fix restart without blocks.log but with retained blocks files

libraries/chain/block_log.cpp

@@ -611,6 +611,12 @@ namespace eosio { namespace chain {
       const auto log_size   = fc::file_size(block_file.get_file_path());
       const auto index_size = fc::file_size(index_file.get_file_path());
 
+      if (log_size == 0 && !catalog.empty()) {
+         this->reset(catalog.last_block_num() + 1, catalog.verifier.chain_id);
+         this->head = read_block_by_num(catalog.last_block_num());
+         return;
+      }
+
       if (log_size) {
          ilog("Log is nonempty");
          block_log_data log_data(block_file.get_file_path());

libraries/chain/include/eosio/chain/log_catalog.hpp

@@ -63,6 +63,12 @@ struct log_catalog {
       return collection.begin()->first;
    }
 
+   uint32_t last_block_num() const {
+      if (empty())
+         return 0;
+      return collection.rbegin()->second.last_block_num;
+   }
+
    static bfs::path make_abosolute_dir(const bfs::path& base_dir, bfs::path new_dir) {
       if (new_dir.is_relative())
          new_dir = base_dir / new_dir;

unittests/restart_chain_tests.cpp

@@ -644,6 +644,14 @@ BOOST_AUTO_TEST_CASE(test_split_log_util2) {
    BOOST_CHECK(bfs::exists( retained_dir / "blocks-51-100.index" ));
    BOOST_CHECK(bfs::exists( retained_dir / "blocks-101-150.log" ));
    BOOST_CHECK(bfs::exists( retained_dir / "blocks-101-150.index" ));
+
+   bfs::remove(blocks_dir/"blocks.log");
+   bfs::remove(blocks_dir/"blocks.index");
+
+   block_log blog({.log_dir = blocks_dir, .retained_dir = retained_dir });
+
+   BOOST_CHECK(blog.version() != 0);
+   BOOST_CHECK(blog.head().get());
 }
 
 BOOST_AUTO_TEST_SUITE_END()