bug#56682: locked narrowing

[Stefan Monnier]

>> >> Because I can't find that code.
>> > What do you mean? it's in maybe_produce_line_number, obviously...
>> My intuition tells me it should be there, but I can't find it.
>
> Really?  I meant this:
>
>       if (!last_line)
>       {
>         /* If possible, reuse data cached by line-number-mode.  */
>         if (it->w->base_line_number > 0
>             && it->w->base_line_pos > 0
>             && it->w->base_line_pos <= IT_CHARPOS (*it)
>             /* line-number-mode always displays narrowed line
>                numbers, so we cannot use its data if the user wants
>                line numbers that disregard narrowing, or if the
>                buffer's narrowing has just changed.  */
>             && !(line_numbers_wide
>                  && (BEG_BYTE != BEGV_BYTE || Z_BYTE != ZV_BYTE))
>             && !current_buffer->clip_changed)
>           {
>             start_from = CHAR_TO_BYTE (it->w->base_line_pos);
>             last_line = it->w->base_line_number - 1;
>           }
>         else
>           start_from = beg_byte;
>         if (!it->lnum_bytepos)
>           first_time = true;
>       }
>       else
>       start_from = it->lnum_bytepos;
>
>       /* Paranoia: what if someone changes the narrowing since the
>        last time display_line was called?  Shouldn't really happen,
>        but who knows what some crazy Lisp invoked by :eval could do?  */
>       if (!(beg_byte <= start_from && start_from <= z_byte))
>       {
>         last_line = 0;
>         start_from = beg_byte;
>       }
>
>       this_line =
>       last_line + display_count_lines_logically (start_from,
>                                                  IT_BYTEPOS (*it),
>                                                  IT_CHARPOS (*it), &bytepos);

This checks for changes in narrowing (via !current_buffer->clip_changed),
but not for changes in the buffer contents (these are apparently
checked in `redisplay_window` and `try_scrolling` instead).

>> >> Apparently `maybe_produce_line_number`
>> >> just uses `w->base_line_number` whenever it's not 0, so apparently it's
>> >> set to 0 elsewhere.
>> > It's set to zero in several places in xdisp.c.
>> I can't seem to find even one of them related to the fact that the
>> buffer was modified.
>
> This:
>
>         if (!just_this_one_p
>             || current_buffer->clip_changed
>             || BEG_UNCHANGED < CHARPOS (startp))
>           /* Forget any recorded base line for line number display.  */
>           w->base_line_number = 0;
>
> And this:
>
>   /* Forget any previously recorded base line for line number display.  */
>   if (!buffer_unchanged_p)
>     w->base_line_number = 0;

Right, I did find them in the end.
Note that these are in code which is not used for `format-mode-line`.

>> > But again, if you don't want to trust the cached values, just use
>> > display_count_lines starting from BOB.
>>
>> That's already what nlinum does (via `line-number-at-pos`).
>
> Then maybe all this discussion is just a waste of breath.

It'll help me improve the code's doc, tho.


        Stefan