accounts/external, signer: EIP-7702 set-code transactions lose authorization list when signed by external signer

[cuiweixie]

System information

Geth version: master (commit 7a73ffe)
OS & Version: all

Description

When a SetCodeTx (EIP-7702, tx type 0x04) is signed through the external
signer path (Clef / ExternalSigner), its authorization list is silently
dropped, and the transaction is round-tripped through a struct that cannot
represent a set-code transaction. The result is a signed transaction that is
either missing its authorizations or reconstructed as the wrong type.

There are two related gaps:

1. ExternalSigner.SignTx never serializes the authorization list.

In accounts/external/backend.go, SetCodeTxType is folded into the same
case as dynamic-fee and blob transactions, so only the fee fields and access
list are copied into SendTxArgs:

// accounts/external/backend.go:215
switch tx.Type() {
case types.LegacyTxType, types.AccessListTxType:
    args.GasPrice = (*hexutil.Big)(tx.GasPrice())
case types.DynamicFeeTxType, types.BlobTxType, types.SetCodeTxType:
    args.MaxFeePerGas = (*hexutil.Big)(tx.GasFeeCap())
    args.MaxPriorityFeePerGas = (*hexutil.Big)(tx.GasTipCap())
...

tx.SetCodeAuthorizations() is never read, so the authorizations are not sent
to the external signer at all.

2. apitypes.SendTxArgs / ToTransaction cannot represent a set-code tx.

SendTxArgs (signer/core/apitypes/types.go:86) has no field for the
authorization list, and ToTransaction (signer/core/apitypes/types.go:137)
has no SetCodeTx branch. Because the switch is ordered
BlobHashes != nil -> MaxFeePerGas != nil -> AccessList != nil -> legacy, a
set-code transaction (which has MaxFeePerGas set) is reconstructed as a plain
DynamicFeeTx. The transaction type changes and any authorizations are lost.

Impact

A wallet that delegates SetCodeTxType signing to Clef / an external signer
gets back a transaction that has lost its EIP-7702 authorizations, or has been
turned into a dynamic-fee transaction. The intended delegation/account-code
change does not take effect.

Suggested fix

• Add an authorization-list field to SendTxArgs, e.g.
  AuthorizationList []types.SetCodeAuthorization \json:"authorizationList,omitempty"``.
• In ExternalSigner.SignTx, add a dedicated case types.SetCodeTxType that
  also sets args.AuthorizationList = tx.SetCodeAuthorizations().
• In ToTransaction, add a branch that builds a *types.SetCodeTx when the
  authorization list is present.

Steps to reproduce

External-sign a set-code transaction carrying one authorization and assert the
returned transaction's type is still SetCodeTxType and its authorization list
is preserved.