Re: asio::strand: Order of execution

From: Vinícius dos Santos Oliveira (vini.ipsmaker_at_[hidden])
Date: 2025-02-11 20:12:09

I found the mistake. ASIO is *not* at fault. You guys can disregard my
previous email. There's no bug to report in ASIO's bug tracker this
time.

Em ter., 11 de fev. de 2025 às 15:17, Vinícius dos Santos Oliveira
<vini.ipsmaker_at_[hidden]> escreveu:
>
> Em seg., 30 de dez. de 2024 Ã s 11:10, Vinnie Falco via Boost
> <boost_at_[hidden]> escreveu:
> > On Mon, Dec 30, 2024 at 2:04 AM Richard Hodges via Boost <
> > boost_at_[hidden]> wrote:
> >
> > > ...execution order is not part of the contract
> > >
> >
> > Yes, it is:
> >
> > https://www.boost.org/doc/libs/1_87_0/doc/html/boost_asio/reference/io_context__strand.html#boost_asio.reference.io_context__strand.order_of_handler_invocation
>
> I think I've found a violation to these rules (which I depend on).
> However I've been failing to produce a minimal test case to send a
> proper bug report. I've exhausted my ideas for the time being, so I've
> come here to ask for help/new ideas that I could attempt.
>
> Given I've failed to produce a minimal test case, I'll have to point
> you guys to code which is larger.
>
> So here I call strand.post(a):
> https://gitlab.com/emilua/emilua/-/blob/v0.11.0/src/actor.ypp#L1038
>
> And here I call strand.post(b):
> https://gitlab.com/emilua/emilua/-/blob/v0.11.0/include/emilua/core.hpp#L1201
>
> strand.post(a) happens before strand.post(b) (I even inserted printf()
> statements locally just to make sure they really do). Therefore a()
> should happen before b(), but that's not what I've been observing. I
> observed b() happening before a() on Windows and Linux (both epoll and
> io_uring). On FreeBSD a() always happens before b(). I don't know what
> ASIO does differently in FreeBSD. Sometimes on Linux I get the desired
> behavior as well, but almost always I get the undesired behaviour. I
> think when the cache is hot I always get the undesired behavior. So
> that's the minimal test case I wrote:
>
> #include <boost/asio.hpp>
> #include <iostream>
> #include <thread>
> #include <memory>
>
> namespace asio = boost::asio;
>
> struct actor
> {
>     actor(asio::io_context& ioc, int nsenders)
>         : work_guard{ioc.get_executor()}
>         , s{ioc}
>         , nsenders{nsenders}
>     {}
>
>     const asio::io_context::strand& strand()
>     {
>         return s;
>     }
>
>     asio::executor_work_guard<asio::io_context::executor_type> work_guard;
>     asio::io_context::strand s;
>     int nsenders;
> };
>
> int main()
> {
>     std::thread t;
>     std::shared_ptr<actor> a;
>     {
>         auto ioc = std::make_shared<asio::io_context>();
>         a = std::make_shared<actor>(*ioc, 2);
>         t = std::thread{[ioc]() mutable {
>             ioc->run();
>             ioc.reset();
>         }};
>     }
>
>     std::cout << "1\n";
>     a->strand().post([a]{
>         std::cout << "2\n";
>         if (--a->nsenders == 0) {
>             a->work_guard.reset();
>         }
>     }, std::allocator<void>{});
>
>     std::cout << "a\n";
>     a->strand().post([a]{
>         std::cout << "b\n";
>         if (--a->nsenders == 0) {
>             a->work_guard.reset();
>         }
>     }, std::allocator<void>{});
>
>     a.reset();
>     t.join();
> }
>
> That's the same algorithm I use in Emilua, but now I cannot observe
> the undesired result. I've tried to insert sleep_for() in a few spots
> in an attempt to mimic the delays/overhead from LuaJIT, but they were
> not enough to reproduce the behavior I observed in Emilua. So... ideas
> on how I can make this minimal test case stress more code branches
> from ASIO?
>
> If you want to reproduce the problem locally, you can attempt the Lua
> code below:
>
> if _CONTEXT ~= 'main' then
>     local inbox = require 'inbox'
>     print(inbox:receive())
>     return
> end
>
> local actor2 = spawn_vm{
>     module = '.',
>
>     -- comment/remove inherit_context=false to make the code work
>     inherit_context = false
> }
>
> actor2:send('hello')
>
> Just run the program with:
>
> emilua path/to/program.lua
>
> The desired output would be the message "hello" printed in stdout
> (which happens very rarely on Linux, and happens every time on
> FreeBSD). The undesired output would be in the likes of:
>
> Main fiber from VM 0x796707f86380 panicked: 'Broadcast the address
> before attempting to receive on it'
> stack traceback:
>         [string "?"]: in function 'receive'
>         /home/vinipsmaker/t5.lua:3: in main chunk
>         [C]: in function ''
>         [string "?"]: in function <[string "?"]:0>
>
>
> --
> Vinícius dos Santos Oliveira
> https://vinipsmaker.github.io/

-- 
Vinícius dos Santos Oliveira
https://vinipsmaker.github.io/