setIpcIndex

        fn setIpcIndex(s: *Storage, ipc_index: Ipc.Index) void {
            // `estimated_total_count` max int indicates the special state that
            // causes `completed_count` to be treated as a file descriptor, so
            // the order here matters.
            @atomicStore(u32, &s.completed_count, @bitCast(ipc_index), .monotonic);
            @atomicStore(u32, &s.estimated_total_count, std.math.maxInt(u32), .release); // synchronizes with acquire in `serialize`
        }