I had one mysql engine connection in clickhouse. 
I shutdown mysql aws RDS instance.

I updated clickhouse using apt get.
Then I restarted clickhouse. Clickhouse wont start.

`ClickHouse client version 20.7.2.30 (official build).`

```
No LSB modules are available.
Distributor ID:	Debian
Description:	Debian GNU/Linux 9.13 (stretch)
Release:	9.13
Codename:	stretch
```

clickhouse server log

```
2020.09.03 12:31:01.937116 [ 947 ] {} <Debug> Application: Shut down storages.
2020.09.03 12:31:01.937566 [ 947 ] {} <Debug> Application: Destroyed global context.
2020.09.03 12:31:01.937693 [ 947 ] {} <Error> Application: DB::Exception: Cannot create MySQL database, because Poco::Exception. Code: 1000, e.code() = 2002, e.displayText() = mysqlxx::ConnectionFailed: Can't connect to MySQL server on '.....rds.amazonaws.com' (110) ((nullptr):0), Stack trace (when copying this message, always include the lines below):

0. Poco::Exception::Exception(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, int) @ 0x1271b990 in /usr/bin/clickhouse
1. ? @ 0x12596556 in /usr/bin/clickhouse
2. mysqlxx::Pool::allocConnection(bool) @ 0x1259fb31 in /usr/bin/clickhouse
3. mysqlxx::Pool::initialize() @ 0x1259fedd in /usr/bin/clickhouse
4. mysqlxx::Pool::get() @ 0x125a00e8 in /usr/bin/clickhouse
5. DB::DatabaseMySQL::fetchTablesWithModificationTime() const @ 0xf46df4a in /usr/bin/clickhouse
6. DB::DatabaseMySQL::fetchTablesIntoLocalCache() const @ 0xf470b57 in /usr/bin/clickhouse
7. DB::DatabaseMySQL::empty() const @ 0xf470c15 in /usr/bin/clickhouse
8. DB::DatabaseFactory::getImpl(DB::ASTCreateQuery const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, DB::Context&) @ 0xf462839 in /usr/bin/clickhouse
9. DB::DatabaseFactory::get(DB::ASTCreateQuery const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, DB::Context&) @ 0xf463052 in /usr/bin/clickhouse
10. DB::InterpreterCreateQuery::createDatabase(DB::ASTCreateQuery&) @ 0xf455fc0 in /usr/bin/clickhouse
11. DB::InterpreterCreateQuery::execute() @ 0xf456f5e in /usr/bin/clickhouse
12. ? @ 0xf8338fa in /usr/bin/clickhouse
13. DB::loadMetadata(DB::Context&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) @ 0xf83441b in /usr/bin/clickhouse
14. DB::Server::main(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&) @ 0xa495dd7 in /usr/bin/clickhouse
15. Poco::Util::Application::run() @ 0x1264a9e7 in /usr/bin/clickhouse
16. DB::Server::run() @ 0xa4558d9 in /usr/bin/clickhouse
17. mainEntryClickHouseServer(int, char**) @ 0xa44a993 in /usr/bin/clickhouse
18. main @ 0xa3dd919 in /usr/bin/clickhouse
19. __libc_start_main @ 0x202e1 in /lib/x86_64-linux-gnu/libc-2.24.so
20. _start @ 0xa3dd02e in /usr/bin/clickhouse
 (version 20.7.2.30 (official build))
2020.09.03 12:31:01.937740 [ 947 ] {} <Informa
```

{
public:
    ~DatabaseMySQL() override;
    DatabaseMySQL(
        ContextPtr context,
        const String & database_name,
        const String & metadata_path,
        const ASTStorage * database_engine_define,
        const String & database_name_in_mysql,
        std::unique_ptr<ConnectionMySQLSettings> settings_,
        mysqlxx::PoolWithFailover && pool);
    String getEngineName() const override { return "MySQL"; }
    bool canContainMergeTreeTables() const override { return false; }
    bool canContainDistributedTables() const override { return false; }
    bool shouldBeEmptyOnDetach() const override { return false; }
    bool empty() const override;
    DatabaseTablesIteratorPtr getTablesIterator(ContextPtr context, const FilterByNameFunction & filter_by_table_name) const override;
    ASTPtr getCreateDatabaseQuery() const override;
    bool isTableExist(const String & name, ContextPtr context) const override;
    StoragePtr tryGetTable(const String & name, ContextPtr context) const override;
    time_t getObjectMetadataModificationTime(const String & name) const override;
    void shutdown() override;
    void drop(ContextPtr ) override;
    String getMetadataPath() const override;
    void createTable(ContextPtr, const String & table_name, const StoragePtr & storage, const ASTPtr & create_query) override;
    void loadStoredObjects(ContextMutablePtr, bool, bool force_attach, bool skip_startup_tables) override;