There are some potential feefilter cleanups mentioned and motivated in comment https://github.com/bitcoin/bitcoin/pull/21509#issuecomment-805808485 . If you address the locking one, please be careful and make sure that `round()` is only called under a lock. See https://github.com/bitcoin/bitcoin/pull/21509#issuecomment-807992199

#### Useful skills:

* Bitcoin protocol messages
* Bitcoin Core P2P design
* Running our unit and functional tests

#### Want to work on this issue?

For guidance on contributing, please read [CONTRIBUTING.md](https://github.com/bitcoin/bitcoin/blob/master/CONTRIBUTING.md) before opening your pull request.

}
        if (pfrom.ExpectServicesFromConn() && !HasAllDesirableServiceFlags(nServices))
        {
            LogPrint(BCLog::NET, "peer=%d does not offer the expected services (%08x offered, %08x expected); disconnecting\n", pfrom.GetId(), nServices, GetDesirableServiceFlags(nServices));
            pfrom.fDisconnect = true;
            return;
        }
        if (nVersion < MIN_PEER_PROTO_VERSION) {
            
            LogPrint(BCLog::NET, "peer=%d using obsolete version %i; disconnecting\n", pfrom.GetId(), nVersion);
            pfrom.fDisconnect = true;
            return;
        }
        if (!vRecv.empty())
            vRecv >> addrFrom >> nNonce;
        if (!vRecv.empty()) {
            std::string strSubVer;
            vRecv >> LIMITED_STRING(strSubVer, MAX_SUBVERSION_LENGTH);
            cleanSubVer = SanitizeString(strSubVer);
        }
        if (!vRecv.empty()) {
            vRecv >> starting_height;
        }
        if (!vRecv.empty())
            vRecv >> fRelay;
        
        if (pfrom.IsInboundConn() && !m_connman.CheckIncomingNonce(nNonce))
        {
            LogPrintf("connected to self at %s, disconnecting\n", pfrom.addr.ToString());
            pfrom.fDisconnect = true;