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// Copyright 2017 The Chromium OS Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
use std::fs::File;
use std::mem;
use std::ops::Deref;
use std::os::unix::io::{AsRawFd, FromRawFd, IntoRawFd, RawFd};
use std::ptr;
use libc::{c_void, dup, eventfd, read, write};
use crate::{errno_result, Result};
/// A safe wrapper around a Linux eventfd (man 2 eventfd).
///
/// An eventfd is useful because it is sendable across processes and can be used for signaling in
/// and out of the KVM API. They can also be polled like any other file descriptor.
#[derive(Debug)]
pub struct EventFd {
eventfd: File,
}
impl EventFd {
/// Creates a new blocking EventFd with an initial value of 0.
pub fn new() -> Result<EventFd> {
// This is safe because eventfd merely allocated an eventfd for our process and we handle
// the error case.
let ret = unsafe { eventfd(0, 0) };
if ret < 0 {
return errno_result();
}
// This is safe because we checked ret for success and know the kernel gave us an fd that we
// own.
Ok(EventFd {
eventfd: unsafe { File::from_raw_fd(ret) },
})
}
/// Adds `v` to the eventfd's count, blocking until this won't overflow the count.
pub fn write(&self, v: u64) -> Result<()> {
// This is safe because we made this fd and the pointer we pass can not overflow because we
// give the syscall's size parameter properly.
let ret = unsafe {
write(
self.as_raw_fd(),
&v as *const u64 as *const c_void,
mem::size_of::<u64>(),
)
};
if ret <= 0 {
return errno_result();
}
Ok(())
}
/// Blocks until the the eventfd's count is non-zero, then resets the count to zero.
pub fn read(&self) -> Result<u64> {
let mut buf: u64 = 0;
let ret = unsafe {
// This is safe because we made this fd and the pointer we pass can not overflow because
// we give the syscall's size parameter properly.
read(
self.as_raw_fd(),
&mut buf as *mut u64 as *mut c_void,
mem::size_of::<u64>(),
)
};
if ret <= 0 {
return errno_result();
}
Ok(buf)
}
/// Clones this EventFd, internally creating a new file descriptor. The new EventFd will share
/// the same underlying count within the kernel.
pub fn try_clone(&self) -> Result<EventFd> {
// This is safe because we made this fd and properly check that it returns without error.
let ret = unsafe { dup(self.as_raw_fd()) };
if ret < 0 {
return errno_result();
}
// This is safe because we checked ret for success and know the kernel gave us an fd that we
// own.
Ok(EventFd {
eventfd: unsafe { File::from_raw_fd(ret) },
})
}
}
impl AsRawFd for EventFd {
fn as_raw_fd(&self) -> RawFd {
self.eventfd.as_raw_fd()
}
}
impl FromRawFd for EventFd {
unsafe fn from_raw_fd(fd: RawFd) -> Self {
EventFd {
eventfd: File::from_raw_fd(fd),
}
}
}
impl IntoRawFd for EventFd {
fn into_raw_fd(self) -> RawFd {
self.eventfd.into_raw_fd()
}
}
/// An `EventFd` wrapper which triggers when it goes out of scope.
///
/// If the underlying `EventFd` fails to trigger during drop, a panic is triggered instead.
pub struct ScopedEvent(EventFd);
impl ScopedEvent {
/// Creates a new `ScopedEvent` which triggers when it goes out of scope.
pub fn new() -> Result<ScopedEvent> {
Ok(EventFd::new()?.into())
}
}
impl From<EventFd> for ScopedEvent {
fn from(e: EventFd) -> Self {
Self(e)
}
}
impl From<ScopedEvent> for EventFd {
fn from(scoped_event: ScopedEvent) -> Self {
// Rust doesn't allow moving out of types with a Drop implementation, so we have to use
// something that copies instead of moves. This is safe because we prevent the drop of
// `scoped_event` using `mem::forget`, so the underlying `EventFd` will not experience a
// double-drop.
let evt = unsafe { ptr::read(&scoped_event.0) };
mem::forget(scoped_event);
evt
}
}
impl Deref for ScopedEvent {
type Target = EventFd;
fn deref(&self) -> &EventFd {
&self.0
}
}
impl Drop for ScopedEvent {
fn drop(&mut self) {
self.write(1).expect("failed to trigger scoped event");
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn new() {
EventFd::new().unwrap();
}
#[test]
fn read_write() {
let evt = EventFd::new().unwrap();
evt.write(55).unwrap();
assert_eq!(evt.read(), Ok(55));
}
#[test]
fn clone() {
let evt = EventFd::new().unwrap();
let evt_clone = evt.try_clone().unwrap();
evt.write(923).unwrap();
assert_eq!(evt_clone.read(), Ok(923));
}
#[test]
fn scoped_event() {
let scoped_evt = ScopedEvent::new().unwrap();
let evt_clone: EventFd = scoped_evt.try_clone().unwrap();
drop(scoped_evt);
assert_eq!(evt_clone.read(), Ok(1));
}
#[test]
fn eventfd_from_scoped_event() {
let scoped_evt = ScopedEvent::new().unwrap();
let evt: EventFd = scoped_evt.into();
evt.write(1).unwrap();
}
}
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