treewide: rename tree to maybe_tree

Using the commit-graph file to walk commit history removes the large
cost of parsing commits during the walk. This exposes a performance
issue: lookup_tree() takes a large portion of the computation time,
even when Git never uses those trees.

In anticipation of lazy-loading these trees, rename the 'tree' member
of struct commit to 'maybe_tree'. This serves two purposes: it hints
at the future role of possibly being NULL even if the commit has a
valid tree, and it allows for unambiguous transformation from simple
member access (i.e. commit->maybe_tree) to method access.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

24 files changed, 65 insertions, 64 deletions

diff --git a/blame.c b/blame.c
index 200e0ad..b78e649 100644
--- a/blame.c
+++ b/blame.c
@@ -553,10 +553,10 @@ static struct blame_origin *find_origin(struct commit *parent,
 	diff_setup_done(&diff_opts);
 
 	if (is_null_oid(&origin->commit->object.oid))
-		do_diff_cache(&parent->tree->object.oid, &diff_opts);
+		do_diff_cache(&parent->maybe_tree->object.oid, &diff_opts);
 	else
-		diff_tree_oid(&parent->tree->object.oid,
-			      &origin->commit->tree->object.oid,
+		diff_tree_oid(&parent->maybe_tree->object.oid,
+			      &origin->commit->maybe_tree->object.oid,
 			      "", &diff_opts);
 	diffcore_std(&diff_opts);
 
@@ -622,10 +622,10 @@ static struct blame_origin *find_rename(struct commit *parent,
 	diff_setup_done(&diff_opts);
 
 	if (is_null_oid(&origin->commit->object.oid))
-		do_diff_cache(&parent->tree->object.oid, &diff_opts);
+		do_diff_cache(&parent->maybe_tree->object.oid, &diff_opts);
 	else
-		diff_tree_oid(&parent->tree->object.oid,
-			      &origin->commit->tree->object.oid,
+		diff_tree_oid(&parent->maybe_tree->object.oid,
+			      &origin->commit->maybe_tree->object.oid,
 			      "", &diff_opts);
 	diffcore_std(&diff_opts);
 
@@ -1257,10 +1257,10 @@ static void find_copy_in_parent(struct blame_scoreboard *sb,
 		diff_opts.flags.find_copies_harder = 1;
 
 	if (is_null_oid(&target->commit->object.oid))
-		do_diff_cache(&parent->tree->object.oid, &diff_opts);
+		do_diff_cache(&parent->maybe_tree->object.oid, &diff_opts);
 	else
-		diff_tree_oid(&parent->tree->object.oid,
-			      &target->commit->tree->object.oid,
+		diff_tree_oid(&parent->maybe_tree->object.oid,
+			      &target->commit->maybe_tree->object.oid,
 			      "", &diff_opts);
 
 	if (!diff_opts.flags.find_copies_harder)
diff --git a/builtin/checkout.c b/builtin/checkout.c
index a52af2e..ca6aa29 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -484,7 +484,7 @@ static int merge_working_tree(const struct checkout_opts *opts,
 
 	resolve_undo_clear();
 	if (opts->force) {
-		ret = reset_tree(new_branch_info->commit->tree, opts, 1, writeout_error);
+		ret = reset_tree(new_branch_info->commit->maybe_tree, opts, 1, writeout_error);
 		if (ret)
 			return ret;
 	} else {
@@ -570,18 +570,18 @@ static int merge_working_tree(const struct checkout_opts *opts,
 			o.verbosity = 0;
 			work = write_tree_from_memory(&o);
 
-			ret = reset_tree(new_branch_info->commit->tree, opts, 1,
+			ret = reset_tree(new_branch_info->commit->maybe_tree, opts, 1,
 					 writeout_error);
 			if (ret)
 				return ret;
 			o.ancestor = old_branch_info->name;
 			o.branch1 = new_branch_info->name;
 			o.branch2 = "local";
-			ret = merge_trees(&o, new_branch_info->commit->tree, work,
-				old_branch_info->commit->tree, &result);
+			ret = merge_trees(&o, new_branch_info->commit->maybe_tree, work,
+				old_branch_info->commit->maybe_tree, &result);
 			if (ret < 0)
 				exit(128);
-			ret = reset_tree(new_branch_info->commit->tree, opts, 0,
+			ret = reset_tree(new_branch_info->commit->maybe_tree, opts, 0,
 					 writeout_error);
 			strbuf_release(&o.obuf);
 			if (ret)
@@ -1002,7 +1002,7 @@ static int parse_branchname_arg(int argc, const char **argv,
 		*source_tree = parse_tree_indirect(rev);
 	} else {
 		parse_commit_or_die(new_branch_info->commit);
-		*source_tree = new_branch_info->commit->tree;
+		*source_tree = new_branch_info->commit->maybe_tree;
 	}
 
