Re: [PATCH v4 02/23] read-cache.c: remove 'const' from index_has_changes()

[Duy Nguyen <pclouds@gmail.com>]

On Mon, Sep 17, 2018 at 6:25 PM Junio C Hamano <gitster@pobox.com> wrote:
>
> Nguyễn Thái Ngọc Duy  <pclouds@gmail.com> writes:
>
> > This function calls do_diff_cache() which eventually needs to set this
> > "istate" to unpack_options->src_index (*). This is an unfortunate fact
> > that unpack_trees() _will_ destroy src_index so we can't really pass a
>
> Wasn't the whole point of introducing src_index and dst_index to
> unpack-trees API so that we can keep the src_index read-only by
> writing the result of merge to a separate in-core dst_index?
>
> What does the above exactly mean by "will destroy src_index"?  Is it
> now fundamental that src_index needs to lack constness, or is it
> something easy to fix?

"destroy" is probably a strong word, but we do modify the src_index, e.g.

 mark_all_ce_unused(o->src_index);
 mark_new_skip_worktree(.., o->src_index...
 move_index_extensions(&o->result, o->src_index);
 invalidate_ce_path();

all these update the source index. It is possible to fix, but I don't
think it's exactly easy and may even incur some performance cost (e.g.
if we stop modify ce_flags in the src_index, then we need to do one
extra lookup to wherever we store these flags).
-- 
Duy