Re: [Libguestfs] [libnbd PATCH] rust: Use mio::poll instead of requiring epoll

From: Eric Blake <eblake(a)redhat.com&gt; CI shows our async handle fails to build on FreeBSD and MacOS (where epoll() is not available as a syscall, and therefore not available as a Rust crate). We can instead accomplish the same level-probing effects by doing a zero-timeout poll with mio (which defers under the hood to epoll on Linux, and kqueue on BSD). Fixes: 223a9965 ("rust: async: Create an async friendly handle type") CC: Tage Johansson <tage.j.lists(a)posteo.net&gt; Signed-off-by: Eric Blake <eblake(a)redhat.com&gt; --- rust/Cargo.toml | 3 ++- rust/src/async_handle.rs | 40 +++++++++++++++++++++++----------------- 2 files changed, 25 insertions(+), 18 deletions(-) - // Use epoll to check the current read/write availability on the fd. + // Use mio poll to check the current read/write availability on the fd. // This is needed because Tokio supports only edge-triggered // notifications but Libnbd requires level-triggered notifications. - let mut revent = epoll::Event { data: 0, events: 0 }; // Setting timeout to 0 means that it will return immediately. - epoll::wait(epfd, 0, std::slice::from_mut(&mut revent))?; - let revents = Events::from_bits(revent.events).unwrap(); - if !revents.contains(Events::EPOLLIN) { - ready_guard.clear_ready_matching(IoReady::READABLE); - } - if !revents.contains(Events::EPOLLOUT) { - ready_guard.clear_ready_matching(IoReady::WRITABLE); + // mio states that it is OS-dependent on whether a single event + // can be both readable and writable, but we survive just fine + // if we only see one direction even when both are available. + poll.registry().register( + &mut SourceFd(&fd), + Token(0), + MioInterest::READABLE | MioInterest::WRITABLE, + )?; + poll.poll(&mut events, Some(Duration::ZERO))?;

+ for event in &events { + if !event.is_readable() { + ready_guard.clear_ready_matching(IoReady::READABLE); + } + if !event.is_writable() { + ready_guard.clear_ready_matching(IoReady::WRITABLE); + } } ready_guard.retain_ready(); + poll.registry().deregister(&mut SourceFd(&fd))?;