Lucas Seiki Oshiro [off-list ref] writes:

This commit is part of the series that introduces the new subcommand
git-repo-info.

The flag `--show-ref-format` from git-rev-parse is used for retrieving
the reference format (i.e. `files` or `reftable`). This way, it is
used for querying repository metadata, fitting in the purpose of
git-repo-info.

Then, add a new field `references.format` to the repo-info subcommand
containing that information.

Nit: I don't think we need the 'Then, ' here, perhaps 'Add ...'.

Shouldn't 'add_field_callback' be renamed, now that we don't add a field
but rather return a value?

+static const char *get_references_format(struct repository *repo)
+{
+	return ref_storage_format_to_name(repo->ref_storage_format);
+}
+
+/* repo_info_fields keys should be in lexicographical order */
+static const struct field repo_info_fields[] = {
+	{ "references.format", get_references_format },
+};
+
+static int repo_info_fields_cmp(const void *va, const void *vb)
+{
+	const struct field *a = va;
+	const struct field *b = vb;
+
+	return strcmp(a->key, b->key);
+}
+
+static get_value_fn *get_value_callback(const char *key)
 {

Nit: A callback generally is a function provided by when a 'fn A' calls
'fn B', providing a 'fn C' which 'fn A' provides.

Here perhaps we can simply rename this to 'get_value_fn_for_key' or
something?

+	const struct field search_key = { key, NULL };
+	const struct field *found = bsearch(&search_key, repo_info_fields,
+					    ARRAY_SIZE(repo_info_fields),
+					    sizeof(struct field),
+					    repo_info_fields_cmp);
+	return found ? found->add_field_callback : NULL;
+}
+
+static int qsort_strcmp(const void *va, const void *vb)
+{
+	const char *a = *(const char **)va;
+	const char *b = *(const char **)vb;
+
+	return strcmp(a, b);
+}
+
+static int print_fields(int argc, const char **argv, struct repository *repo)
+{
+	const char *last = "";
+
+	QSORT(argv, argc, qsort_strcmp);
+
+	for (int i = 0; i < argc; i++) {
+		get_value_fn *callback;
+		const char *key = argv[i];
+		const char *value;
+
+		if (!strcmp(key, last))
+			continue;
+
+		callback = get_value_callback(key);
+
+		if (!callback)
+			return error("key %s not found", key);
+
+		value = callback(repo);
+		printf("%s=%s\n", key, value);

I like this a lot, since we can simply modify this in the future for
different formats. Nice!

[snip]