Fix TSAN data race in ThreadLocalDetailTest.MultiThreadedTest

Summary:
ThreadLocalDetailTest.MultiThreadedTest had a data race on std::vector because multiple threads were accessing and modifying the std::vector without a mutex. I update the code to use indexes such that the vector is not updated concurrently.

Previous run failure that this fixes:

```
stderr:
==================
WARNING: ThreadSanitizer: data race (pid=1249005)
  Write of size 8 at 0x7fff195936d0 by main thread:
    #0 std::vector<std::unique_ptr<folly::test::Barrier, std::default_delete<folly::test::Barrier>>, std::allocator<std::unique_ptr<folly::test::Barrier, std::default_delete<folly::test::Barrier>>>>::pop_back() fbcode/third-party-buck/platform010/build/libgcc/include/c++/trunk/bits/stl_vector.h:1228 (thread_local_detail_test+0x116c28)
    #1 folly::threadlocal_detail::ThreadLocalDetailTest_MultiThreadedTest_Test::TestBody() fbcode/folly/detail/test/ThreadLocalDetailTest.cpp:121 (thread_local_detail_test+0x101aba)
    #2 void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) fbsource/src/gtest.cc:2675 (libthird-party_googletest_1.14.0_gtest.so+0xacf3c)
    #3 testing::Test::Run() fbsource/src/gtest.cc:2692 (libthird-party_googletest_1.14.0_gtest.so+0xacb9c)
    #4 testing::TestInfo::Run() fbsource/src/gtest.cc:2841 (libthird-party_googletest_1.14.0_gtest.so+0xafa4a)
    #5 testing::TestSuite::Run() fbsource/src/gtest.cc:3020 (libthird-party_googletest_1.14.0_gtest.so+0xb4303)
    #6 testing::internal::UnitTestImpl::RunAllTests() fbsource/src/gtest.cc:5925 (libthird-party_googletest_1.14.0_gtest.so+0xd105e)
    facebook#7 bool testing::internal::HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) fbsource/src/gtest.cc:2675 (libthird-party_googletest_1.14.0_gtest.so+0xd08f1)
    facebook#8 testing::UnitTest::Run() fbsource/src/gtest.cc:5489 (libthird-party_googletest_1.14.0_gtest.so+0xd037e)
    facebook#9 RUN_ALL_TESTS() fbsource/gtest/gtest.h:2317 (libcommon_gtest_light_main.so+0x22a7)
    facebook#10 main fbcode/common/gtest/LightMain.cpp:20 (libcommon_gtest_light_main.so+0x2131)

  Previous read of size 8 at 0x7fff195936d0 by thread T123:
    #0 std::vector<std::unique_ptr<folly::test::Barrier, std::default_delete<folly::test::Barrier>>, std::allocator<std::unique_ptr<folly::test::Barrier, std::default_delete<folly::test::Barrier>>>>::size() const fbcode/third-party-buck/platform010/build/libgcc/include/c++/trunk/bits/stl_vector.h:919 (thread_local_detail_test+0x11b0bd)
    #1 std::vector<std::unique_ptr<folly::test::Barrier, std::default_delete<folly::test::Barrier>>, std::allocator<std::unique_ptr<folly::test::Barrier, std::default_delete<folly::test::Barrier>>>>::operator[](unsigned long) fbcode/third-party-buck/platform010/build/libgcc/include/c++/trunk/bits/stl_vector.h:1045 (thread_local_detail_test+0x11d101)
    #2 folly::threadlocal_detail::ThreadLocalDetailTest_MultiThreadedTest_Test::TestBody()::$_0::operator()() const fbcode/folly/detail/test/ThreadLocalDetailTest.cpp:97 (thread_local_detail_test+0x11d08c)
    #3 void std::__invoke_impl<void, folly::threadlocal_detail::ThreadLocalDetailTest_MultiThreadedTest_Test::TestBody()::$_0>(std::__invoke_other, folly::threadlocal_detail::ThreadLocalDetailTest_MultiThreadedTest_Test::TestBody()::$_0&&) fbcode/third-party-buck/platform010/build/libgcc/include/c++/trunk/bits/invoke.h:61 (thread_local_detail_test+0x11cf95)
    #4 std::__invoke_result<folly::threadlocal_detail::ThreadLocalDetailTest_MultiThreadedTest_Test::TestBody()::$_0>::type std::__invoke<folly::threadlocal_detail::ThreadLocalDetailTest_MultiThreadedTest_Test::TestBody()::$_0>(folly::threadlocal_detail::ThreadLocalDetailTest_MultiThreadedTest_Test::TestBody()::$_0&&) fbcode/third-party-buck/platform010/build/libgcc/include/c++/trunk/bits/invoke.h:96 (thread_local_detail_test+0x11cf05)
    #5 void std::thread::_Invoker<std::tuple<folly::threadlocal_detail::ThreadLocalDetailTest_MultiThreadedTest_Test::TestBody()::$_0>>::_M_invoke<0ul>(std::_Index_tuple<0ul>) fbcode/third-party-buck/platform010/build/libgcc/include/c++/trunk/bits/std_thread.h:253 (thread_local_detail_test+0x11cebd)
    #6 std::thread::_Invoker<std::tuple<folly::threadlocal_detail::ThreadLocalDetailTest_MultiThreadedTest_Test::TestBody()::$_0>>::operator()() fbcode/third-party-buck/platform010/build/libgcc/include/c++/trunk/bits/std_thread.h:260 (thread_local_detail_test+0x11ce65)
    facebook#7 std::thread::_State_impl<std::thread::_Invoker<std::tuple<folly::threadlocal_detail::ThreadLocalDetailTest_MultiThreadedTest_Test::TestBody()::$_0>>>::_M_run() fbcode/third-party-buck/platform010/build/libgcc/include/c++/trunk/bits/std_thread.h:211 (thread_local_detail_test+0x11cd79)
    facebook#8 execute_native_thread_routine /home/engshare/third-party2/libgcc/11.x/src/gcc-11.x/x86_64-facebook-linux/libstdc++-v3/src/c++11/../../../.././libstdc++-v3/src/c++11/thread.cc:82:18 (libstdc++.so.6+0xdf4e4) (BuildId: 452d1cdae868baeeb2fdf1ab140f1c219bf50c6e)

  Location is stack of main thread.

  Location is global '??' at 0x7fff19575000 ([stack]+0x1e6d0)

  Thread T123 (tid=1249233, running) created by main thread at:
    #0 pthread_create <null> (thread_local_detail_test+0x156bef)
    #1 __gthread_create /home/engshare/third-party2/libgcc/11.x/src/gcc-11.x/x86_64-facebook-linux/libstdc++-v3/include/x86_64-facebook-linux/bits/gthr-default.h:663:35 (libstdc++.so.6+0xdf80e) (BuildId: 452d1cdae868baeeb2fdf1ab140f1c219bf50c6e)
    #2 std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State>>, void (*)()) /home/engshare/third-party2/libgcc/11.x/src/gcc-11.x/x86_64-facebook-linux/libstdc++-v3/src/c++11/../../../.././libstdc++-v3/src/c++11/thread.cc:147:37 (libstdc++.so.6+0xdf80e)
    #3 folly::threadlocal_detail::ThreadLocalDetailTest_MultiThreadedTest_Test::TestBody() fbcode/folly/detail/test/ThreadLocalDetailTest.cpp:93 (thread_local_detail_test+0x1015cd)
    #4 void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) fbsource/src/gtest.cc:2675 (libthird-party_googletest_1.14.0_gtest.so+0xacf3c)
    #5 testing::Test::Run() fbsource/src/gtest.cc:2692 (libthird-party_googletest_1.14.0_gtest.so+0xacb9c)
    #6 testing::TestInfo::Run() fbsource/src/gtest.cc:2841 (libthird-party_googletest_1.14.0_gtest.so+0xafa4a)
    facebook#7 testing::TestSuite::Run() fbsource/src/gtest.cc:3020 (libthird-party_googletest_1.14.0_gtest.so+0xb4303)
    facebook#8 testing::internal::UnitTestImpl::RunAllTests() fbsource/src/gtest.cc:5925 (libthird-party_googletest_1.14.0_gtest.so+0xd105e)
    facebook#9 bool testing::internal::HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) fbsource/src/gtest.cc:2675 (libthird-party_googletest_1.14.0_gtest.so+0xd08f1)
    facebook#10 testing::UnitTest::Run() fbsource/src/gtest.cc:5489 (libthird-party_googletest_1.14.0_gtest.so+0xd037e)
    facebook#11 RUN_ALL_TESTS() fbsource/gtest/gtest.h:2317 (libcommon_gtest_light_main.so+0x22a7)
    facebook#12 main fbcode/common/gtest/LightMain.cpp:20 (libcommon_gtest_light_main.so+0x2131)

ThreadSanitizer: data race fbcode/third-party-buck/platform010/build/libgcc/include/c++/trunk/bits/stl_vector.h:1228 in std::vector<std::unique_ptr<folly::test::Barrier, std::default_delete<folly::test::Barrier>>, std::allocator<std::unique_ptr<folly::test::Barrier, std::default_delete<folly::test::Barrier>>>>::pop_back()
==================
ThreadSanitizer: reported 1 warnings
```

