fix(window): disallow closing autocmd window in other tabpage

Problem: unlike win_close, win_close_othertab could be used to close the
autocommand window from a different tabpage. This causes aucmd_restbuf to close
the wrong window, potentially causing a crash.

Solution: disallow closing it. Also replace a deprecated use of exc_exec in the
test file.

Fixes #21409.

src/nvim/window.c

@@ -2997,6 +2997,10 @@ bool win_close_othertab(win_T *win, int free_buf, tabpage_T *tp, bool force)
       || (win->w_buffer != NULL && win->w_buffer->b_locked > 0)) {
     return false;  // window is already being closed
   }
+  if (is_aucmd_win(win)) {
+    emsg(_(e_autocmd_close));
+    return false;
+  }
 
   // Check if closing this window would leave only floating windows.
   if (tp->tp_firstwin == win && win->w_next && win->w_next->w_floating) {

test/functional/autocmd/autocmd_spec.lua

@@ -17,7 +17,6 @@ local pcall_err = t.pcall_err
 local fn = n.fn
 local expect = n.expect
 local command = n.command
-local exc_exec = n.exc_exec
 local exec_lua = n.exec_lua
 local retry = t.retry
 local source = n.source
@@ -151,7 +150,10 @@ describe('autocmd', function()
       })
 
       command('autocmd BufLeave * bwipeout yy')
-      eq('Vim(edit):E143: Autocommands unexpectedly deleted new buffer yy', exc_exec('edit yy'))
+      eq(
+        'Vim(edit):E143: Autocommands unexpectedly deleted new buffer yy',
+        pcall_err(command, 'edit yy')
+      )
 
       expect([[
         start of test file xx
@@ -429,6 +431,23 @@ describe('autocmd', function()
     )
   end)
 
+  it('cannot close `aucmd_win` in non-current tabpage', function()
+    exec([[
+      file Xa
+      tabnew Xb
+      call setline(1, 'foo')
+      tabfirst
+      autocmd BufWriteCmd Xb tablast | bwipe! Xa
+    ]])
+    eq(
+      'BufWriteCmd Autocommands for "Xb": Vim(bwipeout):E813: Cannot close autocmd window',
+      pcall_err(command, 'wall')
+    )
+    -- Sanity check: :bwipe failing to close all windows into Xa should keep it loaded.
+    -- (So there's no risk of it being left unloaded inside a window)
+    eq(1, eval('bufloaded("Xa")'))
+  end)
+
   describe('closing last non-floating window in tab from `aucmd_win`', function()
     before_each(function()
       command('edit Xa.txt')