Add rule to compare musig key sets deterministically in leaf_cmp

Author: bigspider

src/common/cleartext.c

@@ -514,6 +514,17 @@ static int leaf_cmp(const ct_leaf_match_t *a, const ct_leaf_match_t *b) {
                 if (ka->type == KEY_EXPRESSION_NORMAL) {
                     if (ka->k.key_index != kb->k.key_index)
                         return (ka->k.key_index < kb->k.key_index) ? -1 : 1;
+                } else {
+                    // KEY_EXPRESSION_MUSIG: order by number of keys, then
+                    // left-to-right by key index
+                    const musig_aggr_key_info_t *ma = r_musig_aggr_key_info(&ka->m.musig_info);
+                    const musig_aggr_key_info_t *mb = r_musig_aggr_key_info(&kb->m.musig_info);
+                    if (ma->n != mb->n) return (ma->n < mb->n) ? -1 : 1;
+                    const uint16_t *ia = r_uint16(&ma->key_indexes);
+                    const uint16_t *ib = r_uint16(&mb->key_indexes);
+                    for (uint16_t j = 0; j < ma->n; j++) {
+                        if (ia[j] != ib[j]) return (ia[j] < ib[j]) ? -1 : 1;
+                    }
                 }
                 break;
             }