 	if (!*source_tree)                   /* case (1): want a tree */
diff --git a/builtin/diff.c b/builtin/diff.c
index 16bfb22..34f18a5 100644
--- a/builtin/diff.c
+++ b/builtin/diff.c
@@ -398,7 +398,7 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 		if (!obj)
 			die(_("invalid object '%s' given."), name);
 		if (obj->type == OBJ_COMMIT)
-			obj = &((struct commit *)obj)->tree->object;
+			obj = &((struct commit *)obj)->maybe_tree->object;
 
 		if (obj->type == OBJ_TREE) {
 			obj->flags |= flags;
diff --git a/builtin/fast-export.c b/builtin/fast-export.c
index 27b2cc1..91e526b 100644
--- a/builtin/fast-export.c
+++ b/builtin/fast-export.c
@@ -578,11 +578,11 @@ static void handle_commit(struct commit *commit, struct rev_info *rev,
 	    get_object_mark(&commit->parents->item->object) != 0 &&
 	    !full_tree) {
 		parse_commit_or_die(commit->parents->item);
-		diff_tree_oid(&commit->parents->item->tree->object.oid,
-			      &commit->tree->object.oid, "", &rev->diffopt);
+		diff_tree_oid(&commit->parents->item->maybe_tree->object.oid,
+			      &commit->maybe_tree->object.oid, "", &rev->diffopt);
 	}
 	else
-		diff_root_tree_oid(&commit->tree->object.oid,
+		diff_root_tree_oid(&commit->maybe_tree->object.oid,
 				   "", &rev->diffopt);
 
 	/* Export the referenced blobs, and remember the marks. */
diff --git a/builtin/log.c b/builtin/log.c
index 46b4ca1..a1fbc60 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -1064,8 +1064,8 @@ static void make_cover_letter(struct rev_info *rev, int use_stdout,
 
 	diff_setup_done(&opts);
 
-	diff_tree_oid(&origin->tree->object.oid,
-		      &head->tree->object.oid,
+	diff_tree_oid(&origin->maybe_tree->object.oid,
+		      &head->maybe_tree->object.oid,
 		      "", &opts);
 	diffcore_std(&opts);
 	diff_flush(&opts);
diff --git a/builtin/reflog.c b/builtin/reflog.c
index ac3dcd7..29d80a3 100644
--- a/builtin/reflog.c
+++ b/builtin/reflog.c
@@ -153,7 +153,7 @@ static int commit_is_complete(struct commit *commit)
 		for (i = 0; i < found.nr; i++) {
 			struct commit *c =
 				(struct commit *)found.objects[i].item;
-			if (!tree_is_complete(&c->tree->object.oid)) {
+			if (!tree_is_complete(&c->maybe_tree->object.oid)) {
 				is_incomplete = 1;
 				c->object.flags |= INCOMPLETE;
 			}
diff --git a/commit-graph.c b/commit-graph.c
index 3ff8c84..49c622c 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -258,7 +258,7 @@ static int fill_commit_in_graph(struct commit *item, struct commit_graph *g, uin
 	item->graph_pos = pos;
 
 	hashcpy(oid.hash, commit_data);
-	item->tree = lookup_tree(&oid);
+	item->maybe_tree = lookup_tree(&oid);
 
 	date_high = get_be32(commit_data + g->hash_len + 8) & 0x3;
 	date_low = get_be32(commit_data + g->hash_len + 12);
@@ -369,7 +369,7 @@ static void write_graph_chunk_data(struct hashfile *f, int hash_len,
 		uint32_t packedDate[2];
 
 		parse_commit(*list);
-		hashwrite(f, (*list)->tree->object.oid.hash, hash_len);
+		hashwrite(f, (*list)->maybe_tree->object.oid.hash, hash_len);
 
 		parent = (*list)->parents;
 
