Re: [PATCH] unpack-trees.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>
> ---
>  I considered adding a test, but since the problem is a warning, not
>  sure how to catch that. And a test would not be able to verify all
>  changes in this patch anyway.
>
>  read-cache.c  |  5 +++--
>  split-index.c | 34 ++++++++++++++++++----------------
>  2 files changed, 21 insertions(+), 18 deletions(-)
>
> diff --git a/read-cache.c b/read-cache.c
> index 0e0c93edc9..d6fb09984f 100644
> --- a/read-cache.c
> +++ b/read-cache.c
> @@ -2894,7 +2894,8 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
>                         return -1;
>         }
>
> -       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 ||
> @@ -3189,7 +3190,7 @@ int write_locked_index(struct index_state *istate, struct lock_file *lock,
>         ret = write_split_index(istate, lock, flags);
>
>         /* Freshen the shared index only if the split-index was written */
> -       if (!ret && !new_shared_index) {
> +       if (!ret && !new_shared_index && !is_null_oid(&si->base_oid)) {
>                 const char *shared_index = git_path("sharedindex.%s",
>                                                     oid_to_hex(&si->base_oid));
>                 freshen_shared_index(shared_index, 1);
> diff --git a/split-index.c b/split-index.c
> index 5820412dc5..a9d13611a4 100644
> --- a/split-index.c
> +++ b/split-index.c
> @@ -440,24 +440,26 @@ void add_split_index(struct index_state *istate)
>  void remove_split_index(struct index_state *istate)
>  {
>         if (istate->split_index) {
> -               /*
> -                * When removing the split index, we need to move
> -                * ownership of the mem_pool associated with the
> -                * base index to the main index. There may be cache entries
> -                * allocated from the base's memory pool that are shared with
> -                * the_index.cache[].
> -                */
> -               mem_pool_combine(istate->ce_mem_pool, istate->split_index->base->ce_mem_pool);
> +               if (istate->split_index->base) {
> +                       /*
> +                        * When removing the split index, we need to move
> +                        * ownership of the mem_pool associated with the
> +                        * base index to the main index. There may be cache entries
> +                        * allocated from the base's memory pool that are shared with
> +                        * the_index.cache[].
> +                        */
> +                       mem_pool_combine(istate->ce_mem_pool,
> +                                        istate->split_index->base->ce_mem_pool);
>
> -               /*
> -                * The split index no longer owns the mem_pool backing
> -                * its cache array. As we are discarding this index,
> -                * mark the index as having no cache entries, so it
> -                * will not attempt to clean up the cache entries or
> -                * validate them.
> -                */
> -               if (istate->split_index->base)
> +                       /*
> +                        * The split index no longer owns the mem_pool backing
> +                        * its cache array. As we are discarding this index,
> +                        * mark the index as having no cache entries, so it
> +                        * will not attempt to clean up the cache entries or
> +                        * validate them.
> +                        */
>                         istate->split_index->base->cache_nr = 0;
> +               }
>
>                 /*
>                  * We can discard the split index because its
> --
> 2.20.1.682.gd5861c6d90