fix(bufwrite): 'nomodified' if buffer changed during :write #42458

Problem:
If a (naughty) autocmd (e.g. a `Progress` handler) changes the buffer
during a slow `:write`, buf_write() still unsets 'modified', even though
the buffer has unwritten changes. `:bdelete` would silently lose the
changes.

Solution:
Reset 'modified' only if `b:changedtick` did not change while writing.

src/nvim/bufwrite.c

@@ -1120,6 +1120,8 @@ int buf_write(buf_T *buf, char *fname, char *sfname, linenr_T start, linenr_T en
   int progress = -1;  // Progress-message percent for a slow write.
   off_T progress_nchars = 0;  // `nchars` at the last progress check.
   const uint64_t write_start = os_hrtime();
+  // Safety: any changes hereafter (eg by Progress autocmds) are not written, so keep 'modified'.
+  const varnumber_T write_tick = buf_get_changedtick(buf);
   msg_scroll = false;               // always overwrite the file message now
 
   char *buffer = verbose_try_malloc(WRITEBUFSIZE);
@@ -1759,11 +1761,12 @@ restore_backup:
                  : msg_with_id(IObuff, msg_id, "bufwrite", NULL, 0, true, true, false), 0);
   }
 
-  // When written everything correctly: reset 'modified'.  Unless not
-  // writing to the original file and '+' is not in 'cpoptions'.
+  // When written everything correctly: reset 'modified'.  Unless not writing to the original file
+  // and not 'cpoptions=+', or the buffer changed (by autocmds!?) while writing.
   if (reset_changed && whole && !append
       && !write_info.bw_conv_error
-      && (overwriting || vim_strchr(p_cpo, kCpoPlus) != NULL)) {
+      && (overwriting || vim_strchr(p_cpo, kCpoPlus) != NULL)
+      && buf_get_changedtick(buf) == write_tick) {
     unchanged(buf, true, false);
     const varnumber_T changedtick = buf_get_changedtick(buf);
     if (buf->b_last_changedtick + 1 == changedtick) {

test/functional/ui/messages_spec.lua

@@ -3957,10 +3957,15 @@ describe('progress-message', function()
       return exec_lua(function(f, cmd)
         local out = {}
         vim.api.nvim_create_autocmd('Progress', {
+          group = vim.api.nvim_create_augroup('test_slow_write', {}),
           callback = function(ev)
             table.insert(out, ev.data.status)
             -- :bwipeout disallowed (E937) while writing.
             out.wiped = out.wiped or pcall(vim.cmd.bwipe, { bang = true })
+            if #out == 1 then
+              -- Progress handler (naughty) edits the buffer DURING :write.
+              vim.api.nvim_buf_set_lines(0, -1, -1, true, { 'not written' })
+            end
           end,
         })
         vim.api.nvim_buf_set_lines(0, 0, -1, true, vim.fn['repeat']({ ('x'):rep(99) }, 2000))
@@ -3978,6 +3983,8 @@ describe('progress-message', function()
     end
 
     eq({ 'running', 'success', false, 'ok' }, slow_write('cat'))
+    -- Buffer changed while writing: still 'modified'.
+    eq(true, api.nvim_get_option_value('modified', {}))
     eq({ 'running', 'failed', false, 'E514' }, slow_write('head -c 100000')) -- Write error (EPIPE).
   end)