diff --git a/commit.c b/commit.c
index 3e39c86..fbc0928 100644
--- a/commit.c
+++ b/commit.c
@@ -335,7 +335,7 @@ int parse_commit_buffer(struct commit *item, const void *buffer, unsigned long s
 	if (get_sha1_hex(bufptr + 5, parent.hash) < 0)
 		return error("bad tree pointer in commit %s",
 			     oid_to_hex(&item->object.oid));
-	item->tree = lookup_tree(&parent);
+	item->maybe_tree = lookup_tree(&parent);
 	bufptr += tree_entry_len + 1; /* "tree " + "hex sha1" + "\n" */
 	pptr = &item->parents;
 
diff --git a/commit.h b/commit.h
index e57ae4b..c4d6e6e 100644
--- a/commit.h
+++ b/commit.h
@@ -22,7 +22,7 @@ struct commit {
 	unsigned int index;
 	timestamp_t date;
 	struct commit_list *parents;
-	struct tree *tree;
+	struct tree *maybe_tree;
 	uint32_t graph_pos;
 };
 
diff --git a/fsck.c b/fsck.c
index 5c8c12d..3228ca5 100644
--- a/fsck.c
+++ b/fsck.c
@@ -396,9 +396,9 @@ static int fsck_walk_commit(struct commit *commit, void *data, struct fsck_optio
 
 	name = get_object_name(options, &commit->object);
 	if (name)
-		put_object_name(options, &commit->tree->object, "%s:", name);
+		put_object_name(options, &commit->maybe_tree->object, "%s:", name);
 
-	result = options->walk((struct object *)commit->tree, OBJ_TREE, data, options);
+	result = options->walk((struct object *)commit->maybe_tree, OBJ_TREE, data, options);
 	if (result < 0)
 		return result;
 	res = result;
@@ -772,7 +772,7 @@ static int fsck_commit_buffer(struct commit *commit, const char *buffer,
 	err = fsck_ident(&buffer, &commit->object, options);
 	if (err)
 		return err;
-	if (!commit->tree) {
+	if (!commit->maybe_tree) {
 		err = report(options, &commit->object, FSCK_MSG_BAD_TREE, "could not load commit's tree %s", sha1_to_hex(tree_sha1));
 		if (err)
 			return err;
diff --git a/http-push.c b/http-push.c
index 7dcd9da..d83479f 100644
--- a/http-push.c
+++ b/http-push.c
@@ -1330,7 +1330,7 @@ static int get_delta(struct rev_info *revs, struct remote_lock *lock)
 	int count = 0;
 
 	while ((commit = get_revision(revs)) != NULL) {
-		p = process_tree(commit->tree, p);
+		p = process_tree(commit->maybe_tree, p);
 		commit->object.flags |= LOCAL;
 		if (!(commit->object.flags & UNINTERESTING))
 			count += add_send_request(&commit->object, lock);
diff --git a/line-log.c b/line-log.c
index cdc2257..e714969 100644
--- a/line-log.c
+++ b/line-log.c
@@ -817,8 +817,8 @@ static void queue_diffs(struct line_log_data *range,
 	assert(commit);
 
 	DIFF_QUEUE_CLEAR(&diff_queued_diff);
-	diff_tree_oid(parent ? &parent->tree->object.oid : NULL,
-		      &commit->tree->object.oid, "", opt);
+	diff_tree_oid(parent ? &parent->maybe_tree->object.oid : NULL,
+		      &commit->maybe_tree->object.oid, "", opt);
 	if (opt->detect_rename) {
 		filter_diffs_for_paths(range, 1);
 		if (diff_might_be_rename())
diff --git a/list-objects.c b/list-objects.c
index 168bef6..bfd09f5 100644
--- a/list-objects.c
+++ b/list-objects.c
@@ -195,7 +195,7 @@ static void mark_edge_parents_uninteresting(struct commit *commit,
 		struct commit *parent = parents->item;
 		if (!(parent->object.flags & UNINTERESTING))
 			continue;
-		mark_tree_uninteresting(parent->tree);
+		mark_tree_uninteresting(parent->maybe_tree);
 		if (revs->edge_hint && !(parent->object.flags & SHOWN)) {
 			parent->object.flags |= SHOWN;
 			show_edge(parent);
@@ -212,7 +212,7 @@ void mark_edges_uninteresting(struct rev_info *revs, show_edge_fn show_edge)
 		struct commit *commit = list->item;
 
