Re: [Libguestfs] [libnbd PATCH 2/2] rust: Use mio::poll instead of requiring epoll

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).

Draining readiness Once a readiness event is received, the corresponding operation must be performed repeatedly until it returns variant std::io::error::ErrorKind::WouldBlock. Unless this is done, there is no guarantee that another readiness event will be delivered, even if further data is received for the event source.

Fixes: 223a9965 ("rust: async: Create an async friendly handle type") CC: Tage Johansson&lt;tage.j.lists(a)posteo.net&gt; Signed-off-by: Eric Blake&lt;eblake(a)redhat.com&gt; --- rust/Cargo.toml | 2 +- rust/src/async_handle.rs | 46 +++++++++++++++++++++++++--------------- 2 files changed, 30 insertions(+), 18 deletions(-) diff --git a/rust/Cargo.toml b/rust/Cargo.toml index 0879b34c..391c80e9 100644 --- a/rust/Cargo.toml +++ b/rust/Cargo.toml @@ -49,7 +49,7 @@ thiserror = "1.0.40" log = { version = "0.4.19", optional = true } libc = "0.2.147" tokio = { optional = true, version = "1.29.1", default-features = false, features = ["rt", "sync", "net"] } -epoll = "4.3.3" +mio = "0.8.0" [features] default = ["log", "tokio"] diff --git a/rust/src/async_handle.rs b/rust/src/async_handle.rs index ab28b910..35b1c0d2 100644 --- a/rust/src/async_handle.rs +++ b/rust/src/async_handle.rs @@ -35,9 +35,11 @@ use crate::sys; use crate::Handle; use crate::{Error, FatalErrorKind, Result}; use crate::{AIO_DIRECTION_BOTH, AIO_DIRECTION_READ, AIO_DIRECTION_WRITE}; -use epoll::Events; +use mio::unix::SourceFd; +use mio::{Events, Interest as MioInterest, Poll, Token}; use std::sync::Arc; use std::sync::Mutex; +use std::time::Duration; use tokio::io::{unix::AsyncFd, Interest, Ready as IoReady}; use tokio::sync::Notify; use tokio::task; @@ -176,12 +178,12 @@ async fn polling_task(handle_data: &HandleData) -> Result<(), FatalErrorKind> { } = handle_data; let fd = handle.aio_get_fd().map_err(Error::to_fatal)?; let tokio_fd = AsyncFd::new(fd)?; - let epfd = epoll::create(false)?; - epoll::ctl( - epfd, - epoll::ControlOptions::EPOLL_CTL_ADD, - fd, - epoll::Event::new(Events::EPOLLIN | Events::EPOLLOUT, 42), + let mut events = Events::with_capacity(1); + let mut poll = Poll::new()?; + poll.registry().register( + &mut SourceFd(&fd), + Token(0), + MioInterest::READABLE | MioInterest::WRITABLE, )?; // The following loop does approximately the following things: @@ -248,19 +250,29 @@ async fn polling_task(handle_data: &HandleData) -> Result<(), FatalErrorKind> { } drop(pending_cmds_lock); - // 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. + match poll.poll(&mut events, Some(Duration::ZERO)) { + Ok(_) => { + 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); + } + } + } + Err(_) => { + ready_guard.clear_ready_matching(IoReady::READABLE); + ready_guard.clear_ready_matching(IoReady::WRITABLE); + } + }; ready_guard.retain_ready(); } }