4 files changed, 12 insertions, 7 deletions
diff --git a/src/nvim/api/buffer.c b/src/nvim/api/buffer.c
index 41d7d8ba6b..690497159a 100644
--- a/src/nvim/api/buffer.c
+++ b/src/nvim/api/buffer.c
@@ -453,6 +453,11 @@ void nvim_buf_set_lines(uint64_t channel_id,
   aco_save_T aco;
   aucmd_prepbuf(&aco, (buf_T *)buf);
 
+  if (!MODIFIABLE(buf)) {
+    api_set_error(err, kErrorTypeException, "Buffer is not 'modifiable'");
+    goto end;
+  }
+
   if (u_save((linenr_T)(start - 1), (linenr_T)end) == FAIL) {
     api_set_error(err, kErrorTypeException, "Failed to save undo information");
     goto end;
diff --git a/src/nvim/option.c b/src/nvim/option.c
index 04349414a2..d1753526a7 100644
--- a/src/nvim/option.c
+++ b/src/nvim/option.c
@@ -6985,6 +6985,7 @@ void save_file_ff(buf_T *buf)
 /// When "ignore_empty" is true don't consider a new, empty buffer to be
 /// changed.
 bool file_ff_differs(buf_T *buf, bool ignore_empty)
+  FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT
 {
   // In a buffer that was never loaded the options are not valid.
   if (buf->b_flags & BF_NEVERLOADED) {
diff --git a/src/nvim/testdir/test_autocmd.vim b/src/nvim/testdir/test_autocmd.vim
index 16bacf4591..5848940a2b 100644
--- a/src/nvim/testdir/test_autocmd.vim
+++ b/src/nvim/testdir/test_autocmd.vim
@@ -1343,6 +1343,8 @@ func Test_Changed_FirstTime()
   call writefile([''], 'Xchanged.txt')
   let buf = term_start([GetVimProg(), '--clean', '-c', 'set noswapfile'], {'term_rows': 3})
   call assert_equal('running', term_getstatus(buf))
+  " Wait for the ruler (in the status line) to be shown.
+  call WaitFor({-> term_getline(buf, 3) =~# '\<All$'})
   " It's only adding autocmd, so that no event occurs.
   call term_sendkeys(buf, ":au! TextChanged <buffer> call writefile(['No'], 'Xchanged.txt')\<cr>")
   call term_sendkeys(buf, "\<C-\\>\<C-N>:qa!\<cr>")
diff --git a/src/nvim/undo.c b/src/nvim/undo.c
index 1a31f6a6c7..c8343941d2 100644
--- a/src/nvim/undo.c
+++ b/src/nvim/undo.c
@@ -2962,7 +2962,7 @@ static char_u *u_save_line(linenr_T lnum)
 ///
 /// @return true if the buffer has changed
 bool bufIsChanged(buf_T *buf)
-  FUNC_ATTR_WARN_UNUSED_RESULT
+  FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT
 {
   return !bt_dontwrite(buf) && (buf->b_changed || file_ff_differs(buf, true));
 }
@@ -2979,15 +2979,12 @@ bool anyBufIsChanged(void)
   return false;
 }
 
-/// Check if the 'modified' flag is set, or 'ff' has changed (only need to
-/// check the first character, because it can only be "dos", "unix" or "mac").
-/// "nofile" and "scratch" type buffers are considered to always be unchanged.
-///
+/// @see bufIsChanged
 /// @return true if the current buffer has changed
 bool curbufIsChanged(void)
+  FUNC_ATTR_WARN_UNUSED_RESULT
 {
-  return (!bt_dontwrite(curbuf)
-          && (curbuf->b_changed || file_ff_differs(curbuf, true)));
+  return bufIsChanged(curbuf);
 }
 
 /// Append the list of undo blocks to a newly allocated list