Waking the current task leads to a deadlock

[piodul]

If a Rust future is co_awaited from C++ and that future decides to wake the current task, then a deadlock will occur.

Found while trying to use futures::FuturesUnordered which wakes the current task here.

I attach a minimal example below.

main.hh:

#pragma once

#include "rust/cxx.h"
#include "cxx-async/include/rust/cxx_async.h"

CXXASYNC_DEFINE_FUTURE(void, RustFutureVoid);

RustFutureVoid run_cpp();

main.cc:

#include "main.hh"
#include "cxx-async-self-wake-bug/src/main.rs.h"

RustFutureVoid run_cpp() {
    co_await run_rust();
}

main.rs:

use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};

use crate::ffi::run_cpp;

#[cxx::bridge]
mod ffi {
    extern "Rust" {
        fn run_rust() -> RustFutureVoid;
    }
    unsafe extern "C++" {
        include!("main.hh");
        fn run_cpp() -> RustFutureVoid;
        type RustFutureVoid = crate::RustFutureVoid;
    }
}

#[cxx_async::bridge]
unsafe impl Future for RustFutureVoid {
    type Output = ();
}

pub fn run_rust() -> RustFutureVoid {
    RustFutureVoid::infallible(YieldFuture { yielded: false })
}

struct YieldFuture {
    yielded: bool,
}

impl Future for YieldFuture {
    type Output = ();

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        if !self.yielded {
            // Yield, but also wake immediately
            self.as_mut().yielded = true;
            cx.waker().wake_by_ref();
            Poll::Pending
        } else {
            Poll::Ready(())
        }
    }
}

fn main() {
    println!("Start!");
    futures::executor::block_on(run_cpp()).unwrap();
    println!("Finish!");
}

Only "Start!" is printed, then the program deadlocks.

[piodul]

This is the backtrace from the moment of the deadlock, according to gdb:

