Method FindNextBlocksToDownload

src/net_processing.cpp:1406–1454

Logic for calculating which blocks to download from a given peer, given our current tip.

// Logic for calculating which blocks to download from a given peer, given our current tip.
void PeerManagerImpl::FindNextBlocksToDownload(const Peer& peer, unsigned int count, std::vector<const CBlockIndex*>& vBlocks, NodeId& nodeStaller)
{
    if (count == 0)
        return;

    vBlocks.reserve(vBlocks.size() + count);
    CNodeState *state = State(peer.m_id);
    assert(state != nullptr);

    // Make sure pindexBestKnownBlock is up to date, we'll need it.
    ProcessBlockAvailability(peer.m_id);

    if (state->pindexBestKnownBlock == nullptr || state->pindexBestKnownBlock->nChainWork < m_chainman.ActiveChain().Tip()->nChainWork || state->pindexBestKnownBlock->nChainWork < m_chainman.MinimumChainWork()) {
        // This peer has nothing interesting.
        return;
    }

    // When syncing with AssumeUtxo and the snapshot has not yet been validated,
    // abort downloading blocks from peers that don't have the snapshot block in their best chain.
    // We can't reorg to this chain due to missing undo data until validation completes,
    // so downloading blocks from it would be futile.
    const CBlockIndex* snap_base{m_chainman.CurrentChainstate().SnapshotBase()};
    if (snap_base && m_chainman.CurrentChainstate().m_assumeutxo == Assumeutxo::UNVALIDATED &&
        state->pindexBestKnownBlock->GetAncestor(snap_base->nHeight) != snap_base) {
        LogDebug(BCLog::NET, "Not downloading blocks from peer=%d, which doesn't have the snapshot block in its best chain.\n", peer.m_id);
        return;
    }

    // Determine the forking point between the peer's chain and our chain:
    // pindexLastCommonBlock is required to be an ancestor of pindexBestKnownBlock, and will be used as a starting point.
    // It is being set to the fork point between the peer's best known block and the current tip, unless it is already set to
    // an ancestor with more work than the fork point.
    auto fork_point = LastCommonAncestor(state->pindexBestKnownBlock, m_chainman.ActiveTip());
    if (state->pindexLastCommonBlock == nullptr ||
        fork_point->nChainWork > state->pindexLastCommonBlock->nChainWork ||
        state->pindexBestKnownBlock->GetAncestor(state->pindexLastCommonBlock->nHeight) != state->pindexLastCommonBlock) {
        state->pindexLastCommonBlock = fork_point;
    }
    if (state->pindexLastCommonBlock == state->pindexBestKnownBlock)
        return;

    const CBlockIndex *pindexWalk = state->pindexLastCommonBlock;
    // Never fetch further than the best block we know the peer has, or more than BLOCK_DOWNLOAD_WINDOW + 1 beyond the last
    // linked block we have in common with this peer. The +1 is so we can detect stalling, namely if we would be able to
    // download that next block if the window were 1 larger.
    int nWindowEnd = state->pindexLastCommonBlock->nHeight + BLOCK_DOWNLOAD_WINDOW;

    FindNextBlocks(vBlocks, peer, state, pindexWalk, count, nWindowEnd, &m_chainman.ActiveChain(), &nodeStaller);
}

void PeerManagerImpl::TryDownloadingHistoricalBlocks(const Peer& peer, unsigned int count, std::vector<const CBlockIndex*>& vBlocks, const CBlockIndex *from_tip, const CBlockIndex* target_block)
{