 		if (commit->object.flags & UNINTERESTING) {
-			mark_tree_uninteresting(commit->tree);
+			mark_tree_uninteresting(commit->maybe_tree);
 			if (revs->edge_hint_aggressive && !(commit->object.flags & SHOWN)) {
 				commit->object.flags |= SHOWN;
 				show_edge(commit);
@@ -227,7 +227,7 @@ void mark_edges_uninteresting(struct rev_info *revs, show_edge_fn show_edge)
 			struct commit *commit = (struct commit *)obj;
 			if (obj->type != OBJ_COMMIT || !(obj->flags & UNINTERESTING))
 				continue;
-			mark_tree_uninteresting(commit->tree);
+			mark_tree_uninteresting(commit->maybe_tree);
 			if (!(obj->flags & SHOWN)) {
 				obj->flags |= SHOWN;
 				show_edge(commit);
@@ -300,8 +300,8 @@ static void do_traverse(struct rev_info *revs,
 		 * an uninteresting boundary commit may not have its tree
 		 * parsed yet, but we are not going to show them anyway
 		 */
-		if (commit->tree)
-			add_pending_tree(revs, commit->tree);
+		if (commit->maybe_tree)
+			add_pending_tree(revs, commit->maybe_tree);
 		show_commit(commit, show_data);
 
 		if (revs->tree_blobs_in_commit_order)
diff --git a/log-tree.c b/log-tree.c
index bdf23c5..99499af 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -806,7 +806,7 @@ static int log_tree_diff(struct rev_info *opt, struct commit *commit, struct log
 		return 0;
 
 	parse_commit_or_die(commit);
-	oid = &commit->tree->object.oid;
+	oid = &commit->maybe_tree->object.oid;
 
 	/* Root commit? */
 	parents = get_saved_parents(opt, commit);
@@ -831,7 +831,7 @@ static int log_tree_diff(struct rev_info *opt, struct commit *commit, struct log
 			 * we merged _in_.
 			 */
 			parse_commit_or_die(parents->item);
-			diff_tree_oid(&parents->item->tree->object.oid,
+			diff_tree_oid(&parents->item->maybe_tree->object.oid,
 				      oid, "", &opt->diffopt);
 			log_tree_diff_flush(opt);
 			return !opt->loginfo;
@@ -846,7 +846,7 @@ static int log_tree_diff(struct rev_info *opt, struct commit *commit, struct log
 		struct commit *parent = parents->item;
 
 		parse_commit_or_die(parent);
-		diff_tree_oid(&parent->tree->object.oid,
+		diff_tree_oid(&parent->maybe_tree->object.oid,
 			      oid, "", &opt->diffopt);
 		log_tree_diff_flush(opt);
 
diff --git a/merge-recursive.c b/merge-recursive.c
index 6ff971f..0f75d75 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -101,7 +101,7 @@ static struct commit *make_virtual_commit(struct tree *tree, const char *comment
 	struct commit *commit = alloc_commit_node();
 
 	set_merge_remote_desc(commit, comment, (struct object *)commit);
-	commit->tree = tree;
+	commit->maybe_tree = tree;
 	commit->object.parsed = 1;
 	return commit;
 }
@@ -2154,7 +2154,8 @@ int merge_recursive(struct merge_options *o,
 		read_cache();
 
 	o->ancestor = "merged common ancestors";
-	clean = merge_trees(o, h1->tree, h2->tree, merged_common_ancestors->tree,
+	clean = merge_trees(o, h1->maybe_tree, h2->maybe_tree,
+			    merged_common_ancestors->maybe_tree,
 			    &mrtree);
 	if (clean < 0) {
 		flush_output(o);
diff --git a/notes-merge.c b/notes-merge.c
index c09c5e0..1d3edc8 100644
--- a/notes-merge.c
+++ b/notes-merge.c
@@ -600,14 +600,14 @@ int notes_merge(struct notes_merge_options *o,
 			printf("No merge base found; doing history-less merge\n");
 	} else if (!bases->next) {
 		base_oid = &bases->item->object.oid;
-		base_tree_oid = &bases->item->tree->object.oid;
+		base_tree_oid = &bases->item->maybe_tree->object.oid;
 		if (o->verbosity >= 4)
 			printf("One merge base found (%.7s)\n",
 			       oid_to_hex(base_oid));
 	} else {
 		/* TODO: How to handle multiple merge-bases? */
 		base_oid = &bases->item->object.oid;
-		base_tree_oid = &bases->item->tree->object.oid;
+		base_tree_oid = &bases->item->maybe_tree->object.oid;
 		if (o->verbosity >= 3)
 			printf("Multiple merge bases found. Using the first "
 				"(%.7s)\n", oid_to_hex(base_oid));
@@ -634,8 +634,8 @@ int notes_merge(struct notes_merge_options *o,
 		goto found_result;
 	}
 
