commit.cocci: refactor code, avoid double rewrite

"maybe" pointer in 'struct commit' is tricky because it can be lazily
initialized to take advantage of commit-graph if available. This makes
it not safe to access directly.

This leads to a rule in commit.cocci to rewrite 'x->maybe_tree' to
'get_commit_tree(x)'. But that rule alone could lead to incorrectly
rewrite assignments, e.g. from

    x->maybe_tree = yes

to

    get_commit_tree(x) = yes

Because of this we have a second rule to revert this effect. Szeder
found out that we could do better by performing the assignment rewrite
rule first, then the remaining is read-only access and handled by the
current first rule.

For this to work, we need to transform "x->maybe_tree = y" to something
that does NOT contain "x->maybe_tree" to avoid the original first
rule. This is where set_commit_tree() comes in.

Helped-by: SZEDER Gábor <szeder.dev@gmail.com>
Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 7 insertions, 2 deletions

diff --git a/commit-graph.c b/commit-graph.c
index 47e9be0a3a..155a270457 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -343,6 +343,11 @@ static void fill_commit_graph_info(struct commit *item, struct commit_graph *g,
 	item->generation = get_be32(commit_data + g->hash_len + 8) >> 2;
 }
 
+static inline void set_commit_tree(struct commit *c, struct tree *t)
+{
+	c->maybe_tree = t;
+}
+
 static int fill_commit_in_graph(struct repository *r,
 				struct commit *item,
 				struct commit_graph *g, uint32_t pos)
@@ -356,7 +361,7 @@ static int fill_commit_in_graph(struct repository *r,
 	item->object.parsed = 1;
 	item->graph_pos = pos;
 
-	item->maybe_tree = NULL;
+	set_commit_tree(item, NULL);
 
 	date_high = get_be32(commit_data + g->hash_len + 8) & 0x3;
 	date_low = get_be32(commit_data + g->hash_len + 12);
@@ -442,7 +447,7 @@ static struct tree *load_tree_for_commit(struct repository *r,
 					   GRAPH_DATA_WIDTH * (c->graph_pos);
 
 	hashcpy(oid.hash, commit_data);
-	c->maybe_tree = lookup_tree(r, &oid);
+	set_commit_tree(c, lookup_tree(r, &oid));
 
 	return c->maybe_tree;
 }