vim-patch:9.1.0112: Remove undo information, when cleaning quickfix buffer

Problem:  When the quickfix buffer has been modified an autocommand
          may invalidate the undo stack (kawarimidoll)
Solution: When clearing the quickfix buffer, also wipe the undo stack

fixes: vim/vim#13905
closes: vim/vim#13928

f0d3d4a426

Co-authored-by: Christian Brabandt <cb@256bit.org>

src/nvim/quickfix.c

@@ -68,6 +68,7 @@
 #include "nvim/strings.h"
 #include "nvim/types_defs.h"
 #include "nvim/ui.h"
+#include "nvim/undo.h"
 #include "nvim/vim_defs.h"
 #include "nvim/window.h"
 
@@ -4142,6 +4143,12 @@ static void qf_fill_buffer(qf_list_T *qfl, buf_T *buf, qfline_T *old_last, int q
     }
 
     // delete all existing lines
+    //
+    // Note: we cannot store undo information, because
+    // qf buffer is usually not allowed to be modified.
+    //
+    // So we need to clean up undo information
+    // otherwise autocommands may invalidate the undo stack
     while ((curbuf->b_ml.ml_flags & ML_EMPTY) == 0) {
       // If deletion fails, this loop may run forever, so
       // signal error and return.
@@ -4150,6 +4157,10 @@ static void qf_fill_buffer(qf_list_T *qfl, buf_T *buf, qfline_T *old_last, int q
         return;
       }
     }
+
+    // Remove all undo information
+    u_blockfree(curbuf);
+    u_clearall(curbuf);
   }
 
   // Check if there is anything to display

test/functional/legacy/autocmd_spec.lua

@@ -0,0 +1,40 @@
+local helpers = require('test.functional.helpers')(after_each)
+local clear = helpers.clear
+local write_file = helpers.write_file
+local command = helpers.command
+local feed = helpers.feed
+local api = helpers.api
+local eq = helpers.eq
+
+before_each(clear)
+
+-- oldtest: Test_autocmd_invalidates_undo_on_textchanged()
+it('no E440 in quickfix window when autocommand invalidates undo', function()
+  write_file(
+    'XTest_autocmd_invalidates_undo_on_textchanged',
+    [[
+    set hidden
+    " create quickfix list (at least 2 lines to move line)
+    vimgrep /u/j %
+
+    " enter quickfix window
+    cwindow
+
+    " set modifiable
+    setlocal modifiable
+
+    " set autocmd to clear quickfix list
+
+    autocmd! TextChanged <buffer> call setqflist([])
+    " move line
+    move+1
+    ]]
+  )
+  finally(function()
+    os.remove('XTest_autocmd_invalidates_undo_on_textchanged')
+  end)
+  command('edit XTest_autocmd_invalidates_undo_on_textchanged')
+  command('so %')
+  feed('G')
+  eq('', api.nvim_get_vvar('errmsg'))
+end)

test/old/testdir/test_autocmd.vim

@@ -3840,4 +3840,32 @@ func Test_autocmd_shortmess()
   delfunc SetupVimTest_shm
 endfunc
 
+func Test_autocmd_invalidates_undo_on_textchanged()
+  CheckRunVimInTerminal
+  let script =<< trim END
+    set hidden
+    " create quickfix list (at least 2 lines to move line)
+    vimgrep /u/j %
+
+    " enter quickfix window
+    cwindow
+
+    " set modifiable
+    setlocal modifiable
+
+    " set autocmd to clear quickfix list
+
+    autocmd! TextChanged <buffer> call setqflist([])
+    " move line
+    move+1
+  END
+  call writefile(script, 'XTest_autocmd_invalidates_undo_on_textchanged', 'D')
+  let buf = RunVimInTerminal('XTest_autocmd_invalidates_undo_on_textchanged', {'rows': 20})
+  call term_sendkeys(buf, ":so %\<cr>")
+  call term_sendkeys(buf, "G")
+  call WaitForAssert({-> assert_match('^XTest_autocmd_invalidates_undo_on_textchanged\s*$', term_getline(buf, 20))}, 1000)
+
+  call StopVimInTerminal(buf)
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab