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.

}
        return;
    }
    if (msg_type == NetMsgType::CMPCTBLOCK)
    {
        
        if (fImporting || fReindex) {
            LogPrint(BCLog::NET, "Unexpected cmpctblock message received from peer %d\n", pfrom.GetId());
            return;
        }
        CBlockHeaderAndShortTxIDs cmpctblock;
        vRecv >> cmpctblock;
        bool received_new_header = false;
        {
        LOCK(cs_main);
        if (!m_chainman.m_blockman.LookupBlockIndex(cmpctblock.header.hashPrevBlock)) {
            
            if (!m_chainman.ActiveChainstate().IsInitialBlockDownload())
                m_connman.PushMessage(&pfrom, msgMaker.Make(NetMsgType::GETHEADERS, m_chainman.ActiveChain().GetLocator(pindexBestHeader), uint256()));
            return;
        }
        if (!m_chainman.m_blockman.LookupBlockIndex(cmpctblock.header.GetHash())) {
            received_new_header = true;
        }
        }
        const CBlockIndex *pindex = nullptr;
        BlockValidationState state;
        if (!m_chainman.ProcessNewBlockHeaders({cmpctblock.header}, state, m_chainparams, &pindex)) {
            if (state.IsInvalid()) {
                MaybePunishNodeForBlock(pfrom.GetId(), state,  true, "invalid header via cmpctblock");