#0  futex_wait (private=0, expected=2, futex_word=0x5555555f9940) at ../sysdeps/nptl/futex-internal.h:146
#1  __GI___lll_lock_wait (futex=futex@entry=0x5555555f9940, private=0) at lowlevellock.c:49
#2  0x00007ffff7ab1412 in lll_mutex_lock_optimized (mutex=0x5555555f9940) at pthread_mutex_lock.c:48
#3  ___pthread_mutex_lock (mutex=0x5555555f9940) at pthread_mutex_lock.c:93
#4  0x00005555555869ca in __gthread_mutex_lock (__mutex=0x5555555f9940) at /usr/include/c++/12/x86_64-redhat-linux/bits/gthr-default.h:749
#5  0x0000555555586cce in std::mutex::lock (this=0x5555555f9940) at /usr/include/c++/12/bits/std_mutex.h:100
#6  0x0000555555588ce6 in std::lock_guard<std::mutex>::lock_guard (this=0x7fffffffc9b8, __m=...) at /usr/include/c++/12/bits/std_mutex.h:229
#7  0x0000555555588620 in rust::async::RustFutureReceiver<RustFutureVoid>::wake (this=0x5555555f9940, coroutine=0x5555555f99a0)
    at /home/piodul/code/cxx-async-self-wake-bug/target/debug/build/cxx-async-391fb75ee5f1fc50/out/cxxbridge/crate/cxx-async/include/rust/cxx_async.h:852
#8  0x0000555555587d9a in rust::async::RustAwaiter<RustFutureVoid>::await_suspend(std::__n4861::coroutine_handle<void>)::{lambda(rust::async::SuspendedCoroutine*)#1}::operator()(rust::async::SuspendedCoroutine*) const (__closure=0x5555555f99e0, coroutine=0x5555555f99a0)
    at /home/piodul/code/cxx-async-self-wake-bug/target/debug/build/cxx-async-391fb75ee5f1fc50/out/cxxbridge/crate/cxx-async/include/rust/cxx_async.h:879
#9  0x00005555555895c2 in std::__invoke_impl<rust::async::FutureWakeStatus, rust::async::RustAwaiter<RustFutureVoid>::await_suspend(std::__n4861::coroutine_handle<void>)::{lambda(rust::async::SuspendedCoroutine*)#1}&, rust::async::SuspendedCoroutine*>(std::__invoke_other, rust::async::RustAwaiter<RustFutureVoid>::await_suspend(std::__n4861::coroutine_handle<void>)::{lambda(rust::async::SuspendedCoroutine*)#1}&, rust::async::SuspendedCoroutine*&&) (__f=...) at /usr/include/c++/12/bits/invoke.h:61
#10 0x00005555555891cf in std::__invoke_r<rust::async::FutureWakeStatus, rust::async::RustAwaiter<RustFutureVoid>::await_suspend(std::__n4861::coroutine_handle<void>)::{lambda(rust::async::SuspendedCoroutine*)#1}&, rust::async::SuspendedCoroutine*>(rust::async::RustAwaiter<RustFutureVoid>::await_suspend(std::__n4861::coroutine_handle<void>)::{lambda(rust::async::SuspendedCoroutine*)#1}&, rust::async::SuspendedCoroutine*&&) (__fn=...) at /usr/include/c++/12/bits/invoke.h:114
#11 0x0000555555588e29 in std::_Function_handler<rust::async::FutureWakeStatus (rust::async::SuspendedCoroutine*), rust::async::RustAwaiter<RustFutureVoid>::await_suspend(std::__n4861::coroutine_handle<void>)::{lambda(rust::async::SuspendedCoroutine*)#1}>::_M_invoke(std::_Any_data const&, rust::async::SuspendedCoroutine*&&) (__functor=..., 
    __args#0=@0x7fffffffcab0: 0x5555555f99a0) at /usr/include/c++/12/bits/std_function.h:290
#12 0x0000555555587c19 in std::function<rust::async::FutureWakeStatus (rust::async::SuspendedCoroutine*)>::operator()(rust::async::SuspendedCoroutine*) const (
    this=0x5555555f99b0, __args#0=0x5555555f99a0) at /usr/include/c++/12/bits/std_function.h:591
#13 0x000055555558728f in rust::async::SuspendedCoroutine::wake (this=0x5555555f99a0)
    at /home/piodul/code/cxx-async-self-wake-bug/target/debug/build/cxx-async-391fb75ee5f1fc50/out/cxxbridge/crate/cxx-async/include/rust/cxx_async.h:688
#14 0x0000555555594bb9 in cxxasync_suspended_coroutine_wake_by_ref (ptr=0x5555555f99a0 "\003") at src/cxx_async.cpp:47
#15 0x000055555559451b in cxx_async::rust_suspended_coroutine_wake_by_ref (address=0x5555555f99a0) at src/lib.rs:892
#16 0x00005555555814c2 in core::task::wake::Waker::wake_by_ref (self=0x7fffffffcda8) at /builddir/build/BUILD/rustc-1.67.1-src/library/core/src/task/wake.rs:291
#17 0x00005555555825b3 in cxx_async_self_wake_bug::{impl#0}::poll (self=..., cx=0x7fffffffcd18) at src/main.rs:40
#18 0x0000555555584c23 in cxx_async::IntoCxxAsyncFuture::infallible::{async_block#0}<cxx_async_self_wake_bug::RustFutureVoid, cxx_async_self_wake_bug::YieldFuture> ()
    at /home/piodul/.cargo/git/checkouts/cxx-async-30622540fc81757a/c8f4b77/cxx-async/src/lib.rs:610
#19 0x00005555555832b7 in core::future::future::{impl#1}::poll<alloc::boxed::Box<(dyn core::future::future::Future<Output=core::result::Result<(), cxx_async::CxxAsyncException>> + core::marker::Send), alloc::alloc::Global>> (self=..., cx=0x7fffffffcd18) at /builddir/build/BUILD/rustc-1.67.1-src/library/core/src/future/future.rs:124
#20 0x00005555555826a1 in cxx_async_self_wake_bug::{impl#4}::poll (self=..., cx=0x7fffffffcd18) at src/main.rs:20
#21 0x00005555555846bb in cxx_async::future_poll::{closure#0}<cxx_async_self_wake_bug::RustFutureVoid, ()> ()
    at /home/piodul/.cargo/git/checkouts/cxx-async-30622540fc81757a/c8f4b77/cxx-async/src/lib.rs:835
#22 0x00005555555815f1 in core::panic::unwind_safe::{impl#23}::call_once<u32, cxx_async::future_poll::{closure_env#0}<cxx_async_self_wake_bug::RustFutureVoid, ()>> (
