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

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fRevertToInv = true;
                        break;
                    }
                    pBestIndex = pindex;
                    if (fFoundStartingHeader) {
                        
                        vHeaders.push_back(pindex->GetBlockHeader());
                    } else if (PeerHasHeader(&state, pindex)) {
                        continue; 
                    } else if (pindex->pprev == nullptr || PeerHasHeader(&state, pindex->pprev)) {
                        
                        
                        fFoundStartingHeader = true;
                        vHeaders.push_back(pindex->GetBlockHeader());
                    } else {
                        
                        
                        fRevertToInv = true;
                        break;
                    }
                }
            }
            if (!fRevertToInv && !vHeaders.empty()) {