CI found one issue [1].
Here is the stack trace for invalid read:
<details>
<summary>stack trace</summary>
```
0: DB::TemporaryFileLazySource::TemporaryFileLazySource(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, DB::Block const&) [inlined] std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__is_long(this="") const at string:1445:22
1: DB::TemporaryFileLazySource::TemporaryFileLazySource(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, DB::Block const&) [inlined] std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::basic_string(this="", __str="") at string:1927
2: DB::TemporaryFileLazySource::TemporaryFileLazySource(this=0x00007f3aec105f58, path_="", header_=0x00007f38ffd93b40) at TemporaryFileLazySource.cpp:11
3: DB::SortedBlocksWriter::streamFromFile(std::__1::unique_ptr<Poco::TemporaryFile, std::__1::default_delete<Poco::TemporaryFile> > const&) const [inlined] DB::TemporaryFileLazySource* std::__1::construct_at<DB::TemporaryFileLazySource, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, DB::Block, DB::TemporaryFileLazySource*>(__args=0x00007f38ffd91560) at construct_at.h:38:50
4: DB::SortedBlocksWriter::streamFromFile(std::__1::unique_ptr<Poco::TemporaryFile, std::__1::default_delete<Poco::TemporaryFile> > const&) const [inlined] void std::__1::allocator_traits<std::__1::allocator<DB::TemporaryFileLazySource> >::construct<DB::TemporaryFileLazySource, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, DB::Block, void, void>(__args=0x00007f38ffd91560) at allocator_traits.h:298
5: DB::SortedBlocksWriter::streamFromFile(std::__1::unique_ptr<Poco::TemporaryFile, std::__1::default_delete<Poco::TemporaryFile> > const&) const [inlined] std::__1::__shared_ptr_emplace<DB::TemporaryFileLazySource, std::__1::allocator<DB::TemporaryFileLazySource> >::__shared_ptr_emplace<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, DB::Block>(this=0x00007f3aec105f40, __args=0x00007f38ffd91560) at shared_ptr.h:293
6: DB::SortedBlocksWriter::streamFromFile(std::__1::unique_ptr<Poco::TemporaryFile, std::__1::default_delete<Poco::TemporaryFile> > const&) const [inlined] std::__1::shared_ptr<DB::TemporaryFileLazySource> std::__1::allocate_shared<DB::TemporaryFileLazySource, std::__1::allocator<DB::TemporaryFileLazySource>, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, DB::Block, void>(__args=<unavailable>, __args=<unavailable>) at shared_ptr.h:954
7: DB::SortedBlocksWriter::streamFromFile(std::__1::unique_ptr<Poco::TemporaryFile, std::__1::default_delete<Poco::TemporaryFile> > const&) const [inlined] std::__1::shared_ptr<DB::TemporaryFileLazySource> std::__1::make_shared<DB::TemporaryFileLazySource, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, DB::Block, void>(__args=<unavailable>, __args=<unavailable>) at shared_ptr.h:963
8: DB::SortedBlocksWriter::streamFromFile(this=<unavailable>, file=<unavailable>) const at SortedBlocksWriter.cpp:238
9: DB::SortedBlocksWriter::premerge(this=<unavailable>) at SortedBlocksWriter.cpp:209:32
```
</details>
[1]: https://s3.amazonaws.com/clickhouse-test-reports/41046/adea92f847373d1fcfd733d8979c63024f9b80bf/stress_test__asan_.html
So the problem here is that there was empty unique_ptr<> reference to
temporary file, because of empty block that accepted by
SortedBlocksWriter::insert(), but insert() is not a problem the problem
is premerge() that steals blocks from insert() and do not have check
that there are some rows. However this check exists in
SortedBlocksWriter::flush(), and in that case temporary file is not
created.
Signed-off-by: Azat Khuzhin <a.khuzhin@semrush.com>