Reviewed By: yfeldblum

Differential Revision: D56643303

fbshipit-source-id: cc364817ae79bc6418cc07faa35e1bcd21830c66

Author: skrueger

folly/detail/test/ThreadLocalDetailTest.cpp

@@ -85,11 +85,11 @@ TEST_F(ThreadLocalDetailTest, MultiThreadedTest) {
   ASSERT_GE(meta.totalElementWrappers_.load(), count);
 
   std::vector<std::thread> threads;
-  std::vector<std::unique_ptr<test::Barrier>> threadBarriers;
+  std::vector<std::unique_ptr<test::Barrier>> threadBarriers(count);
   test::Barrier allThreadsBarriers{count + 1};
 
   for (int32_t i = 0; i < count; ++i) {
-    threadBarriers.push_back(std::make_unique<test::Barrier>(2));
+    threadBarriers[i] = std::make_unique<test::Barrier>(2);
     threads.push_back(std::thread([&, index = i]() {
       // This thread's vector will sized to have index elements at least.
       *helper.elements[index] = index;
@@ -112,13 +112,14 @@ TEST_F(ThreadLocalDetailTest, MultiThreadedTest) {
 
   auto upperBound = [](int32_t numThreads) { return (numThreads + 2) * count; };
 
+  int32_t threadBarriersIndex = count - 1;
   while (!threads.empty()) {
     ASSERT_GE(meta.totalElementWrappers_.load(), lowerBound(threads.size()));
     ASSERT_LE(meta.totalElementWrappers_.load(), upperBound(threads.size()));
-    threadBarriers.back()->wait();
+    threadBarriers[threadBarriersIndex]->wait();
     threads.back().join();
     threads.pop_back();
-    threadBarriers.pop_back();
+    threadBarriersIndex -= 1;
   }
 }