//! The `Pid` type.

#![allow(unsafe_code)]

use core::num::NonZeroI32;

/// A process identifier as a raw integer.

pub type RawPid = i32;

/// `pid_t`—A non-zero Unix process ID.

///

/// This is a pid, and not a pidfd. It is not a file descriptor, and the

/// process it refers to could disappear at any time and be replaced by

/// another, unrelated, process.

///

/// On Linux, `Pid` values are also used to identify threads.

#[repr(transparent)]

#[derive(Copy, Clone, Eq, PartialEq, Debug, Hash)]

pub struct Pid(NonZeroI32);

impl Pid {

    /// A `Pid` corresponding to the init process (pid 1).

    pub const INIT: Self = Self(match NonZeroI32::new(1) {

        Some(n) => n,

        None => panic!("unreachable"),

    });

    /// Converts a `RawPid` into a `Pid`.

    ///

    /// Returns `Some` for positive values, and `None` for zero values.

    ///

    /// This is safe because a `Pid` is a number without any guarantees for the

    /// kernel. Non-child `Pid`s are always racy for any syscalls, but can only

    /// cause logic errors. If you want race-free access to or control of

    /// non-child processes, please consider other mechanisms like [pidfd] on

    /// Linux.

    ///

    /// Passing a negative number doesn't invoke undefined behavior, but it

    /// may cause unexpected behavior.

    ///

    /// [pidfd]: https://man7.org/linux/man-pages/man2/pidfd_open.2.html

    #[inline]

    pub const fn from_raw(raw: RawPid) -> Option<Self> {

        debug_assert!(raw >= 0);

        match NonZeroI32::new(raw) {

            Some(non_zero) => Some(Self(non_zero)),

            None => None,

        }

    }

    /// Converts a known positive `RawPid` into a `Pid`.

    ///

    /// Passing a negative number doesn't invoke undefined behavior, but it

    /// may cause unexpected behavior.

    ///

    /// # Safety

    ///

    /// The caller must guarantee `raw` is non-zero.

    #[inline]

    pub const unsafe fn from_raw_unchecked(raw: RawPid) -> Self {

        debug_assert!(raw > 0);

        Self(NonZeroI32::new_unchecked(raw))

    }

    /// Creates a `Pid` holding the ID of the given child process.

    #[cfg(feature = "std")]

    #[inline]

    pub fn from_child(child: &std::process::Child) -> Self {

        let id = child.id();

        // SAFETY: We know the returned ID is valid because it came directly

        // from an OS API.

        unsafe { Self::from_raw_unchecked(id as i32) }

    }

    /// Converts a `Pid` into a `NonZeroI32`.

    #[inline]

    pub const fn as_raw_nonzero(self) -> NonZeroI32 {

        self.0

    }

    /// Converts an `Option<Pid>` into a `RawPid`.

    #[inline]

    pub const fn as_raw(pid: Option<Self>) -> RawPid {

        match pid {

            Some(pid) => pid.0.get(),

            None => 0,

        }

    }

    /// Test whether this pid represents the init process ([`Pid::INIT`]).

    #[inline]

    pub const fn is_init(self) -> bool {

        self.0.get() == Self::INIT.0.get()

    }

}

#[cfg(test)]

mod tests {

    use super::*;

    #[test]

    fn test_sizes() {

        use core::mem::transmute;

        assert_eq_size!(RawPid, NonZeroI32);

        assert_eq_size!(RawPid, Pid);

        assert_eq_size!(RawPid, Option<Pid>);

        // Rustix doesn't depend on `Option<Pid>` matching the ABI of a raw integer

        // for correctness, but it should work nonetheless.

        const_assert_eq!(0 as RawPid, unsafe {

            transmute::<Option<Pid>, RawPid>(None)

        });

        const_assert_eq!(4567 as RawPid, unsafe {

            transmute::<Option<Pid>, RawPid>(Some(Pid::from_raw_unchecked(4567)))

        });

    }

    #[test]

    fn test_ctors() {

        use std::num::NonZeroI32;

        assert!(Pid::from_raw(0).is_none());

        assert_eq!(

            Pid::from_raw(77).unwrap().as_raw_nonzero(),

            NonZeroI32::new(77).unwrap()

        );

        assert_eq!(Pid::as_raw(Pid::from_raw(77)), 77);

    }

    #[test]

    fn test_specials() {

        assert!(Pid::from_raw(1).unwrap().is_init());

        assert_eq!(Pid::from_raw(1).unwrap(), Pid::INIT);

    }

}