On Sat, Feb 9, 2019 at 3:23 AM Nguyễn Thái Ngọc Duy <pclouds@xxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxx>
> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@xxxxxxxxx>
Thanks for digging this down and fixing it. When I saw this split
index bug bisect to me that my heart sank a little; there's so much of
that code I don't understand. Nice to see you've already come along
and fixed it all up. :-)
> ---
> 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
>
