blockchain: fix inconsistent reorg behavior

If the chain is reorganized because the newly submit block has higher
justified block number, it can be reverted back to the old chain prior
to reorg because the old blocks cause ErrKnownBlock during insertion
via the insertChain method, which invokes writeKnownBlock to set the
old chain as the canonical chain again.

core/blockchain.go

@@ -1665,7 +1665,7 @@ func (bc *BlockChain) insertChain(chain types.Blocks, verifySeals bool) (int, er
 		)
 		for block != nil && bc.skipBlock(err, it) {
 			externTd = new(big.Int).Add(externTd, block.Difficulty())
-			if localTd.Cmp(externTd) < 0 {
+			if bc.reorgNeeded(current, localTd, block, externTd) {
 				break
 			}
 			log.Debug("Ignoring already known block", "number", block.Number(), "hash", block.Hash())
@@ -1734,7 +1734,19 @@ func (bc *BlockChain) insertChain(chain types.Blocks, verifySeals bool) (int, er
 		}
 	}()
 
-	for ; block != nil && err == nil || errors.Is(err, ErrKnownBlock); block, err = it.next() {
+	var (
+		current  = bc.CurrentBlock()
+		localTd  = bc.GetTd(current.Hash(), current.NumberU64())
+		externTd = common.Big0
+	)
+
+	if block != nil {
+		externTd = bc.GetTd(block.ParentHash(), block.NumberU64()-1)
+	}
+
+	for ; (block != nil && err == nil) || errors.Is(err, ErrKnownBlock); block, err = it.next() {
+		// err == ErrknownBlock means block != nil
+		externTd = new(big.Int).Add(externTd, block.Difficulty())
 		// If the chain is terminating, stop processing blocks
 		if bc.insertStopped() {
 			log.Debug("Abort during block processing")
@@ -1751,7 +1763,7 @@ func (bc *BlockChain) insertChain(chain types.Blocks, verifySeals bool) (int, er
 		// just skip the block (we already validated it once fully (and crashed), since
 		// its header and body was already in the database). But if the corresponding
 		// snapshot layer is missing, forcibly rerun the execution to build it.
-		if bc.skipBlock(err, it) {
+		if bc.skipBlock(err, it) && bc.reorgNeeded(current, localTd, block, externTd) {
 			logger := log.Debug
 			if bc.chainConfig.Clique == nil {
 				logger = log.Warn