-	result = merge_from_diffs(o, base_tree_oid, &local->tree->object.oid,
-				  &remote->tree->object.oid, local_tree);
+	result = merge_from_diffs(o, base_tree_oid, &local->maybe_tree->object.oid,
+				  &remote->maybe_tree->object.oid, local_tree);
 
 	if (result != 0) { /* non-trivial merge (with or without conflicts) */
 		/* Commit (partial) result */
diff --git a/packfile.c b/packfile.c
index b1d33b6..3eb9c4a 100644
--- a/packfile.c
+++ b/packfile.c
@@ -1925,7 +1925,7 @@ static int add_promisor_object(const struct object_id *oid,
 		struct commit *commit = (struct commit *) obj;
 		struct commit_list *parents = commit->parents;
 
-		oidset_insert(set, &commit->tree->object.oid);
+		oidset_insert(set, &commit->maybe_tree->object.oid);
 		for (; parents; parents = parents->next)
 			oidset_insert(set, &parents->item->object.oid);
 	} else if (obj->type == OBJ_TAG) {
diff --git a/pretty.c b/pretty.c
index f7ce490..42095ea 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1161,10 +1161,10 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 		strbuf_addstr(sb, diff_get_color(c->auto_color, DIFF_RESET));
 		return 1;
 	case 'T':		/* tree hash */
-		strbuf_addstr(sb, oid_to_hex(&commit->tree->object.oid));
+		strbuf_addstr(sb, oid_to_hex(&commit->maybe_tree->object.oid));
 		return 1;
 	case 't':		/* abbreviated tree hash */
-		strbuf_add_unique_abbrev(sb, commit->tree->object.oid.hash,
+		strbuf_add_unique_abbrev(sb, commit->maybe_tree->object.oid.hash,
 					 c->pretty_ctx->abbrev);
 		return 1;
 	case 'P':		/* parent hashes */
diff --git a/ref-filter.c b/ref-filter.c
index 99a45be..a59654c 100644
--- a/ref-filter.c
+++ b/ref-filter.c
@@ -815,7 +815,7 @@ static void grab_commit_values(struct atom_value *val, int deref, struct object
 		if (deref)
 			name++;
 		if (!strcmp(name, "tree")) {
-			v->s = xstrdup(oid_to_hex(&commit->tree->object.oid));
+			v->s = xstrdup(oid_to_hex(&commit->maybe_tree->object.oid));
 		}
 		else if (!strcmp(name, "numparent")) {
 			v->value = commit_list_count(commit->parents);
diff --git a/revision.c b/revision.c
index 5c1cb72..f61d1c9 100644
--- a/revision.c
+++ b/revision.c
@@ -439,8 +439,8 @@ static void file_change(struct diff_options *options,
 static int rev_compare_tree(struct rev_info *revs,
 			    struct commit *parent, struct commit *commit)
 {
-	struct tree *t1 = parent->tree;
-	struct tree *t2 = commit->tree;
+	struct tree *t1 = parent->maybe_tree;
+	struct tree *t2 = commit->maybe_tree;
 
 	if (!t1)
 		return REV_TREE_NEW;
@@ -476,7 +476,7 @@ static int rev_compare_tree(struct rev_info *revs,
 static int rev_same_tree_as_empty(struct rev_info *revs, struct commit *commit)
 {
 	int retval;
-	struct tree *t1 = commit->tree;
+	struct tree *t1 = commit->maybe_tree;
 
 	if (!t1)
 		return 0;
@@ -614,7 +614,7 @@ static void try_to_simplify_commit(struct rev_info *revs, struct commit *commit)
 	if (!revs->prune)
 		return;
 
