commit-graph: Use the commit-graph in revwalks

This change makes revwalks a bit faster by using the `commit-graph` file
(if present). This is thanks to the `commit-graph` allow much faster
parsing of the commit information by requiring near-zero I/O (aside from
reading a few dozen bytes off of a `mmap(2)`-ed file) for each commit,
instead of having to read the ODB, inflate the commit, and parse it.

This is done by modifying `git_commit_list_parse()` and letting it use
the ODB-owned commit-graph file.

Part of: #5757

1 files changed, 28 insertions, 0 deletions

diff --git a/src/commit_list.c b/src/commit_list.c
index 2e103ec22..dfdd5daab 100644
--- a/src/commit_list.c
+++ b/src/commit_list.c
@@ -124,6 +124,7 @@ static int commit_quick_parse(
 		return -1;
 	}
 
+	node->generation = 0;
 	node->time = commit->committer->when.time;
 	node->out_degree = (uint16_t) git_array_size(commit->parent_ids);
 	node->parents = alloc_parents(walk, node, node->out_degree);
@@ -143,11 +144,38 @@ static int commit_quick_parse(
 int git_commit_list_parse(git_revwalk *walk, git_commit_list_node *commit)
 {
 	git_odb_object *obj;
+	git_commit_graph_file *cgraph = NULL;
 	int error;
 
 	if (commit->parsed)
 		return 0;
 
+	/* Let's try to use the commit graph first. */
+	git_odb__get_commit_graph(&cgraph, walk->odb);
+	if (cgraph) {
+		git_commit_graph_entry e;
+
+		error = git_commit_graph_entry_find(&e, cgraph, &commit->oid, GIT_OID_RAWSZ);
+		if (error == 0 && git__is_uint16(e.parent_count)) {
+			size_t i;
+			commit->generation = (uint32_t)e.generation;
+			commit->time = e.commit_time;
+			commit->out_degree = (uint16_t)e.parent_count;
+			commit->parents = alloc_parents(walk, commit, commit->out_degree);
+			GIT_ERROR_CHECK_ALLOC(commit->parents);
+
+			for (i = 0; i < commit->out_degree; ++i) {
+				git_commit_graph_entry parent;
+				error = git_commit_graph_entry_parent(&parent, cgraph, &e, i);
+				if (error < 0)
+					return error;
+				commit->parents[i] = git_revwalk__commit_lookup(walk, &parent.sha1);
+			}
+			commit->parsed = 1;
+			return 0;
+		}
+	}
+
 	if ((error = git_odb_read(&obj, walk->odb, &commit->oid)) < 0)
 		return error;