Node

const Node = struct {
    /// Only meant to be accessed indirectly via the methods supplied by this type,
    /// except if the node is owned by the thread accessing it.
    /// Must always be an even number to accommodate `resize` bit.
    size: Size,
    /// Any increase of `end_index` has to use acquire semantics;
    /// any decrease of `end_index` that invalidates (formerly) active allocations
    /// has to use release semantics.
    /// This guarantees that all accesses to memory that's about to be freed
    /// happen-before the free is published.
    /// Since `size` can only grow and never shrink, memory access depending on
    /// any `end_index` <= any `size` can never be OOB.
    end_index: usize,
    /// This field should only be accessed if the node is owned by the thread
    /// accessing it.
    next: ?*Node,

    const Size = packed struct(usize) {
        resizing: bool,
        _: @Int(.unsigned, @bitSizeOf(usize) - 1) = 0,

        fn fromInt(int: usize) Size {
            assert(int >= @sizeOf(Node));
            const size: Size = @bitCast(int);
            assert(!size.resizing);
            return size;
        }

        fn toInt(size: Size) usize {
            var int = size;
            int.resizing = false;
            return @bitCast(int);
        }

        comptime {
            assert(Size{ .resizing = true } == @as(Size, @bitCast(@as(usize, 1))));
        }
    };

    fn loadBuf(node: *Node) []u8 {
        // `size` can only ever grow, so the buffer returned by this function is
        // always valid memory.
        const size = @atomicLoad(Size, &node.size, .monotonic);
        return @as([*]u8, @ptrCast(node))[0..size.toInt()][@sizeOf(Node)..];
    }

    /// Returns allocated slice or `null` if node is already (being) resized.
    fn beginResize(node: *Node) ?[]u8 {
        const size = @atomicRmw(Size, &node.size, .Or, .{ .resizing = true }, .acquire); // syncs with release in `endResize`
        if (size.resizing) return null;
        return @as([*]u8, @ptrCast(node))[0..size.toInt()];
    }

    fn endResize(node: *Node, size: usize, prev_size: usize) void {
        assert(size >= prev_size); // nodes must not shrink
        assert(@atomicLoad(Size, &node.size, .unordered).toInt() == prev_size);
        return @atomicStore(Size, &node.size, .fromInt(size), .release); // syncs with acquire in `beginResize`
    }

    /// Not threadsafe.
    fn allocatedSliceUnsafe(node: *Node) []u8 {
        return @as([*]u8, @ptrCast(node))[0..node.size.toInt()];
    }
}