Re: [PATCH v2] read-cache.c: fix writing "link" index ext with null base oid

> Since commit 7db118303a (unpack_trees: fix breakage when o->src_index !=
> o->dst_index - 2018-04-23) and changes in merge code to use separate
> index_state for source and destination, when doing a merge with split
> index activated, we may run into this line in unpack_trees():
>
>     o->result.split_index = init_split_index(&o->result);
>
> This is by itself not wrong. But this split index information is not
> fully populated (and it's only so when move_cache_to_base_index() is
> called, aka force splitting the index, or loading index_state from a
> file). Both "base_oid" and "base" in this case remain null.
>
> So when writing the main index down, we link to this index with null
> oid (default value after init_split_index()), which also means "no split
> index" internally. This triggers an incorrect base index refresh:
>
>     warning: could not freshen shared index '.../sharedindex.0{40}'
>
> This patch makes sure we will not refresh null base_oid (because the
> file is never there). It also makes sure not to write "link" extension
> with null base_oid in the first place (no point having it at
> all). Read code already has protection against null base_oid.
>
> There is also another side fix in remove_split_index() that causes a
> crash when doing "git update-index --no-split-index" when base_oid in
> the index file is null. In this case we will not load
> istate->split_index->base but we dereference it anyway and are rewarded
> with a segfault. This should not happen anymore, but it's still wrong to
> dereference a potential NULL pointer, especially when we do check for
> NULL pointer in the next code.
>
> Reported-by: Luke Diamand <luke@diamand.org>
> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
> ---
>  v2 added a new test

>  read-cache.c           |  5 +++--
>  split-index.c          | 34 ++++++++++++++++++----------------
>  t/t1700-split-index.sh | 18 ++++++++++++++++++
>  3 files changed, 39 insertions(+), 18 deletions(-)
>
> diff --git a/read-cache.c b/read-cache.c
> index 8f644f68b4..d140b44f8f 100644
> --- a/read-cache.c
> +++ b/read-cache.c
> @@ -2520,7 +2520,8 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
>  		return err;
>
>  	/* Write extension data here */
> -	if (!strip_extensions && istate->split_index) {
> +	if (!strip_extensions && istate->split_index &&
> +	    !is_null_oid(&istate->split_index->base_oid)) {
>  		struct strbuf sb = STRBUF_INIT;
>
>  		err = write_link_extension(&sb, istate) < 0 ||