    self=..., _args=()) at /builddir/build/BUILD/rustc-1.67.1-src/library/core/src/panic/unwind_safe.rs:271
#23 0x0000555555583061 in std::panicking::try::do_call<core::panic::unwind_safe::AssertUnwindSafe<cxx_async::future_poll::{closure_env#0}<cxx_async_self_wake_bug::RustFutureVoid, ()>>, u32> (data=0x7fffffffcef0) at /builddir/build/BUILD/rustc-1.67.1-src/library/std/src/panicking.rs:483
#24 0x000055555558310b in __rust_try ()
#25 0x0000555555582ecc in std::panicking::try<u32, core::panic::unwind_safe::AssertUnwindSafe<cxx_async::future_poll::{closure_env#0}<cxx_async_self_wake_bug::RustFutureVoid, ()>>> (f=...) at /builddir/build/BUILD/rustc-1.67.1-src/library/std/src/panicking.rs:447
#26 0x0000555555582dd8 in std::panic::catch_unwind<core::panic::unwind_safe::AssertUnwindSafe<cxx_async::future_poll::{closure_env#0}<cxx_async_self_wake_bug::RustFutureVoid, ()>>, u32> (f=...) at /builddir/build/BUILD/rustc-1.67.1-src/library/std/src/panic.rs:137
#27 0x00005555555844fe in cxx_async::future_poll<cxx_async_self_wake_bug::RustFutureVoid, ()> (this=..., result=0x5555555f9978, waker_data=0x5555555f99a0) at /home/piodul/.cargo/git/checkouts/cxx-async-30622540fc81757a/c8f4b77/cxx-async/src/lib.rs:833
#28 0x0000555555588660 in rust::async::RustFutureReceiver<RustFutureVoid>::wake (this=0x5555555f9940, coroutine=0x5555555f99a0) at /home/piodul/code/cxx-async-self-wake-bug/target/debug/build/cxx-async-391fb75ee5f1fc50/out/cxxbridge/crate/cxx-async/include/rust/cxx_async.h:861
#29 0x0000555555587d9a in rust::async::RustAwaiter<RustFutureVoid>::await_suspend(std::__n4861::coroutine_handle<void>)::{lambda(rust::async::SuspendedCoroutine*)#1}::operator()(rust::async::SuspendedCoroutine*) const (__closure=0x5555555f99e0, coroutine=0x5555555f99a0) at /home/piodul/code/cxx-async-self-wake-bug/target/debug/build/cxx-async-391fb75ee5f1fc50/out/cxxbridge/crate/cxx-async/include/rust/cxx_async.h:879
#30 0x00005555555895c2 in std::__invoke_impl<rust::async::FutureWakeStatus, rust::async::RustAwaiter<RustFutureVoid>::await_suspend(std::__n4861::coroutine_handle<void>)::{lambda(rust::async::SuspendedCoroutine*)#1}&, rust::async::SuspendedCoroutine*>(std::__invoke_other, rust::async::RustAwaiter<RustFutureVoid>::await_suspend(std::__n4861::coroutine_handle<void>)::{lambda(rust::async::SuspendedCoroutine*)#1}&, rust::async::SuspendedCoroutine*&&) (__f=...) at /usr/include/c++/12/bits/invoke.h:61
#31 0x00005555555891cf in std::__invoke_r<rust::async::FutureWakeStatus, rust::async::RustAwaiter<RustFutureVoid>::await_suspend(std::__n4861::coroutine_handle<void>)::{lambda(rust::async::SuspendedCoroutine*)#1}&, rust::async::SuspendedCoroutine*>(rust::async::RustAwaiter<RustFutureVoid>::await_suspend(std::__n4861::coroutine_handle<void>)::{lambda(rust::async::SuspendedCoroutine*)#1}&, rust::async::SuspendedCoroutine*&&) (__fn=...) at /usr/include/c++/12/bits/invoke.h:114
#32 0x0000555555588e29 in std::_Function_handler<rust::async::FutureWakeStatus (rust::async::SuspendedCoroutine*), rust::async::RustAwaiter<RustFutureVoid>::await_suspend(std::__n4861::coroutine_handle<void>)::{lambda(rust::async::SuspendedCoroutine*)#1}>::_M_invoke(std::_Any_data const&, rust::async::SuspendedCoroutine*&&) (__functor=..., __args#0=@0x7fffffffd210: 0x5555555f99a0) at /usr/include/c++/12/bits/std_function.h:290
#33 0x0000555555587c19 in std::function<rust::async::FutureWakeStatus (rust::async::SuspendedCoroutine*)>::operator()(rust::async::SuspendedCoroutine*) const (this=0x5555555f99b0, __args#0=0x5555555f99a0) at /usr/include/c++/12/bits/std_function.h:591
#34 0x000055555558728f in rust::async::SuspendedCoroutine::wake (this=0x5555555f99a0) at /home/piodul/code/cxx-async-self-wake-bug/target/debug/build/cxx-async-391fb75ee5f1fc50/out/cxxbridge/crate/cxx-async/include/rust/cxx_async.h:688
#35 0x00005555555872aa in rust::async::SuspendedCoroutine::initial_suspend (this=0x5555555f99a0) at /home/piodul/code/cxx-async-self-wake-bug/target/debug/build/cxx-async-391fb75ee5f1fc50/out/cxxbridge/crate/cxx-async/include/rust/cxx_async.h:696
#36 0x0000555555587f0c in rust::async::RustAwaiter<RustFutureVoid>::await_suspend (this=0x5555555f97f0, next=...) at /home/piodul/code/cxx-async-self-wake-bug/target/debug/build/cxx-async-391fb75ee5f1fc50/out/cxxbridge/crate/cxx-async/include/rust/cxx_async.h:881
#37 0x00005555555876e6 in run_cpp(_Z7run_cppv.Frame *) (frame_ptr=0x5555555f97b0) at src/main.cc:5
#38 0x00005555555874dd in run_cpp () at src/main.cc:4
#39 0x00005555555867bf in cxxbridge1$run_cpp (return$=0x7fffffffd3e0) at /home/piodul/code/cxx-async-self-wake-bug/target/debug/build/cxx-async-self-wake-bug-578275989b282d27/out/cxxbridge/sources/cxx-async-self-wake-bug/src/main.rs.cc:79
#40 0x00005555555822b8 in cxx_async_self_wake_bug::ffi::run_cpp () at src/main.rs:15
#41 0x0000555555582606 in cxx_async_self_wake_bug::main () at src/main.rs:50

It looks like rust::async::RustFutureReceiver<RustFutureVoid>::wake is called recursively and it tries to acquire the same lock again.