-	if (!commit->tree)
+	if (!commit->maybe_tree)
 		return;
 
 	if (!commit->parents) {
diff --git a/sequencer.c b/sequencer.c
index 6ca4499..9269607 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -501,8 +501,8 @@ static int do_recursive_merge(struct commit *base, struct commit *next,
 	o.show_rename_progress = 1;
 
 	head_tree = parse_tree_indirect(head);
-	next_tree = next ? next->tree : empty_tree();
-	base_tree = base ? base->tree : empty_tree();
+	next_tree = next ? next->maybe_tree : empty_tree();
+	base_tree = base ? base->maybe_tree : empty_tree();
 
 	for (xopt = opts->xopts; xopt != opts->xopts + opts->xopts_nr; xopt++)
 		parse_merge_opt(&o, *xopt);
@@ -562,7 +562,7 @@ static int is_index_unchanged(void)
 			return error(_("unable to update cache tree"));
 
 	return !oidcmp(&active_cache_tree->oid,
-		       &head_commit->tree->object.oid);
+		       &head_commit->maybe_tree->object.oid);
 }
 
 static int write_author_script(const char *message)
@@ -1119,7 +1119,7 @@ static int try_to_commit(struct strbuf *msg, const char *author,
 	}
 
 	if (!(flags & ALLOW_EMPTY) && !oidcmp(current_head ?
-					      &current_head->tree->object.oid :
+					      &current_head->maybe_tree->object.oid :
 					      &empty_tree_oid, &tree)) {
 		res = 1; /* run 'git commit' to display error message */
 		goto out;
@@ -1217,12 +1217,12 @@ static int is_original_commit_empty(struct commit *commit)
 		if (parse_commit(parent))
 			return error(_("could not parse parent commit %s"),
 				oid_to_hex(&parent->object.oid));
-		ptree_oid = &parent->tree->object.oid;
+		ptree_oid = &parent->maybe_tree->object.oid;
 	} else {
 		ptree_oid = the_hash_algo->empty_tree; /* commit is root */
 	}
 
-	return !oidcmp(ptree_oid, &commit->tree->object.oid);
+	return !oidcmp(ptree_oid, &commit->maybe_tree->object.oid);
 }
 
 /*
diff --git a/sha1_name.c b/sha1_name.c
index e7c18ff..6ca2a2d 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -896,7 +896,7 @@ struct object *peel_to_type(const char *name, int namelen,
 		if (o->type == OBJ_TAG)
 			o = ((struct tag*) o)->tagged;
 		else if (o->type == OBJ_COMMIT)
-			o = &(((struct commit *) o)->tree->object);
+			o = &(((struct commit *) o)->maybe_tree->object);
 		else {
 			if (name)
 				error("%.*s: expected %s type, but the object "
diff --git a/tree.c b/tree.c
index b224115..dbc5e0b 100644
--- a/tree.c
+++ b/tree.c
@@ -109,7 +109,7 @@ static int read_tree_1(struct tree *tree, struct strbuf *base,
 				    oid_to_hex(entry.oid),
 				    base->buf, entry.path);
 
-			oidcpy(&oid, &commit->tree->object.oid);
+			oidcpy(&oid, &commit->maybe_tree->object.oid);
 		}
 		else
 			continue;
@@ -248,7 +248,7 @@ struct tree *parse_tree_indirect(const struct object_id *oid)
 		if (obj->type == OBJ_TREE)
 			return (struct tree *) obj;
 		else if (obj->type == OBJ_COMMIT)
-			obj = &(((struct commit *) obj)->tree->object);
+			obj = &(((struct commit *) obj)->maybe_tree->object);
 		else if (obj->type == OBJ_TAG)
 			obj = ((struct tag *) obj)->tagged;
 		else
diff --git a/walker.c b/walker.c
index dffb9c8..1d5f305 100644
--- a/walker.c
+++ b/walker.c
@@ -87,7 +87,7 @@ static int process_commit(struct walker *walker, struct commit *commit)
 	walker_say(walker, "walk %s\n", oid_to_hex(&commit->object.oid));
 
 	if (walker->get_tree) {
-		if (process(walker, &commit->tree->object))
+		if (process(walker, &commit->maybe_tree->object))
 			return -1;
 		if (!walker->get_all)
 			walker->get_tree = 0;