[libcxx] [test] Fix spurious failures in the thread detach test on Windows

[mstorsjo]

Summary

Make sure that the detached thread has started up before exiting
the process.

If the detached thread hasn't started up at all, and the main thread
exists, global data structures in the process are torn down, which
then can cause crashes when the thread starts up late after required
mutexes have been destroyed. (In particular, the mutex used internally
in _Init_thread_header, which is used in the intialization of
thread_local_data()::p, can cause crashes if the main thread already
has finished and progressed far with destruction.)

[ldionne]

Thanks for fixing!

libcxx/test/std/thread/thread.threads/thread.thread.class/thread.thread.member/detach.pass.cpp

89

Can you add a comment?

[mstorsjo]

Added a comment.

libcxx/test/std/thread/thread.threads/thread.thread.class/thread.thread.member/detach.pass.cpp

void operator()()
         op_run = true;
         done_ = true;
     }
 };
 
 int G::n_alive = 0;
 bool G::op_run = false;
 
-void foo() {}
+void foo() { done = true; }
 
 int main(int, char**)
 {
     {
         G g;
         std::thread t0 = support::make_test_thread(g);
         assert(t0.joinable());
         t0.detach();
         assert(!t0.joinable());
         while (!done) {}
         assert(G::op_run);
         assert(G::n_alive == 1);
     }
     assert(G::n_alive == 0);
+    done = false;
 #ifndef TEST_HAS_NO_EXCEPTIONS
     {
         std::thread t0 = support::make_test_thread(foo);
         assert(t0.joinable());
         t0.detach();
         assert(!t0.joinable());
         try {
             t0.detach();
         } catch (std::system_error const&) {
         }
+        // Wait to make sure that the detached thread has started up.
+        // Without this, we could exit main and start destructing global
+        // resources that are needed when the thread starts up, while the
+        // detached thread would start up only later.
+        while (!done) {}
     }
 #endif
 
   return 0;
 }