fix incomplete pending block handling logic regarding API requests

[aidan-kwon]

Proposed changes

This PR contains two different fixes regarding "pending" block requests.

1. panic because of the absence of pending block
   Panic was reported in Invalid memory address or nil pointer dereference #1841 regarding API servicing with "pending" block parameter. The panic can be triggered when a pending block has never been created yet but a user requests API with "pending" parameter.

2. occasional API error with "invalid signature length" error message
   During the synchronization, a node uses a worker's snapshot to return a pending block. The block is created with the worker's current data in updateSnapshot() method. Since EN doesn't create a pending block correctly, however, the current.header doesn't have enough information including a miner's signature. Therefore, eth_getBlockByNumber and other API fail to recover a block proposer's address and return an error message. This PR set the previous block header for a pending block of EN/PN to support the API correctly.

Types of changes

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• Bugfix
• New feature or enhancement
• Others

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• I have read the CONTRIBUTING GUIDELINES doc
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• I have added tests that prove my fix is effective or that my feature works
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• Any dependent changes have been merged and published in downstream modules

Related issues

Resolve #1842

[kjeom]

I didn't find any negative effect when setting the previous block header.

[aidan-kwon]

@kjeom You mean that you can't find the current problem related to block header, right?

1. Currently, an incomplete block header is used for makeCurrent and the method set the header to worker.current.header.
   

   klaytn/work/worker.go

   Lines 529 to 543 in 6833919

   	header := &types.Header{
   	ParentHash: parent.Hash(),
   	Number: nextBlockNum,
   	Extra: self.extra,
   	Time: big.NewInt(tstamp),
   	}
   	if self.config.IsMagmaForkEnabled(nextBlockNum) {
   	header.BaseFee = nextBaseFee
   	}
   	if err := self.engine.Prepare(self.chain, header); err != nil {
   	logger.Error("Failed to prepare header for mining", "err", err)
   	return
   	}
   	// Could potentially happen if starting to mine in an odd state.
   	err = self.makeCurrent(parent, header)

2. worker.snapshotBlock consists of worker.current.header and others
   

   klaytn/work/worker.go

   Lines 616 to 620 in 6833919

   	self.snapshotBlock = types.NewBlock(
   	self.current.header,
   	self.current.txs,
   	self.current.receipts,
   	)

3. worker.snapshotBlock can be returned as a pending block
   

   klaytn/work/worker.go

   Lines 220 to 225 in 6833919

   	func (self *worker) pendingBlock() *types.Block {
   	if atomic.LoadInt32(&self.mining) == 0 {
   	// return a snapshot to avoid contention on currentMu mutex
   	self.snapshotMu.RLock()
   	defer self.snapshotMu.RUnlock()
   	return self.snapshotBlock

4. In the JSON marshaling logic, the pending block's header is used to calculate a block proposer address
   

   klaytn/api/api_ethereum.go

   Lines 1463 to 1474 in 6833919

   	func (api *EthereumAPI) rpcMarshalHeader(head *types.Header) (map[string]interface{}, error) {
   	var proposer common.Address
   	var err error
   	b := api.publicKlayAPI.b
   	if head.Number.Sign() != 0 {
   	proposer, err = b.Engine().Author(head)
   	if err != nil {
   	// miner is the field Klaytn should provide the correct value. It's not the field dummy value is allowed.
   	logger.Error("Failed to fetch author during marshaling header", "err", err.Error())
   	return nil, err
   	}

Therefore, an incomplete block header assigned to worker.current.header can cause panic. Since EN/PN doesn't finish the pending block generation process, an alternative header is needed for worker.current.header

[kjeom]

@aidan-kwon Oh, sorry for confusing. I just noted what I have finished for review.
Anyway thanks for explaining it in detail.

I just wanted to inform this line to other reviewers to check if there's something that I didn't find