commit-graph.c: handle corrupt/missing trees

Apply similar treatment as in the previous commit to handle an unchecked
call to 'get_commit_tree_oid()'. Previously, a NULL return value from
this function would be immediately dereferenced with '->hash', and then
cause a segfault.

Before dereferencing to access the 'hash' member, check the return value
of 'get_commit_tree_oid()' to make sure that it is not NULL.

To make this check correct, a related change is also needed in
'commit.c', which is to check the return value of 'get_commit_tree'
before taking its address. If 'get_commit_tree' returns NULL, we
encounter an undefined behavior when taking the address of the return
value of 'get_commit_tree' and then taking '->object.oid'. (On my system,
this is memory address 0x8, which is obviously wrong).

Fix this by making sure that 'get_commit_tree' returns something
non-NULL before digging through a structure that is not there, thus
preventing a segfault down the line in the commit graph code.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-graph.c

@@ -839,6 +839,7 @@ static void write_graph_chunk_data(struct hashfile *f, int hash_len,
 
 	while (list < last) {
 		struct commit_list *parent;
+		struct object_id *tree;
 		int edge_value;
 		uint32_t packedDate[2];
 		display_progress(ctx->progress, ++ctx->progress_cnt);
@@ -846,7 +847,11 @@ static void write_graph_chunk_data(struct hashfile *f, int hash_len,
 		if (parse_commit_no_graph(*list))
 			die(_("unable to parse commit %s"),
 				oid_to_hex(&(*list)->object.oid));
-		hashwrite(f, get_commit_tree_oid(*list)->hash, hash_len);
+		tree = get_commit_tree_oid(*list);
+		if (!tree)
+			die(_("unable to get tree for %s"),
+				oid_to_hex(&(*list)->object.oid));
+		hashwrite(f, tree->hash, hash_len);
 
 		parent = (*list)->parents;
 

commit.c

@@ -358,7 +358,8 @@ struct tree *repo_get_commit_tree(struct repository *r,
 
 struct object_id *get_commit_tree_oid(const struct commit *commit)
 {
-	return &get_commit_tree(commit)->object.oid;
+	struct tree *tree = get_commit_tree(commit);
+	return tree ? &tree->object.oid : NULL;
 }
 
 void release_commit_memory(struct parsed_object_pool *pool, struct commit *c)

t/t5318-commit-graph.sh

@@ -607,7 +607,7 @@ test_expect_success 'corrupt commit-graph write (broken parent)' '
 	)
 '
 
-test_expect_failure 'corrupt commit-graph write (missing tree)' '
+test_expect_success 'corrupt commit-graph write (missing tree)' '
 	rm -rf repo &&
 	git init repo &&
 	(