Currently it is possible to stupidly lose mempool data by calling the rpc "savemempool" before the previous stored mempool has been read and processed to the memory mempool.

I haven't looked at the code yet, but at minimum one ugly solution would be to have a global g_is_mempool_read_finished (or something of the short) that starts false and gets sets to true once the reading is reading processing of mempool.dat is successfully finished. The rpc savemempool checks this global on the start and returns a an error while it remains false.

{
    if (request.fHelp || request.params.size() != 0)
        throw std::runtime_error(
            "gettxoutsetinfo\n"
            "\nReturns statistics about the unspent transaction output set.\n"
            "Note this call may take some time.\n"
            "\nResult:\n"
            "{\n"
            "  \"height\":n,     (numeric) The current block height (index)\n"
            "  \"bestblock\": \"hex\",   (string) the best block hash hex\n"
            "  \"transactions\": n,      (numeric) The number of transactions\n"
            "  \"txouts\": n,            (numeric) The number of output transactions\n"
            "  \"bogosize\": n,          (numeric) A meaningless metric for UTXO set size\n"
            "  \"hash_serialized_2\": \"hash\", (string) The serialized hash\n"
            "  \"disk_size\": n,         (numeric) The estimated size of the chainstate on disk\n"
            "  \"total_amount\": x.xxx          (numeric) The total amount\n"
            "}\n"
            "\nExamples:\n"
            + HelpExampleCli("gettxoutsetinfo", "")
            + HelpExampleRpc("gettxoutsetinfo", "")