429,7 @@ describe('messages2', function()
     exec_lua(function()
       vim.schedule(function()
         print('foo')
-        vim.uv.sleep(100)
-        print('bar')
+        vim.fn.getchar()
       end)
     end)
     screen:expect([[
@@ -438,13 +437,6 @@ describe('messages2', function()
       {1:~                                                    }|*12
       foo                                                  |
     ]])
-    screen:expect([[
-      ^                                                     |
-      {1:~                                                    }|*10
-      {3:                                                     }|
-      foo                                                  |
-      bar                                                  |
-    ]])
   end)
 
   it('properly formatted carriage return messages', function()
@@ -477,4 +469,46 @@ describe('messages2', function()
       {15:bar}{9:baz}                                               |
     ]])
   end)
+
+  it('can show message during textlock', function()
+    exec_lua(function()
+      _G.omnifunc = function()
+        print('x!')
+        vim.cmd.sleep('100m')
+      end
+      vim.bo.omnifunc = 'v:lua.omnifunc'
+    end)
+    feed('i<C-X>')
+    screen:expect([[
+      ^                                                     |
+      {1:~                                                    }|*12
+      {5:-- ^X mode (^]^D^E^F^I^K^L^N^O^P^Rs^U^V^Y)}           |
+    ]])
+    feed('<C-O>')
+    screen:expect([[
+      ^                                                     |
+      {1:~                                                    }|*12
+      x!                                                   |
+    ]])
+    screen:expect([[
+      ^                                                     |
+      {1:~                                                    }|*12
+      {5:-- Omni completion (^O^N^P) }{9:Pattern not found}        |
+    ]])
+    exec_lua(function()
+      vim.keymap.set('n', '<F1>', function()
+        print('i hate locks so much!!!!')
+        vim.cmd.messages()
+      end, { expr = true })
+    end)
+    feed('<Esc><F1>')
+    screen:expect([[
+                                                           |
+      {1:~                                                    }|*8
+      {3:                                                     }|
+      x^!                                                   |
+      x!                                                   |
+      i hate locks so much!!!!                             |*2
+    ]])
+  end)
 end)