fix(api): disallow some more functions during textlock

Problem: nvim_buf_set_text(), nvim_open_term() and termopen() all change buffer
text, which is forbidden during textlock. Additionally, nvim_open_term() and
termopen() may be used to convert the cmdwin buffer into a terminal buffer,
which is weird.

Solution: Allow nvim_buf_set_text() and nvim_open_term() in the cmdwin, but
disallow nvim_open_term() from converting the cmdwin buffer into a terminal
buffer. termopen() is not allowed in the cmdwin (as it always operates on
curbuf), so just check text_locked().

Also happens to improve the error in #21055: nvim_buf_set_text() was callable
during textlock, but happened to check textlock indirectly via u_save();
however, this caused the error to be overwritten by an unhelpful "Failed to
save undo information" message when msg_list == NULL (e.g: an `<expr>` mapping
invoked outside of do_cmdline()).

1 files changed, 4 insertions, 1 deletions

diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index f348a61075..7c3c5cc274 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -8385,7 +8385,10 @@ static void f_termopen(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
   if (check_secure()) {
     return;
   }
-
+  if (text_locked()) {
+    text_locked_msg();
+    return;
+  }
   if (curbuf->b_changed) {
     emsg(_("Can only call this function in an unmodified buffer"));
     return;