Method FindMostWorkChain

src/validation.cpp:3128–3181

* Return the tip of the chain with the most work in it, that isn't * known to be invalid (it's however far from certain to be valid). */

 * known to be invalid (it's however far from certain to be valid).
 */
CBlockIndex* Chainstate::FindMostWorkChain()
{
    AssertLockHeld(::cs_main);
    do {
        CBlockIndex *pindexNew = nullptr;

        // Find the best candidate header.
        {
            std::set<CBlockIndex*, CBlockIndexWorkComparator>::reverse_iterator it = setBlockIndexCandidates.rbegin();
            if (it == setBlockIndexCandidates.rend())
                return nullptr;
            pindexNew = *it;
        }

        // Check whether all blocks on the path between the currently active chain and the candidate are valid.
        // Just going until the active chain is an optimization, as we know all blocks in it are valid already.
        bool fInvalidAncestor = false;
        for (CBlockIndex *pindexTest = pindexNew; pindexTest && !m_chain.Contains(*pindexTest); pindexTest = pindexTest->pprev) {
            assert(pindexTest->HaveNumChainTxs() || pindexTest->nHeight == 0);

            // Pruned nodes may have entries in setBlockIndexCandidates for
            // which block files have been deleted.  Remove those as candidates
            // for the most work chain if we come across them; we can't switch
            // to a chain unless we have all the non-active-chain parent blocks.
            bool fFailedChain = pindexTest->nStatus & BLOCK_FAILED_VALID;
            bool fMissingData = !(pindexTest->nStatus & BLOCK_HAVE_DATA);
            if (fFailedChain || fMissingData) {
                // Candidate chain is not usable (either invalid or missing data)
                if (fFailedChain && (m_chainman.m_best_invalid == nullptr || pindexNew->nChainWork > m_chainman.m_best_invalid->nChainWork)) {
                    m_chainman.m_best_invalid = pindexNew;
                }
                // Remove the entire chain from the set.
                for (CBlockIndex *pindexFailed = pindexNew; pindexFailed != pindexTest; pindexFailed = pindexFailed->pprev) {
                    // If we're missing data and not a descendant of an invalid block,
                    // then add back to m_blocks_unlinked, so that if the block arrives in the future
                    // we can try adding to setBlockIndexCandidates again.
                    if (fMissingData && !fFailedChain) {
                        // Avoid duplicate entries in m_blocks_unlinked. If the same entry is
                        // processed twice in ReceivedBlockTransactions(), it may be re-added to
                        // setBlockIndexCandidates with a modified nSequenceId, breaking ordering
                        // guarantees and leading to undefined behavior.
                        m_blockman.AddUnlinkedBlock(pindexFailed);
                    }
                    setBlockIndexCandidates.erase(pindexFailed);
                }
                setBlockIndexCandidates.erase(pindexTest);
                fInvalidAncestor = true;
                break;
            }
        }
        if (!fInvalidAncestor)
            return pindexNew;
    } while(true);
}

/** Delete all entries in setBlockIndexCandidates that are worse than the current tip. */
void Chainstate::PruneBlockIndexCandidates() {