Summary:
The patch adds a resource management/RAII class called `ThreadGuard`,
which can be used to ensure that the managed thread is joined when the
`ThreadGuard` is destroyed, regardless of whether it is due to the
object going out of scope, an early return, an exception etc. This is
important because if an `std::thread` object is destroyed without having
been joined (or detached) first, the process is aborted (via
`std::terminate`).
For now, `ThreadGuard` is only used in the test case
`ExternalSSTFileTest.PickedLevelBug`; however, it could come in handy
elsewhere in the codebase as well (both in test code and "real" code).
Case in point: in the `PickedLevelBug` test case, with the earlier code we
could end up in the above situation when the following assertion (which is
before the threads are joined) is triggered:
```
ASSERT_FALSE(bg_compact_started.load());
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/8112
Test Plan:
```
make check
gtest-parallel --repeat=10000 ./external_sst_file_test --gtest_filter="*PickedLevelBug"
```
Reviewed By: riversand963
Differential Revision: D27343185
Pulled By: ltamasi
fbshipit-source-id: 2a8c3aa68bc78cc03ec0dbae909fb25c2cd15c69
