Fixed bug with declaring S3 disk at root of bucket

[excitoon]

I hereby agree to the terms of the CLA available at: https://yandex.ru/legal/cla/?lang=en

Changelog category (leave one):

• Bug Fix

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Fixed bug with declaring S3 disk at root of bucket. Earlier, it reported an error:

[heather] 2021.05.10 02:11:11.932234 [ 72790 ] {2ff80b7b-ec53-41cb-ac35-19bb390e1759} <Error> executeQuery: Code: 36, e.displayText() = DB::Exception: Key name is empty in path style S3 URI:  (http://172.17.0.2/bucket/) (version 21.6.1.1) (from 127.0.0.1:47994) (in query: SELECT policy_name FROM system.storage_policies), 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) @ 0x1118c68c in /home/vladimir/ClickHouse-bin/programs/clickhouse
1. DB::Exception::Exception(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, int, bool) @ 0x8b18d3a in /home/vladimir/ClickHouse-bin/programs/clickhouse
2. DB::S3::URI::URI(Poco::URI const&) @ 0xe2e3f55 in /home/vladimir/ClickHouse-bin/programs/clickhouse
3. std::__1::shared_ptr<DB::IDisk> std::__1::__function::__policy_invoker<std::__1::shared_ptr<DB::IDisk> (std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, Poco::Util::AbstractConfiguration const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::shared_ptr<DB::Context const>)>::__call_impl<std::__1::__function::__default_alloc_func<DB::registerDiskS3(DB::DiskFactory&)::$_1, std::__1::shared_ptr<DB::IDisk> (std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, Poco::Util::AbstractConfiguration const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::shared_ptr<DB::Context const>)> >(std::__1::__function::__policy_storage const*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, Poco::Util::AbstractConfiguration const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::shared_ptr<DB::Context const>&&) @ 0xe7aea43 in /home/vladimir/ClickHouse-bin/programs/clickhouse
4. DB::DiskFactory::create(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, Poco::Util::AbstractConfiguration const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::shared_ptr<DB::Context const>) const @ 0xe7a7dca in /home/vladimir/ClickHouse-bin/programs/clickhouse
5. DB::DiskSelector::DiskSelector(Poco::Util::AbstractConfiguration const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::shared_ptr<DB::Context const>) @ 0xe7a4a04 in /home/vladimir/ClickHouse-bin/programs/clickhouse
6. void std::__1::allocator<DB::DiskSelector>::construct<DB::DiskSelector, Poco::Util::AbstractConfiguration const&, char const* const&, std::__1::shared_ptr<DB::Context const> >(DB::DiskSelector*, Poco::Util::AbstractConfiguration const&, char const* const&, std::__1::shared_ptr<DB::Context const>&&) @ 0xe83d29f in /home/vladimir/ClickHouse-bin/programs/clickhouse
7. DB::Context::getDiskSelector(std::__1::lock_guard<std::__1::mutex>&) const @ 0xe826797 in /home/vladimir/ClickHouse-bin/programs/clickhouse
8. DB::Context::getStoragePolicySelector(std::__1::lock_guard<std::__1::mutex>&) const @ 0xe81b6fd in /home/vladimir/ClickHouse-bin/programs/clickhouse
9. DB::Context::getPoliciesMap() const @ 0xe826af3 in /home/vladimir/ClickHouse-bin/programs/clickhouse
10. DB::StorageSystemStoragePolicies::read(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&, std::__1::shared_ptr<DB::StorageInMemoryMetadata const> const&, DB::SelectQueryInfo&, std::__1::shared_ptr<DB::Context>, DB::QueryProcessingStage::Enum, unsigned long, unsigned int) @ 0xe3f070f in /home/vladimir/ClickHouse-bin/programs/clickhouse
11. DB::IStorage::read(DB::QueryPlan&, 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&, std::__1::shared_ptr<DB::StorageInMemoryMetadata const> const&, DB::SelectQueryInfo&, std::__1::shared_ptr<DB::Context>, DB::QueryProcessingStage::Enum, unsigned long, unsigned int) @ 0xef3b1ae in /home/vladimir/ClickHouse-bin/programs/clickhouse
12. DB::InterpreterSelectQuery::executeFetchColumns(DB::QueryProcessingStage::Enum, DB::QueryPlan&) @ 0xeb0f9ea in /home/vladimir/ClickHouse-bin/programs/clickhouse
13. DB::InterpreterSelectQuery::executeImpl(DB::QueryPlan&, std::__1::shared_ptr<DB::IBlockInputStream> const&, std::__1::optional<DB::Pipe>) @ 0xeb08799 in /home/vladimir/ClickHouse-bin/programs/clickhouse
14. DB::InterpreterSelectQuery::buildQueryPlan(DB::QueryPlan&) @ 0xeb0805b in /home/vladimir/ClickHouse-bin/programs/clickhouse
15. DB::InterpreterSelectWithUnionQuery::buildQueryPlan(DB::QueryPlan&) @ 0xec53865 in /home/vladimir/ClickHouse-bin/programs/clickhouse
16. DB::InterpreterSelectWithUnionQuery::execute() @ 0xec545bf in /home/vladimir/ClickHouse-bin/programs/clickhouse
17. DB::executeQueryImpl(char const*, char const*, std::__1::shared_ptr<DB::Context>, bool, DB::QueryProcessingStage::Enum, bool, DB::ReadBuffer*) @ 0xedae753 in /home/vladimir/ClickHouse-bin/programs/clickhouse
18. DB::executeQuery(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::shared_ptr<DB::Context>, bool, DB::QueryProcessingStage::Enum, bool) @ 0xedad41a in /home/vladimir/ClickHouse-bin/programs/clickhouse
19. DB::TCPHandler::runImpl() @ 0xf3d69d9 in /home/vladimir/ClickHouse-bin/programs/clickhouse
20. DB::TCPHandler::run() @ 0xf3e22d9 in /home/vladimir/ClickHouse-bin/programs/clickhouse
21. Poco::Net::TCPServerConnection::start() @ 0x110ff8c7 in /home/vladimir/ClickHouse-bin/programs/clickhouse
22. Poco::Net::TCPServerDispatcher::run() @ 0x110ffd5c in /home/vladimir/ClickHouse-bin/programs/clickhouse
23. Poco::PooledThread::run() @ 0x111e74f9 in /home/vladimir/ClickHouse-bin/programs/clickhouse
24. Poco::ThreadImpl::runnableEntry(void*) @ 0x111e5206 in /home/vladimir/ClickHouse-bin/programs/clickhouse
25. start_thread @ 0x9450 in /usr/lib/x86_64-linux-gnu/libpthread-2.33.so
26. __clone @ 0x117d53 in /usr/lib/x86_64-linux-gnu/libc-2.33.so

on SELECT policy_name FROM system.storage_policies.

Detailed description / Documentation draft:

...

By adding documentation, you'll allow users to try your new feature immediately, not when someone else will have time to document it later. Documentation is necessary for all features that affect user experience in any way. You can add brief documentation draft above, or add documentation right into your patch as Markdown files in docs folder.

If you are doing this for the first time, it's recommended to read the lightweight Contributing to ClickHouse Documentation guide first.

Information about CI checks: https://clickhouse.tech/docs/en/development/continuous-integration/