accounts/external, internal/ethapi: fixes for London tx signing (#23274)

Ticket #23273 found a flaw where we were unable to sign legacy-transactions
using the external signer, even if we're still on non-london network. That's
fixed in this PR.

Additionally, I found that even when supplying all parameters, it was impossible
to sign a london-transaction on an unsynched node. It's a pretty common usecase
that someone wants to sign a transaction using an unsynced 'vanilla' node,
providing all necessary data. Our setDefaults, however, insisted on checking the
current block against the config. This PR therefore adds a case, so that if both
MaxPriorityFeePerGas and MaxFeePerGas are provided, we accept them as given.

OBS This PR fixes a regression -- on current master, we are unable to sign a
london-transaction unless the node is synched, which may break scenarios where
geth (or clef) is used as a cold wallet.

Fixes #23273 

accounts/external/backend.go

@@ -211,11 +211,14 @@ func (api *ExternalSigner) SignTx(account accounts.Account, tx *types.Transactio
 		To:    to,
 		From:  common.NewMixedcaseAddress(account.Address),
 	}
-	if tx.GasFeeCap() != nil {
+	switch tx.Type() {
+	case types.LegacyTxType, types.AccessListTxType:
+		args.GasPrice = (*hexutil.Big)(tx.GasPrice())
+	case types.DynamicFeeTxType:
 		args.MaxFeePerGas = (*hexutil.Big)(tx.GasFeeCap())
 		args.MaxPriorityFeePerGas = (*hexutil.Big)(tx.GasTipCap())
-	} else {
-		args.GasPrice = (*hexutil.Big)(tx.GasPrice())
+	default:
+		return nil, fmt.Errorf("Unsupported tx type %d", tx.Type())
 	}
 	// We should request the default chain id that we're operating with
 	// (the chain we're executing on)

internal/ethapi/transaction_args.go

@@ -80,40 +80,45 @@ func (args *TransactionArgs) setDefaults(ctx context.Context, b Backend) error {
 	}
 	// After london, default to 1559 unless gasPrice is set
 	head := b.CurrentHeader()
-	if b.ChainConfig().IsLondon(head.Number) && args.GasPrice == nil {
-		if args.MaxPriorityFeePerGas == nil {
-			tip, err := b.SuggestGasTipCap(ctx)
-			if err != nil {
-				return err
+	// If user specifies both maxPriorityfee and maxFee, then we do not
+	// need to consult the chain for defaults. It's definitely a London tx.
+	if args.MaxPriorityFeePerGas == nil || args.MaxFeePerGas == nil {
+		// In this clause, user left some fields unspecified.
+		if b.ChainConfig().IsLondon(head.Number) && args.GasPrice == nil {
+			if args.MaxPriorityFeePerGas == nil {
+				tip, err := b.SuggestGasTipCap(ctx)
+				if err != nil {
+					return err
+				}
+				args.MaxPriorityFeePerGas = (*hexutil.Big)(tip)
 			}
-			args.MaxPriorityFeePerGas = (*hexutil.Big)(tip)
-		}
-		if args.MaxFeePerGas == nil {
-			gasFeeCap := new(big.Int).Add(
-				(*big.Int)(args.MaxPriorityFeePerGas),
-				new(big.Int).Mul(head.BaseFee, big.NewInt(2)),
-			)
-			args.MaxFeePerGas = (*hexutil.Big)(gasFeeCap)
-		}
-		if args.MaxFeePerGas.ToInt().Cmp(args.MaxPriorityFeePerGas.ToInt()) < 0 {
-			return fmt.Errorf("maxFeePerGas (%v) < maxPriorityFeePerGas (%v)", args.MaxFeePerGas, args.MaxPriorityFeePerGas)
-		}
-	} else {
-		if args.MaxFeePerGas != nil || args.MaxPriorityFeePerGas != nil {
-			return errors.New("maxFeePerGas or maxPriorityFeePerGas specified but london is not active yet")
-		}
-		if args.GasPrice == nil {
-			price, err := b.SuggestGasTipCap(ctx)
-			if err != nil {
-				return err
+			if args.MaxFeePerGas == nil {
+				gasFeeCap := new(big.Int).Add(
+					(*big.Int)(args.MaxPriorityFeePerGas),
+					new(big.Int).Mul(head.BaseFee, big.NewInt(2)),
+				)
+				args.MaxFeePerGas = (*hexutil.Big)(gasFeeCap)
+			}
+			if args.MaxFeePerGas.ToInt().Cmp(args.MaxPriorityFeePerGas.ToInt()) < 0 {
+				return fmt.Errorf("maxFeePerGas (%v) < maxPriorityFeePerGas (%v)", args.MaxFeePerGas, args.MaxPriorityFeePerGas)
+			}
+		} else {
+			if args.MaxFeePerGas != nil || args.MaxPriorityFeePerGas != nil {
+				return errors.New("maxFeePerGas or maxPriorityFeePerGas specified but london is not active yet")
 			}
-			if b.ChainConfig().IsLondon(head.Number) {
-				// The legacy tx gas price suggestion should not add 2x base fee
-				// because all fees are consumed, so it would result in a spiral
-				// upwards.
-				price.Add(price, head.BaseFee)
+			if args.GasPrice == nil {
+				price, err := b.SuggestGasTipCap(ctx)
+				if err != nil {
+					return err
+				}
+				if b.ChainConfig().IsLondon(head.Number) {
+					// The legacy tx gas price suggestion should not add 2x base fee
+					// because all fees are consumed, so it would result in a spiral
+					// upwards.
+					price.Add(price, head.BaseFee)
+				}
+				args.GasPrice = (*hexutil.Big)(price)
 			}
-			args.GasPrice = (*hexutil.Big)(price)
 		}
 	}
 	if args.Value == nil {