Fix rados_shutdown hang forever when using library radosstriper

Dear list,

I have met this when I was using radosstriper C API. My program is
roughly like this:

1.  rados_striper_aio_write
2.  rados_aio_flush
3.  rados_aio_wait_for_safe
4.  rados_aio_release
5.  rados_striper_destroy
6.  rados_ioctx_destroy
7.  rados_shutdown /*Hangs here*/

In most time, this works well, But the programm occasionally
hangs forever. Output of gstack:

Thread 1 (Thread 0x7fe0afba0760 (LWP 18509)):
0 0x000000330f20822d in pthread_join () from /lib64/libpthread.so.0
1 0x000000347566cea2 in Thread::join(void**) () from /usr/lib64/librados.so.2
2 0x00000034755ac535 in librados::RadosClient::shutdown() () from /usr/lib64/librados.so.2
3 0x0000003475592269 in rados_shutdown () from /usr/lib64/librados.so.2
4 0x0000000000402349 in main ()

Thread 4 (Thread 0x7fe0ab14d700 (LWP 18541)):
0 0x000000330f20e264 in __lll_lock_wait () from /lib64/libpthread.so.0
1 0x000000330f209508 in _L_lock_854 () from /lib64/libpthread.so.0
2 0x000000330f2093d7 in pthread_mutex_lock () from /lib64/libpthread.so.0
3 0x0000003475633af1 in Mutex::Lock(bool) () from /usr/lib64/librados.so.2
4 0x00000034755abd37 in librados::RadosClient::put() () from /usr/lib64/librados.so.2
5 0x0000003475592501 in librados::Rados::shutdown() () from /usr/lib64/librados.so.2
6 0x00007fe0afbba9f7 in libradosstriper::RadosStriperImpl::CompletionData::~CompletionData() () from /usr/lib64/libradosstriper.so.1
7 0x00007fe0afbbaad9 in libradosstriper::RadosStriperImpl::WriteCompletionData::~WriteCompletionData() () from /usr/lib64/libradosstriper.so.1
8 0x00007fe0afbc1d75 in RefCountedObject::put() () from /usr/lib64/libradosstriper.so.1
9 0x00007fe0afbc224d in libradosstriper::MultiAioCompletionImpl::safe_request(long) () from /usr/lib64/libradosstriper.so.1
10 0x00000034755c5ce8 in librados::C_AioSafe::finish(int) () from /usr/lib64/librados.so.2
11 0x00000034755a0e89 in Context::complete(int) () from /usr/lib64/librados.so.2
12 0x000000347564d4c8 in Finisher::finisher_thread_entry() () from /usr/lib64/librados.so.2
13 0x000000330f2079d1 in start_thread () from /lib64/libpthread.so.0
14 0x000000330eae886d in clone () from /lib64/libc.so.6

It is obvious that librados::Rados::shutdown is not a thread-safe
function here. It will hang forever. The culprit of this is when
CompletionData is released, it will first notify
"rados_aio_wait_for_safe" to continue, and CompletionData will call
put() to release other data. But if the main thread(Thread 1 here) runs
fast enough, rados_striper_destroy will be executed before other
thread(Thread 4 here)'s releasing refcnf. In this situation, main thread
runs Rados::shutdown() while other thread runs Rados::shutdown() in the
same time.

My suggestion here is to let rados_striper_destroy block until all refcount
of RadosStriperImp became 1, and it finally release the
RadosStriperImpl.