node: extend node::TxBroadcast with a 3rd option

Extend `node::TxBroadcast` with a 3rd option to not add the
transaction to the mempool and broadcast privately.

This is a non-functional change - `BroadcastTransaction()` will not
do anything if the 3rd options is passed and is not used by any of
its callers.

src/node/transaction.cpp

@@ -74,13 +74,14 @@ TransactionError BroadcastTransaction(NodeContext& node,
             wtxid = mempool_tx->GetWitnessHash();
         } else {
             // Transaction is not already in the mempool.
-            if (max_tx_fee > 0) {
+            const bool check_max_fee{max_tx_fee > 0};
+            if (check_max_fee || broadcast_method == TxBroadcast::NO_MEMPOOL_PRIVATE_BROADCAST) {
                 // First, call ATMP with test_accept and check the fee. If ATMP
                 // fails here, return error immediately.
                 const MempoolAcceptResult result = node.chainman->ProcessTransaction(tx, /*test_accept=*/ true);
                 if (result.m_result_type != MempoolAcceptResult::ResultType::VALID) {
                     return HandleATMPError(result.m_state, err_string);
-                } else if (result.m_base_fees.value() > max_tx_fee) {
+                } else if (check_max_fee && result.m_base_fees.value() > max_tx_fee) {
                     return TransactionError::MAX_FEE_EXCEEDED;
                 }
             }
@@ -104,6 +105,8 @@ TransactionError BroadcastTransaction(NodeContext& node,
                     node.mempool->AddUnbroadcastTx(txid);
                 }
                 break;
+            case TxBroadcast::NO_MEMPOOL_PRIVATE_BROADCAST:
+                break;
             }
 
             if (wait_callback && node.validation_signals) {
@@ -135,6 +138,8 @@ TransactionError BroadcastTransaction(NodeContext& node,
     case TxBroadcast::MEMPOOL_AND_BROADCAST_TO_ALL:
         node.peerman->InitiateTxBroadcastToAll(txid, wtxid);
         break;
+    case TxBroadcast::NO_MEMPOOL_PRIVATE_BROADCAST:
+        break;
     }
 
     return TransactionError::OK;

src/node/types.h

@@ -108,6 +108,9 @@ enum class TxBroadcast : uint8_t {
     MEMPOOL_AND_BROADCAST_TO_ALL,
     /// Add the transaction to the mempool, but don't broadcast to anybody.
     MEMPOOL_NO_BROADCAST,
+    /// Omit the mempool and directly send the transaction via a few dedicated connections to
+    /// peers on privacy networks.
+    NO_MEMPOOL_PRIVATE_BROADCAST,
 };
 
 } // namespace node

src/wallet/wallet.cpp

@@ -1984,6 +1984,9 @@ bool CWallet::SubmitTxMemoryPoolAndRelay(CWalletTx& wtx,
     case node::TxBroadcast::MEMPOOL_NO_BROADCAST:
         what = "to mempool without broadcast";
         break;
+    case node::TxBroadcast::NO_MEMPOOL_PRIVATE_BROADCAST:
+        what = "for private broadcast without adding to the mempool";
+        break;
     }
     WalletLogPrintf("Submitting wtx %s %s\n", wtx.GetHash().ToString(), what);
     // We must set TxStateInMempool here. Even though it will also be set later by the