commit-graph: Introduce `git_commit_graph_needs_refresh()`

This change introduces a function that allows the caller to know whether
the `commit-graph` file has not been modified since it was parsed.

Part of: #5757

src/commit_graph.c

@@ -333,6 +333,40 @@ static int git_commit_graph_entry_get_byindex(
 	return 0;
 }
 
+bool git_commit_graph_needs_refresh(const git_commit_graph_file *cgraph, const char *path)
+{
+	git_file fd = -1;
+	struct stat st;
+	ssize_t bytes_read;
+	git_oid cgraph_checksum = {{0}};
+
+	if (path == NULL)
+		path = git_buf_cstr(&cgraph->filename);
+
+	/* TODO: properly open the file without access time using O_NOATIME */
+	fd = git_futils_open_ro(path);
+	if (fd < 0)
+		return true;
+
+	if (p_fstat(fd, &st) < 0) {
+		p_close(fd);
+		return true;
+	}
+
+	if (!S_ISREG(st.st_mode) || !git__is_sizet(st.st_size)
+	    || (size_t)st.st_size != cgraph->graph_map.len) {
+		p_close(fd);
+		return true;
+	}
+
+	bytes_read = p_pread(fd, cgraph_checksum.id, GIT_OID_RAWSZ, st.st_size - GIT_OID_RAWSZ);
+	p_close(fd);
+	if (bytes_read != GIT_OID_RAWSZ)
+		return true;
+
+	return !git_oid_equal(&cgraph_checksum, &cgraph->checksum);
+}
+
 int git_commit_graph_entry_find(
 		git_commit_graph_entry *e,
 		const git_commit_graph_file *cgraph,

src/commit_graph.h

@@ -89,6 +89,14 @@ typedef struct git_commit_graph_entry {
 } git_commit_graph_entry;
 
 int git_commit_graph_open(git_commit_graph_file **cgraph_out, const char *path);
+
+/*
+ * Returns whether the commit_graph_file needs to be reloaded since the
+ * contents of the commit-graph file have changed on disk. If `path` is NULL,
+ * the filename stored in `cgraph` will be used.
+ */
+bool git_commit_graph_needs_refresh(const git_commit_graph_file *cgraph, const char *path);
+
 int git_commit_graph_entry_find(
 		git_commit_graph_entry *e,
 		const git_commit_graph_file *cgraph,

tests/graph/commit_graph.c

@@ -15,6 +15,7 @@ void test_graph_commit_graph__parse(void)
 	cl_git_pass(git_repository_open(&repo, cl_fixture("testrepo.git")));
 	cl_git_pass(git_buf_joinpath(&commit_graph_path, git_repository_path(repo), "objects/info/commit-graph"));
 	cl_git_pass(git_commit_graph_open(&cgraph, git_buf_cstr(&commit_graph_path)));
+	cl_assert_equal_i(git_commit_graph_needs_refresh(cgraph, git_buf_cstr(&commit_graph_path)), 0);
 
 	cl_git_pass(git_oid_fromstr(&id, "5001298e0c09ad9c34e4249bc5801c75e9754fa5"));
 	cl_git_pass(git_commit_graph_entry_find(&e, cgraph, &id, GIT_OID_HEXSZ));