consensus/beacon: fix isPostMerge for mainnet (#31191)

This fixes a regression introduced in #31153 where we didn't consider
mainnet to be in PoS, causing #31190.
The problem is, `params.MainnetChainConfig` does not have a defined
`MergeNetsplitBlock`, so it isn't considered to be in PoS in
`CalcDifficulty`.

consensus/beacon/consensus.go

@@ -75,9 +75,11 @@ func New(ethone consensus.Engine) *Beacon {
 // isPostMerge reports whether the given block number is assumed to be post-merge.
 // Here we check the MergeNetsplitBlock to allow configuring networks with a PoW or
 // PoA chain for unit testing purposes.
-func isPostMerge(config *params.ChainConfig, block uint64) bool {
+func isPostMerge(config *params.ChainConfig, blockNum uint64, timestamp uint64) bool {
 	mergedAtGenesis := config.TerminalTotalDifficulty != nil && config.TerminalTotalDifficulty.Sign() == 0
-	return mergedAtGenesis || config.MergeNetsplitBlock != nil && block >= config.MergeNetsplitBlock.Uint64()
+	return mergedAtGenesis ||
+		config.MergeNetsplitBlock != nil && blockNum >= config.MergeNetsplitBlock.Uint64() ||
+		config.ShanghaiTime != nil && timestamp >= *config.ShanghaiTime
 }
 
 // Author implements consensus.Engine, returning the verified author of the block.
@@ -327,7 +329,7 @@ func (beacon *Beacon) verifyHeaders(chain consensus.ChainHeaderReader, headers [
 // Prepare implements consensus.Engine, initializing the difficulty field of a
 // header to conform to the beacon protocol. The changes are done inline.
 func (beacon *Beacon) Prepare(chain consensus.ChainHeaderReader, header *types.Header) error {
-	if !isPostMerge(chain.Config(), header.Number.Uint64()) {
+	if !isPostMerge(chain.Config(), header.Number.Uint64(), header.Time) {
 		return beacon.ethone.Prepare(chain, header)
 	}
 	header.Difficulty = beaconDifficulty
@@ -437,7 +439,7 @@ func (beacon *Beacon) SealHash(header *types.Header) common.Hash {
 // the difficulty that a new block should have when created at time
 // given the parent block's time and difficulty.
 func (beacon *Beacon) CalcDifficulty(chain consensus.ChainHeaderReader, time uint64, parent *types.Header) *big.Int {
-	if !isPostMerge(chain.Config(), parent.Number.Uint64()+1) {
+	if !isPostMerge(chain.Config(), parent.Number.Uint64()+1, time) {
 		return beacon.ethone.CalcDifficulty(chain, time, parent)
 	}
 	return beaconDifficulty