refactor: move getFeePayer to STObject

include/xrpl/protocol/STObject.h

@@ -217,6 +217,11 @@ public:
     [[nodiscard]] AccountID
     getAccountID(SField const& field) const;
 
+    /** The account responsible for the fee and authorization: the delegate when
+        sfDelegate is present, otherwise the account. */
+    [[nodiscard]] AccountID
+    getFeePayer() const;
+
     [[nodiscard]] Blob
     getFieldVL(SField const& field) const;
     [[nodiscard]] STAmount const&

include/xrpl/protocol/STTx.h

@@ -84,9 +84,6 @@ public:
     [[nodiscard]] std::uint32_t
     getSeqValue() const;
 
-    [[nodiscard]] AccountID
-    getFeePayer() const;
-
     [[nodiscard]] boost::container::flat_set<AccountID>
     getMentionedAccounts() const;
 

include/xrpl/tx/transactors/system/Batch.h

@@ -25,9 +25,6 @@ public:
     static NotTEC
     preflightSigValidated(PreflightContext const& ctx);
 
-    static NotTEC
-    checkBatchSign(PreclaimContext const& ctx);
-
     static NotTEC
     checkSign(PreclaimContext const& ctx);
 
@@ -62,6 +59,12 @@ public:
         ttLOAN_MANAGE,
         ttLOAN_PAY,
     });
+
+private:
+    // Skips signature verification for inner txns, so keep it private: it must
+    // only be reached through Batch::checkSign.
+    static NotTEC
+    checkBatchSign(PreclaimContext const& ctx);
 };
 
 }  // namespace xrpl

src/libxrpl/protocol/STObject.cpp

@@ -635,6 +635,20 @@ STObject::getAccountID(SField const& field) const
     return getFieldByValue<STAccount>(field);
 }
 
+AccountID
+STObject::getFeePayer() const
+{
+    // If sfDelegate is present, the delegate account is the payer
+    // note: if a delegate is specified, its authorization to act on behalf of the account is
+    // enforced in `Transactor::invokeCheckPermission`
+    // cryptographic signature validity is checked separately (e.g., in `Transactor::checkSign`)
+    if (isFieldPresent(sfDelegate))
+        return getAccountID(sfDelegate);
+
+    // Default payer
+    return getAccountID(sfAccount);
+}
+
 Blob
 STObject::getFieldVL(SField const& field) const
 {

src/libxrpl/protocol/STTx.cpp

@@ -214,20 +214,6 @@ STTx::getSeqValue() const
     return getSeqProxy().value();
 }
 
-AccountID
-STTx::getFeePayer() const
-{
-    // If sfDelegate is present, the delegate account is the payer
-    // note: if a delegate is specified, its authorization to act on behalf of the account is
-    // enforced in `Transactor::invokeCheckPermission`
-    // cryptographic signature validity is checked separately (e.g., in `Transactor::checkSign`)
-    if (isFieldPresent(sfDelegate))
-        return getAccountID(sfDelegate);
-
-    // Default payer
-    return getAccountID(sfAccount);
-}
-
 void
 STTx::sign(
     PublicKey const& publicKey,

src/libxrpl/tx/transactors/system/Batch.cpp

@@ -421,7 +421,7 @@ Batch::preflightSigValidated(PreflightContext const& ctx)
     {
         // A delegated inner is signed by the delegate, not the account holder,
         // so the delegate is the required signer when present.
-        AccountID const authorizer = rb[~sfDelegate].value_or(rb[sfAccount]);
+        AccountID const authorizer = rb.getFeePayer();
 
         // The outer account signs the batch itself, so it is never added to the
         // required signers.