Fixed zero-offset protection for PBFT (where it can be normal)

Author: valdok

explorer/adapter.cpp

@@ -471,7 +471,14 @@ class Adapter : public Node::IObserver, public IAdapter {
 
             if (bForceReset)
             {
-                if (!lst.empty() && (lst.back().second.m_TotalOffset == sd.m_Extra0.m_TotalOffset))
+                // Historically explorer node could corrupt itself, it usually manifests in consecutive blocks having the same State Extra, in particular same Total Offset.
+                // Here we try to detect this.
+                // One situation which could be legit: it's a network running on PBFT consensus algorithm. Completely empty blocks are normal.
+                // But, due to cannibalization, this can happen only to the last block.
+                //
+                // hence we check that our backtravel list has at least 2 previous entries
+
+                if ((lst.size() >= 2) && (lst.back().second.m_TotalOffset == sd.m_Extra0.m_TotalOffset))
                 {
                     CorruptionException exc;
                     exc.m_sErr = "zero offset detected. Full Reset is required to recover";

node/processor.cpp

@@ -3147,7 +3147,12 @@ bool NodeProcessor::HandleBlockInternal(const HeightHash& id, const Block::Syste
 
 	if (bFirstTime)
 	{
-		if (block.m_Offset.m_Value == Zero)
+		// Protect against blocks with invalid offset (historically happened in explorer node)
+
+		// Block with zero offset can be created in PBFT consensus algorithm, empty blocks are normal.
+		// However only the last block can be empty (due to cannibalization).
+		// Hence it's not applicable to Fast-Sync case.
+		if ((s.m_Number.v <= m_SyncData.m_TxoLo.v) && (block.m_Offset.m_Value == Zero))
 		{
 			BEAM_LOG_WARNING() << id << " has zero offset";
 			return false;