fix(cmdline): prevent cmdline_show events after exiting cmdline #32033

Problem:  If a (vim.ui_attach) cmdline_hide callback triggers a redraw,
          it may cause cmdline_show events for an already exited cmdline.
Solution: Avoid emitting cmdline_show event when ccline.cmdbuff is
          already NULL. Unset ccline.cmdbuff before emitting cmdline_hide.

src/nvim/ex_getln.c

@@ -951,6 +951,8 @@ theend:
   kv_destroy(ccline.last_colors.colors);
 
   char *p = ccline.cmdbuff;
+  // Prevent show events triggered by a (vim.ui_attach) hide callback.
+  ccline.cmdbuff = NULL;
 
   if (ui_has(kUICmdline)) {
     ui_call_cmdline_hide(ccline.level, s->gotesc);
@@ -965,8 +967,6 @@ theend:
 
   if (did_save_ccline) {
     restore_cmdline(&save_ccline);
-  } else {
-    ccline.cmdbuff = NULL;
   }
 
   return (uint8_t *)p;
@@ -3415,6 +3415,10 @@ static void draw_cmdline(int start, int len)
 
 static void ui_ext_cmdline_show(CmdlineInfo *line)
 {
+  if (line->cmdbuff == NULL) {
+    return;
+  }
+
   Arena arena = ARENA_EMPTY;
   Array content;
   if (cmdline_star) {

test/functional/lua/ui_event_spec.lua

@@ -168,18 +168,67 @@ describe('vim.ui_attach', function()
       vim.ui_attach(ns, { ext_messages = true }, function(ev)
         if ev == 'msg_show' then
           vim.schedule(function() vim.cmd.redraw() end)
-        else
+        elseif ev:find('cmdline') then
-          vim.cmd.redraw()
+          _G.cmdline = _G.cmdline + (ev == 'cmdline_show' and 1 or 0)
+          vim.api.nvim_buf_set_lines(0, 0, -1, false, { tostring(_G.cmdline) })
+          vim.cmd('redraw')
         end
-        _G.cmdline = _G.cmdline + (ev == 'cmdline_show' and 1 or 0)
       end
     )]])
+    screen:expect([[
+      ^                                        |
+      {1:~                                       }|*4
+    ]])
     feed(':')
-    n.assert_alive()
+    screen:expect({
-    eq(2, exec_lua('return _G.cmdline'))
+      grid = [[
-    n.assert_alive()
+        ^1                                       |
+        {1:~                                       }|*4
+      ]],
+      cmdline = { {
+        content = { { '' } },
+        firstc = ':',
+        pos = 0,
+      } },
+    })
     feed('version<CR><CR>v<Esc>')
-    n.assert_alive()
+    screen:expect({
+      grid = [[
+        ^2                                       |
+        {1:~                                       }|*4
+      ]],
+      cmdline = { { abort = false } },
+    })
+    feed([[:call confirm("Save changes?", "&Yes\n&No\n&Cancel")<CR>]])
+    screen:expect({
+      grid = [[
+        ^5                                       |
+        {1:~                                       }|*4
+      ]],
+      cmdline = {
+        {
+          content = { { '' } },
+          hl_id = 10,
+          pos = 0,
+          prompt = '[Y]es, (N)o, (C)ancel: ',
+        },
+      },
+      messages = {
+        {
+          content = { { '\nSave changes?\n', 6, 10 } },
+          history = false,
+          kind = 'confirm',
+        },
+      },
+    })
+    feed('n')
+    screen:expect({
+      grid = [[
+        ^5                                       |
+        {1:~                                       }|*4
+      ]],
+      cmdline = { { abort = false } },
+    })
   end)
 
   it("preserved 'incsearch/command' screen state after :redraw from ext_cmdline", function()