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>

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);

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

t/t1700-split-index.sh

@@ -447,4 +447,22 @@ test_expect_success 'writing split index with null sha1 does not write cache tre
 	test_line_count = 0 cache-tree.out
 '
 
+test_expect_success 'do not refresh null base index' '
+	test_create_repo merge &&
+	(
+		cd merge &&
+		test_commit initial &&
+		git checkout -b side-branch &&
+		test_commit extra &&
+		git checkout master &&
+		git update-index --split-index &&
+		test_commit more &&
+		# must not write a new shareindex, or we wont catch the problem
+		git -c splitIndex.maxPercentChange=100 merge --no-edit side-branch 2>err &&
+		# i.e. do not expect warnings like
+		# could not freshen shared index .../shareindex.00000...
+		test_must_be_empty err
+	)
+'
+
 test_done