[Bug libstdc++/60966] std::call_once sometime hangs

[thomas.sanchz at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60966

--- Comment #4 from Thomas Sanchez <thomas.sanchz at gmail dot com> ---
(In reply to Jonathan Wakely from comment #3)
> > > N.B. the std::move in Manager::cancelConnection is redundant, the return
> > > value from cancel_connection is already an rvalue.
> > 
> > Yes, I know, the problem is I have some hard time to avoid doing this as it
> > is more for me and what I expect the compiler to do or the semantics I want
> > the code to have :)
> 
> OK, but it makes the code worse. Here the compiler will elide the move
> constructor (aka return value optimisation):
> 
>   auto future = cancel_connection(c);
> 
> Whereas here the compiler cannot do that and must call a move constructor:
> 
>   auto future = std::move(cancel_connection(c));

I did not meant to say I should continue, far from here :) Thank for the
notice!

Anyway, I pushed the actual code in this branch:
https://github.com/daedric/httpp/tree/bug-promise-test

I was able to do a small test that can *sometimes* reproduce the bug:
https://github.com/daedric/httpp/blob/bug-promise-test/tests/client/promise.cpp


Since I was able to trigger the problem, here are what you asked for:
>From the get side, (from here
https://github.com/daedric/httpp/blob/bug-promise-test/src/httpp/http/client/Manager.cpp#L113)

(gdb) print future
$1 = {
  <std::__basic_future<void>> = {
    <std::__future_base> = {<No data fields>}, 
    members of std::__basic_future<void>: 
    _M_state = std::shared_ptr (count 2, weak 0) 0x805468
  }, <No data fields>}
(gdb) print promise
$2 = {
  _M_future = std::shared_ptr (count 2, weak 0) 0x805468, 
  _M_storage = std::unique_ptr<std::__future_base::_Result<void>> containing
0x805780
}
(gdb) print &promise
$3 = (std::promise<void> *) 0x7fffffffce90


>From the thread calling the pthread_once:

#4  0x0000000000500d67 in std::promise<void>::set_value (this=0x7fffffffce90)
at /usr/include/c++/4.8/future:1197
1197        _M_future->_M_set_result(std::move(__setter));
(gdb) print *this
$4 = {
  _M_future = std::shared_ptr (count 2, weak 0) 0x805468, 
  _M_storage = std::unique_ptr<std::__future_base::_Result<void>> containing
0x805780
}

This is seems to be correct.
This has been produced with commit: 850ebba9a72d102b54de6912e820889618a4f30d

I'm testing now with the lambdas replaced :)





http://reproducible.io/