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.

static const bool DEFAULT_PROXYRANDOMIZE = true;
static const bool DEFAULT_REST_ENABLE = false;
static const bool DEFAULT_STOPAFTERBLOCKIMPORT = false;
std::unique_ptr<CConnman> g_connman;
std::unique_ptr<PeerLogicValidation> peerLogic;
#if ENABLE_ZMQ
static CZMQNotificationInterface* pzmqNotificationInterface = nullptr;
#endif
#ifdef WIN32
#define MIN_CORE_FILEDESCRIPTORS 0
#else
#define MIN_CORE_FILEDESCRIPTORS 150
#endif
static const char* FEE_ESTIMATES_FILENAME="fee_estimates.dat";
std::atomic<bool> fRequestShutdown(false);
std::atomic<bool> fDumpMempoolLater(false);
void StartShutdown()
{
    fRequestShutdown = true;
}
bool ShutdownRequested()
{
    return fRequestShutdown;
}
/**
 * This is a minimally invasive approach to shutdown on LevelDB read errors from the
 * chainstate, while keeping user interface out of the common library, which is shared
 * between bitcoind, and bitcoin-qt and non-server tools.
*/
class CCoinsViewErrorCatcher final : public CCoinsViewBacked