Repository: bitcoin/bitcoin
File: src/validation.cpp (C++)

Code before:
            pindex->nStatus |= BLOCK_FAILED_VALID;
            m_blockman.m_dirty_blockindex.insert(pindex);
            setBlockIndexCandidates.erase(pindex);
            m_chainman.m_failed_blocks.insert(pindex);
        }

        // If any new blocks somehow arrived while we were disconnecting
        // (above), then the pre-calculation of what should go into
        // setBlockIndexCandidates may have missed entries. This would
        // technically be an inconsistency in the block index, but if we clean
        // it up here, this should be an essentially unobservable error.
        // Loop back over all block index entries and add any missing entries
        // to setBlockIndexCandidates.
        for (auto& [_, block_index] : m_blockman.m_block_index) {
            if (block_index.IsValid(BLOCK_VALID_TRANSACTIONS) && block_index.HaveNumChainTxs() && !setBlockIndexCandidates.value_comp()(&block_index, m_chain.Tip())) {
                setBlockIndexCandidates.insert(&block_index);
            }
        }

        InvalidChainFound(to_mark_failed);
    }

    // Only notify about a new block tip if the active chain was modified.
    if (pindex_was_in_chain) {
        // Ignoring return value for now, this could be changed to bubble up
        // kernel::Interrupted value to the caller so the caller could
        // distinguish between completed and interrupted operations. It might
        // also make sense for the blockTip notification to have an enum
        // parameter indicating the source of the tip change so hooks can
        // distinguish user-initiated invalidateblock changes from other
        // changes.
        (void)m_chainman.GetNotifications().blockTip(
            /*state=*/GetSynchronizationState(m_chainman.IsInitialBlockDownload(), m_chainman.m_blockman.m_blockfiles_indexed),
            /*index=*/*to_mark_failed->pprev,
            /*verification_progress=*/WITH_LOCK(m_chainman.GetMutex(), return m_chainman.GuessVerificationProgress(to_mark_failed->pprev)));

        // Fire ActiveTipChange now for the current chain tip to make sure clients are notified.
        // ActivateBestChain may call this as well, but not necessarily.
        if (m_chainman.m_options.signals) {
            m_chainman.m_options.signals->ActiveTipChange(*Assert(m_chain.Tip()), m_chainman.IsInitialBlockDownload());
        }
    }
    return true;
}

void Chainstate::SetBlockFailureFlags(CBlockIndex* invalid_block)
{
    AssertLockHeld(cs_main);

    for (auto& [_, block_index] : m_blockman.m_block_index) {
        if (invalid_block != &block_index && block_index.GetAncestor(invalid_block->nHeight) == invalid_block) {

Review comment (suggestion):
Can the `m_best_header` update logic be merged into the while loop that handles the `nStatus` updates? Not sure if it is necessarily better; but I was thinking of something like:

Adding this just before the while loop:
```c++
const bool best_header_needs_update = m_chainman.m_best_header->GetAncestor(invalid_walk_tip->nHeight) == invalid_walk_tip;
if (best_header_needs_update) {
  m_chainman.m_best_header = invalid_walk_tip->pprev;
}
``` 

And then here:
```suggestion
            if (best_header_needs_update &&
                !CBlockIndexWorkComparator()(candidate_it->second, m_chainman.m_best_header)) {
                m_chainman.m_best_header = candidate_it->second;
            }
            ++candidate_it;
        }
```

Code after:

        while (candidate_it != highpow_outofchain_headers.end()) {
            CBlockIndex* candidate{candidate_it->second};
            if (candidate->GetAncestor(invalid_walk_tip->nHeight) == invalid_walk_tip) {
                // Children of failed blocks should be marked as BLOCK_FAILED_CHILD instead.
                candidate->nStatus &= ~BLOCK_FAILED_VALID;
                candidate->nStatus |= BLOCK_FAILED_CHILD;
                m_blockman.m_dirty_blockindex.insert(candidate);
                // If invalidated, the block is irrelevant for setBlockIndexCandidates
                // and for m_best_header and can be removed from the cache.
                candidate_it = highpow_outofchain_headers.erase(candidate_it);
                continue;
            }
            if (!CBlockIndexWorkComparator()(candidate, invalid_walk_tip->pprev) &&
                candidate->IsValid(BLOCK_VALID_TRANSACTIONS) &&
                candidate->HaveNumChainTxs()) {
                setBlockIndexCandidates.insert(candidate);
                // Do not remove candidate from the highpow_outofchain_headers cache, because it might be a descendant of the block being invalidated
                // which needs to be marked failed later.
            }
            if (best_header_needs_update &&
                m_chainman.m_best_header->nChainWork < candidate->nChainWork) {
                m_chainman.m_best_header = candidate;
            }
            ++candidate_it;
        }

        // Track the last disconnected block, so we can correct its BLOCK_FAILED_CHILD status in future
        // iterations, or, if it's the last one, call InvalidChainFound on it.
        to_mark_failed = invalid_walk_tip;
    }

    m_chainman.CheckBlockIndex();

    {
        LOCK(cs_main);
        if (m_chain.Contains(to_mark_failed)) {
            // If the to-be-marked invalid block is in the active chain, something is interfering and we can't proceed.
            return false;
        }

        // Mark pindex as invalid if it never was in the main chain
        if (!pindex_was_in_chain && !(pindex->nStatus & BLOCK_FAILED_MASK)) {
            pindex->nStatus |= BLOCK_FAILED_VALID;
            m_blockman.m_dirty_blockindex.insert(pindex);
            setBlockIndexCandidates.erase(pindex);
        }

        // If any new blocks somehow arrived while we were disconnecting
        // (above), then the pre-calculation of what should go into
        // setBlockIndexCandidates may have missed entries. This would