commit.c: add repo_get_commit_tree()

Remove the implicit dependency on the_repository in this function.
It will be used in sha1-name.c functions when they are updated to take
any 'struct repository'. get_commit_tree() remains as a compat wrapper,
to be slowly replaced later.

Any access to "maybe_tree" field directly will result in _broken_ code
after running through commit.cocci because we can't know what is the
right repository to use.

the_repository would be correct most of the time. But we're relying less
and less on the_repository and that assumption may no longer be
true. The transformation now is more of a poor man replacement for a C++
compiler catching access to private fields.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -345,7 +345,8 @@ static inline void set_commit_tree(struct commit *c, struct tree *t)
 	c->maybe_tree = t;
 }
 
-struct tree *get_commit_tree(const struct commit *commit)
+struct tree *repo_get_commit_tree(struct repository *r,
+				  const struct commit *commit)
 {
 	if (commit->maybe_tree || !commit->object.parsed)
 		return commit->maybe_tree;
@@ -353,7 +354,7 @@ struct tree *get_commit_tree(const struct commit *commit)
 	if (commit->graph_pos == COMMIT_NOT_FROM_GRAPH)
 		BUG("commit has NULL tree, but was not loaded from commit-graph");
 
-	return get_commit_tree_in_graph(the_repository, commit);
+	return get_commit_tree_in_graph(r, commit);
 }
 
 struct object_id *get_commit_tree_oid(const struct commit *commit)

commit.h

@@ -32,7 +32,7 @@ struct commit {
 
 	/*
 	 * If the commit is loaded from the commit-graph file, then this
-	 * member may be NULL. Only access it through get_commit_tree()
+	 * member may be NULL. Only access it through repo_get_commit_tree()
 	 * or get_commit_tree_oid().
 	 */
 	struct tree *maybe_tree;
@@ -143,7 +143,8 @@ void repo_unuse_commit_buffer(struct repository *r,
  */
 void free_commit_buffer(struct parsed_object_pool *pool, struct commit *);
 
-struct tree *get_commit_tree(const struct commit *);
+struct tree *repo_get_commit_tree(struct repository *, const struct commit *);
+#define get_commit_tree(c) repo_get_commit_tree(the_repository, c)
 struct object_id *get_commit_tree_oid(const struct commit *);
 
 /*

contrib/coccinelle/commit.cocci

@@ -23,12 +23,12 @@ expression s;
 // These excluded functions must access c->maybe_tree direcly.
 // Note that if c->maybe_tree is written somewhere outside of these
 // functions, then the recommended transformation will be bogus with
-// get_commit_tree() on the LHS.
+// repo_get_commit_tree() on the LHS.
 @@
-identifier f !~ "^(get_commit_tree|get_commit_tree_in_graph_one|load_tree_for_commit|set_commit_tree)$";
+identifier f !~ "^(repo_get_commit_tree|get_commit_tree_in_graph_one|load_tree_for_commit|set_commit_tree)$";
 expression c;
 @@
   f(...) {<...
 - c->maybe_tree
-+ get_commit_tree(c)
++ repo_get_commit_tree(specify_the_right_repo_here, c)
   ...>}