Mutex

pub const Mutex = extern struct {
    state: std.atomic.Value(State),

    pub const init: Mutex = .{ .state = .init(.unlocked) };

    pub const State = enum(u32) {
        unlocked,
        locked_once,
        contended,
    };

    pub fn tryLock(m: *Mutex) bool {
        return m.state.cmpxchgStrong(.unlocked, .locked_once, .acquire, .monotonic) == null;
    }

    pub fn lock(m: *Mutex, io: Io) Cancelable!void {
        const initial_state = m.state.cmpxchgStrong(
            .unlocked,
            .locked_once,
            .acquire,
            .monotonic,
        ) orelse {
            @branchHint(.likely);
            return;
        };
        if (initial_state == .contended) {
            try io.futexWait(State, &m.state.raw, .contended);
        }
        while (m.state.swap(.contended, .acquire) != .unlocked) {
            try io.futexWait(State, &m.state.raw, .contended);
        }
    }

    /// Same as `lock`, except does not introduce a cancelation point.
    ///
    /// For a description of cancelation and cancelation points, see `Future.cancel`.
    pub fn lockUncancelable(m: *Mutex, io: Io) void {
        const initial_state = m.state.cmpxchgStrong(
            .unlocked,
            .locked_once,
            .acquire,
            .monotonic,
        ) orelse {
            @branchHint(.likely);
            return;
        };
        if (initial_state == .contended) {
            io.futexWaitUncancelable(State, &m.state.raw, .contended);
        }
        while (m.state.swap(.contended, .acquire) != .unlocked) {
            io.futexWaitUncancelable(State, &m.state.raw, .contended);
        }
    }

    pub fn unlock(m: *Mutex, io: Io) void {
        switch (m.state.swap(.unlocked, .release)) {
            .unlocked => unreachable,
            .locked_once => {},
            .contended => {
                @branchHint(.unlikely);
                io.futexWake(State, &m.state.raw, 1);
            },
        }
    }
}