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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 3 insertions, 2 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 ||
@@ -2794,7 +2795,7 @@ 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);