// Copyright 2019 Intel Corporation. All Rights Reserved.
// SPDX-License-Identifier: Apache-2.0
//
// 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-BSD-3-Clause file.

use std::fs::File;
use std::os::unix::io::{AsRawFd, FromRawFd, RawFd};
use std::{io, mem, result};

use libc::{c_void, dup, eventfd, read, write};

/// A safe wrapper around a Linux eventfd (man 2 eventfd).
pub struct EventFd {
    eventfd: File,
}

impl EventFd {
    /// Creates a new blocking EventFd with an initial value.
    ///
    /// `flag`: The initial value. Refer to Linux eventfd(2).
    pub fn new(flag: i32) -> result::Result<EventFd, io::Error> {
        // This is safe because eventfd merely allocated an eventfd for
        // our process and we handle the error case.
        let ret = unsafe { eventfd(0, flag) };
        if ret < 0 {
            Err(io::Error::last_os_error())
        } else {
            // 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, does not block if the result will overflow the count
    pub fn write(&self, v: u64) -> result::Result<(), io::Error> {
        // 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 {
            Err(io::Error::last_os_error())
        } else {
            Ok(())
        }
    }

    /// Tries to read from the eventfd, does not block if the counter is zero
    pub fn read(&self) -> result::Result<u64, io::Error> {
        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 {
            Err(io::Error::last_os_error())
        } else {
            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::Result<EventFd, io::Error> {
        // 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 {
            Err(io::Error::last_os_error())
        } else {
            // 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),
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use libc::EFD_NONBLOCK;

    #[test]
    fn test_new() {
        EventFd::new(EFD_NONBLOCK).unwrap();
        EventFd::new(0).unwrap();
    }

    #[test]
    fn test_read_write() {
        let evt = EventFd::new(EFD_NONBLOCK).unwrap();
        evt.write(55).unwrap();
        assert_eq!(evt.read().unwrap(), 55);
    }

    #[test]
    fn test_write_overflow() {
        let evt = EventFd::new(EFD_NONBLOCK).unwrap();
        evt.write(std::u64::MAX - 1).unwrap();
        let r = evt.write(1);
        match r {
            Err(ref inner) if inner.kind() == io::ErrorKind::WouldBlock => (),
            _ => panic!("Unexpected"),
        }
    }
    #[test]
    fn test_read_nothing() {
        let evt = EventFd::new(EFD_NONBLOCK).unwrap();
        let r = evt.read();
        match r {
            Err(ref inner) if inner.kind() == io::ErrorKind::WouldBlock => (),
            _ => panic!("Unexpected"),
        }
    }
    #[test]
    fn test_clone() {
        let evt = EventFd::new(EFD_NONBLOCK).unwrap();
        let evt_clone = evt.try_clone().unwrap();
        evt.write(923).unwrap();
        assert_eq!(evt_clone.read().unwrap(), 923);
    }
}