validation: in invalidateblock, calculate m_best_header right away

Before, m_best_header would be calculated only after disconnecting
multiple blocks, letting go of cs_main in the meantime.
This is in preparation for adding checks to CheckBlockIndex()
requiring that m_best_header is the most-work header not known to be invalid.

Co-authored-by: stringintech <[email protected]>

Author: mzumsande

src/validation.cpp

@@ -3756,7 +3756,15 @@ bool Chainstate::InvalidateBlock(BlockValidationState& state, CBlockIndex* pinde
         // Mark out-of-chain descendants of the invalidated block as invalid
         // (possibly replacing a pre-existing BLOCK_FAILED_VALID with BLOCK_FAILED_CHILD)
         // Add any equal or more work headers that are not invalidated to setBlockIndexCandidates
+        // Recalculate m_best_header if it became invalid.
         auto candidate_it = highpow_outofchain_headers.lower_bound(invalid_walk_tip->pprev->nChainWork);
+
+        const bool best_header_needs_update{m_chainman.m_best_header->GetAncestor(invalid_walk_tip->nHeight) == invalid_walk_tip};
+        if (best_header_needs_update) {
+            // pprev is definitely still valid at this point, but there may be better ones
+            m_chainman.m_best_header = invalid_walk_tip->pprev;
+        }
+
         while (candidate_it != highpow_outofchain_headers.end()) {
             CBlockIndex* candidate{candidate_it->second};
             if (candidate->GetAncestor(invalid_walk_tip->nHeight) == invalid_walk_tip) {
@@ -3765,7 +3773,7 @@ bool Chainstate::InvalidateBlock(BlockValidationState& state, CBlockIndex* pinde
                 candidate->nStatus |= BLOCK_FAILED_CHILD;
                 m_blockman.m_dirty_blockindex.insert(candidate);
                 // If invalidated, the block is irrelevant for setBlockIndexCandidates
-                // and can be removed from the cache.
+                // and for m_best_header and can be removed from the cache.
                 candidate_it = highpow_outofchain_headers.erase(candidate_it);
                 continue;
             }
@@ -3776,6 +3784,10 @@ bool Chainstate::InvalidateBlock(BlockValidationState& state, CBlockIndex* pinde
                 // 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;
         }