Handle base fee calculation for transition block

Author: piersy

consensus/misc/eip1559/eip1559.go

@@ -58,6 +58,12 @@ func VerifyEIP1559Header(config *params.ChainConfig, parent, header *types.Heade
 // CalcBaseFee calculates the basefee of the header.
 // The time belongs to the new block to check if Canyon is activted or not
 func CalcBaseFee(config *params.ChainConfig, parent *types.Header, time uint64) *big.Int {
+	// If this is the cel2 transition block then we use the initial base fee
+	// from the Cel2Config of the chain config.
+	if config.Cel2Time != nil && *config.Cel2Time == time {
+		return new(big.Int).SetUint64(config.Cel2Config.TransitionBlockBaseFee)
+	}
+
 	// If the current block is the first EIP-1559 block, return the InitialBaseFee.
 	if !config.IsLondon(parent.Number) {
 		return new(big.Int).SetUint64(params.InitialBaseFee)

params/celo_config.go

@@ -0,0 +1,19 @@
+package params
+
+// Cel2Config holds config required when running a cel2 chain.
+type Cel2Config struct {
+	// TransitionBlockBaseFee is the base fee for the transition block in a
+	// migrated chain. it must be set for migrated chains during the migration
+	// to the value in the transition block. For non migrated chains it does
+	// not need to be set.This is required because we need
+	// eip1559.CalcBaseFee(config *params.ChainConfig, parent *types.Header,
+	// time uint64) to be able to return the correct base fee for the
+	// transition block and CalcBaseFee does not have access to the current
+	// header so cannot know what the base fee should be. We can't just use the
+	// base fee of the parent either because if we are transitioning at a
+	// pre-gingerbread block then it won't have a base fee, so this seems like
+	// the least invasive approach. Alternatively we could change the signature
+	// of CalcBaseFee to include the current header but that would require
+	// changing code in a number of places.
+	TransitionBlockBaseFee uint64
+}

params/config.go

@@ -434,6 +434,8 @@ type ChainConfig struct {
 
 	Cel2Time *uint64 `json:"cel2Time,omitempty"` // Cel2 switch time (nil = no fork, 0 = already on optimism cel2)
 
+	Cel2Config *Cel2Config `json:"cel2Config,omitempty"` // Cel2 config
+
 	// TerminalTotalDifficulty is the amount of total difficulty reached by
 	// the network that triggers the consensus upgrade.
 	TerminalTotalDifficulty *big.Int `json:"terminalTotalDifficulty,omitempty"`