blockstorage: Don't move cursor backwards in UpdateBlockInfo

Previously, it was possible to move the cursor back to an older file
during reindex if blocks are enocuntered out of order during reindex.
This would means that MaxBlockfileNum() would be incorrect, and
a wrong DB_LAST_BLOCK could be written to disk.

This improves the logic by only ever moving the cursor forward (if possible)
but not backwards.

Co-authored-by: Martin Zumsande <mzumsande@gmail.com>

src/node/blockstorage.cpp

@@ -943,24 +943,19 @@ void BlockManager::UpdateBlockInfo(const CBlock& block, unsigned int nHeight, co
 {
     LOCK(cs_LastBlockFile);
 
+    // Update the cursor so it points to the last file.
+    const BlockfileType chain_type{BlockfileTypeForHeight(nHeight)};
+    auto& cursor{m_blockfile_cursors[chain_type]};
+    if (!cursor || cursor->file_num < pos.nFile) {
+        m_blockfile_cursors[chain_type] = BlockfileCursor{pos.nFile};
+    }
+
+    // Update the file information with the current block.
     const unsigned int added_size = ::GetSerializeSize(TX_WITH_WITNESS(block));
-    const BlockfileType chain_type = BlockfileTypeForHeight(nHeight);
-    // Check that chain type is NORMAL, because this function is only
-    // called during reindexing, and reindexing deletes snapshot chainstates, so
-    // chain_type will not be SNAPSHOT. Also check that cursor exists, because
-    // the normal cursor should never be null.
-    Assume(chain_type == BlockfileType::NORMAL);
-    Assume(m_blockfile_cursors[chain_type]);
     const int nFile = pos.nFile;
     if (static_cast<int>(m_blockfile_info.size()) <= nFile) {
         m_blockfile_info.resize(nFile + 1);
     }
-
-    const int last_blockfile = m_blockfile_cursors[chain_type]->file_num;
-    if (nFile != last_blockfile) {
-        // No undo data yet in the new file, so reset our undo-height tracking.
-        m_blockfile_cursors[chain_type] = BlockfileCursor{nFile};
-    }
     m_blockfile_info[nFile].AddBlock(nHeight, block.GetBlockTime());
     m_blockfile_info[nFile].nSize = std::max(pos.nPos + added_size, m_blockfile_info[nFile].nSize);
     m_dirty_fileinfo.insert(nFile);