bug#59693: 29.0.50; treesitter in base buffer doesn't respond to modifications in indirect buffer correctly

[Eli Zaretskii <eliz@gnu.org>]

> From: Yuan Fu <casouri@gmail.com>
> Date: Sat, 3 Dec 2022 23:20:59 -0800
> Cc: miha@kamnitnik.top,
>  59693@debbugs.gnu.org
> 
> >>> We don’t want indirect buffer and base buffers to share parsers, since they can have different narrowing, and semantically indirect buffers should share anything but the text with the base buffer.
> >> 
> >> Yes, the parsers should not be shared.
> >> 
> >>> How about this: we change current_buffer->parser_list from a plain list of parsers to a cons (PARSER-LIST . INDIRECT-PARSER-LIST), where PARSER-LIST is as before. But for base buffers, INDIRECT-PARSER-LIST includes all the parsers of its indirect buffers; and for indirect buffers, INDIRECT-PARSER-LIST is nil.
> >> 
> >> You can maybe have the indirect buffers in the list, not their parsers.
> >> That could make it easier to access other treesit-related information of the
> >> indirect buffers, if needed.
> > 
> > Good idea, it’s easier to know when to remove the reference with buffers, aka when buffer is killed.
> 
> I now have a patch that fixes this problem. WDYT? I added a new buffer field since it’s cleaner than turning ts_parser_list into a cons, hopefully that’s not frowned upon. 

Thanks.

If we are adding to the buffer object a field that holds the list of
indirect buffers, then:

  . the name of the field should not include "treesit" in it, and it
    shouldn't be conditioned on HAVE_TREE_SITTER
  . I wonder if a flat list is a good idea, i.e. scalable enough? also,
    treesit_reap_indirect_buffers conses a lot as result
  . I vaguely remember that adding built-in fields to the buffer object had
    some caveats, but I don't recall the details (did you bootstrap?)

Stefan, any comments on this?  Are there better ideas of how to track buffer
text changes in indirect buffers?