[PATCH 2/3] commit-graph: introduce `repo_find_commit_pos_in_graph()`

[Taylor Blau <me@ttaylorr.com>]

Low-level callers in systems that are adjacent to the commit-graph (like
the changed-path Bloom filter code) could benefit from being able to
call a function like `parse_commit_in_graph()` without modifying the
corresponding commit slab data.

This is useful in contexts where that slab data is being used to prepare
for an upcoming commit-graph write, where Git must be careful to avoid
clobbering any of that data during a read operation.

Introduce a low-level variant of `parse_commit_in_graph()` which returns
the graph position of a given commit only, without modifying any of the
slab data.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 commit-graph.c | 12 +++++++++---
 commit-graph.h | 15 +++++++++++++++
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/commit-graph.c b/commit-graph.c
index 92d4503336..1c34ae1ea4 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -889,6 +889,14 @@ static int find_commit_pos_in_graph(struct commit *item, struct commit_graph *g,
 	}
 }
 
+int repo_find_commit_pos_in_graph(struct repository *r, struct commit *c,
+				  uint32_t *pos)
+{
+	if (!prepare_commit_graph(r))
+		return 0;
+	return find_commit_pos_in_graph(c, r->objects->commit_graph, pos);
+}
+
 struct commit *lookup_commit_in_graph(struct repository *repo, const struct object_id *id)
 {
 	struct commit *commit;
@@ -946,9 +954,7 @@ int parse_commit_in_graph(struct repository *r, struct commit *item)
 void load_commit_graph_info(struct repository *r, struct commit *item)
 {
 	uint32_t pos;
-	if (!prepare_commit_graph(r))
-		return;
-	if (find_commit_pos_in_graph(item, r->objects->commit_graph, &pos))
+	if (repo_find_commit_pos_in_graph(r, item, &pos))
 		fill_commit_graph_info(item, r->objects->commit_graph, pos);
 }
 
diff --git a/commit-graph.h b/commit-graph.h
index 2e3ac35237..f23b9e9026 100644
--- a/commit-graph.h
+++ b/commit-graph.h
@@ -40,6 +40,21 @@ int open_commit_graph(const char *graph_file, int *fd, struct stat *st);
  */
 int parse_commit_in_graph(struct repository *r, struct commit *item);
 
+/*
+ * Fills `*pos` with the graph position of `c`, and returns 1 if `c` is
+ * found in the commit-graph belonging to `r`, or 0 otherwise.
+ * Initializes the commit-graph belonging to `r` if it hasn't been
+ * already.
+ *
+ * Note: this is a low-level helper that does not alter any slab data
+ * associated with `c`. Useful in circumstances where the slab data is
+ * already being modified (e.g., writing the commit-graph itself).
+ *
+ * In most cases, callers should use `parse_commit_in_graph()` instead.
+ */
+int repo_find_commit_pos_in_graph(struct repository *r, struct commit *c,
+				  uint32_t *pos);
+
 /*
  * Look up the given commit ID in the commit-graph. This will only return a
  * commit if the ID exists both in the graph and in the object database such
-- 
2.37.0.1.g1379af2e9d