loom 0.2.11

Permutation testing for concurrent code
Documentation 
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#![allow(warnings)]
use crate::rt::object::Object;
use crate::rt::{self, Access, Synchronize};

use std::sync::atomic::Ordering::{Acquire, Release};

#[derive(Debug, Copy, Clone)]
pub(crate) struct Arc {
    obj: Object,
}

#[derive(Debug)]
pub(super) struct State {
    /// Reference count
    ref_cnt: usize,

    /// Causality transfers between threads
    ///
    /// Only updated on on ref dec and acquired before drop
    synchronize: Synchronize,

    /// Tracks access to the arc object
    last_ref_inc: Option<Access>,
    last_ref_dec: Option<Access>,
}

/// Actions performed on the Arc
///
/// Clones are only dependent with inspections. Drops are dependent between each
/// other.
#[derive(Debug, Copy, Clone)]
pub(super) enum Action {
    /// Clone the arc
    RefInc,

    /// Drop the Arc
    RefDec,
    /*
    /// Inspect internals (such as get ref count). This is done with SeqCst
    /// causality
    Inspect,
    */
}

impl Arc {
    pub(crate) fn new() -> Arc {
        rt::execution(|execution| {
            let obj = execution.objects.insert_arc(State {
                ref_cnt: 1,
                synchronize: Synchronize::new(execution.max_threads),
                last_ref_inc: None,
                last_ref_dec: None,
            });

            Arc { obj }
        })
    }

    pub(crate) fn ref_inc(self) {
        self.obj.branch(Action::RefInc);

        rt::execution(|execution| {
            let state = self.obj.arc_mut(&mut execution.objects);
            state.ref_cnt = state.ref_cnt.checked_add(1).expect("overflow");
        })
    }

    /// Validate a `get_mut` call
    pub(crate) fn get_mut(self) -> bool {
        self.obj.branch(Action::RefDec);

        rt::execution(|execution| {
            let state = self.obj.arc_mut(&mut execution.objects);

            assert!(state.ref_cnt >= 1, "Arc is released");

            // Synchronize the threads
            state.synchronize.sync_load(&mut execution.threads, Acquire);

            if state.ref_cnt == 1 {
                true
            } else {
                false
            }
        })
    }

    /// Returns true if the memory should be dropped.
    pub(crate) fn ref_dec(self) -> bool {
        self.obj.branch(Action::RefDec);

        rt::execution(|execution| {
            let state = self.obj.arc_mut(&mut execution.objects);

            assert!(state.ref_cnt >= 1, "Arc is already released");

            // Decrement the ref count
            state.ref_cnt -= 1;

            // Synchronize the threads.
            state
                .synchronize
                .sync_store(&mut execution.threads, Release);

            if state.ref_cnt == 0 {
                // Final ref count, the arc will be dropped. This requires
                // acquiring the causality
                //
                // In the real implementation, this is done with a fence.
                state.synchronize.sync_load(&mut execution.threads, Acquire);
                true
            } else {
                false
            }
        })
    }
}

impl State {
    pub(super) fn check_for_leaks(&self) {
        assert_eq!(0, self.ref_cnt, "Arc leaked");
    }

    pub(super) fn last_dependent_accesses<'a>(
        &'a self,
        action: Action,
    ) -> Box<dyn Iterator<Item = &'a Access> + 'a> {
        match action {
            // RefIncs are not dependent w/ RefDec, only inspections
            Action::RefInc => Box::new([].into_iter()),
            Action::RefDec => Box::new(self.last_ref_dec.iter()),
        }
    }

    pub(super) fn set_last_access(&mut self, action: Action, access: Access) {
        match action {
            Action::RefInc => self.last_ref_inc = Some(access),
            Action::RefDec => self.last_ref_dec = Some(access),
        }
    }
}