[cfe-dev] [libc++] Patch to fix futures unit test hangs on Linux

[Howard Hinnant]

On May 24, 2012, at 11:57 PM, Andrew C. Morrow wrote:

> Hi -
> 
> I've recently been trying to get the mainline libc++ unit tests to
> pass on Linux (Ubuntu 12.04 x86_64). The following tests
> 
>    libc++ :: thread/futures/futures.promise/alloc_ctor.pass.cpp
>    libc++ :: thread/futures/futures.promise/move_assign.pass.cpp
>    libc++ :: thread/futures/futures.promise/move_ctor.pass.cpp
>    libc++ :: thread/futures/futures.shared_future/dtor.pass.cpp
>    libc++ :: thread/futures/futures.unique_future/dtor.pass.cpp
> 
> were hanging indefinitely, with stacks like this:
> 
> #0  __lll_lock_wait_private () at
> ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:93
> #1  0x00007ffff7bc8bf7 in _L_lock_21 () from
> /lib/x86_64-linux-gnu/libpthread.so.0
> #2  0x00007ffff7bc8a6e in __pthread_cond_destroy (cond=0x40e050) at
> pthread_cond_destroy.c:33
> #3  0x00007ffff7fcefdb in std::__1::__assoc_sub_state::~__assoc_sub_state() ()
>   from libc++.so.1
> #4  0x00000000004027df in
> std::__1::__assoc_sub_state_alloc<test_allocator<void>
>> ::__on_zero_shared() ()
> #5  0x00007ffff7f9529d in std::__1::__shared_count::__release_shared() ()
>   from libc++.so.1
> #6  0x00007ffff7fce93d in std::__1::promise<void>::~promise() ()
>   from libc++.so.1
> #7  0x0000000000401a88 in main ()
> 
> I noticed that in __assoc_sub_state_alloc<Alloc>::__on_zero_shared
> there are actually two explicit calls to the destructor for 'this',
> which seems wrong to me. After arbitrarily removing the call to
> this->~base(), leaving just the one call to
> this->~assoc_sub_state_alloc(), the tests listed above no longer hang
> in my environment. I've attached that edit as a patch.

Thanks Andrew!  Your fix looks right on to me.  I searched <future> for similar bugs but did not find any.  Committed revision 157476.

Howard