wallet, rpc: add anti-fee-sniping to send and sendall

Author: ishaanam

src/wallet/rpc/spend.cpp

@@ -92,8 +92,21 @@ std::set<int> InterpretSubtractFeeFromOutputInstructions(const UniValue& sffo_in
     return sffo_set;
 }
 
-static UniValue FinishTransaction(const std::shared_ptr<CWallet> pwallet, const UniValue& options, const CMutableTransaction& rawTx)
+static UniValue FinishTransaction(const std::shared_ptr<CWallet> pwallet, const UniValue& options, CMutableTransaction& rawTx)
 {
+    bool can_anti_fee_snipe = !options.exists("locktime");
+
+    for (const CTxIn& tx_in : rawTx.vin) {
+        // Checks sequence values consistent with DiscourageFeeSniping
+        can_anti_fee_snipe = can_anti_fee_snipe && (tx_in.nSequence == CTxIn::MAX_SEQUENCE_NONFINAL || tx_in.nSequence == MAX_BIP125_RBF_SEQUENCE);
+    }
+
+    if (can_anti_fee_snipe) {
+        LOCK(pwallet->cs_wallet);
+        FastRandomContext rng_fast;
+        DiscourageFeeSniping(rawTx, rng_fast, pwallet->chain(), pwallet->GetLastBlockHash(), pwallet->GetLastBlockHeight());
+    }
+
     // Make a blank psbt
     PartiallySignedTransaction psbtx(rawTx);
 
@@ -1257,7 +1270,7 @@ RPCHelpMan send()
                           }},
                         },
                     },
-                    {"locktime", RPCArg::Type::NUM, RPCArg::Default{0}, "Raw locktime. Non-0 value also locktime-activates inputs"},
+                    {"locktime", RPCArg::Type::NUM, RPCArg::DefaultHint{"locktime close to block height to prevent fee sniping"}, "Raw locktime. Non-0 value also locktime-activates inputs"},
                     {"lock_unspents", RPCArg::Type::BOOL, RPCArg::Default{false}, "Lock selected unspent outputs"},
                     {"psbt", RPCArg::Type::BOOL,  RPCArg::DefaultHint{"automatic"}, "Always return a PSBT, implies add_to_wallet=false."},
                     {"subtract_fee_from_outputs", RPCArg::Type::ARR, RPCArg::Default{UniValue::VARR}, "Outputs to subtract the fee from, specified as integer indices.\n"
@@ -1326,7 +1339,8 @@ RPCHelpMan send()
             rawTx.vout.clear();
             auto txr = FundTransaction(*pwallet, rawTx, recipients, options, coin_control, /*override_min_fee=*/false);
 
-            return FinishTransaction(pwallet, options, CMutableTransaction(*txr.tx));
+            CMutableTransaction tx = CMutableTransaction(*txr.tx);
+            return FinishTransaction(pwallet, options, tx);
         }
     };
 }
@@ -1374,7 +1388,7 @@ RPCHelpMan sendall()
                                 },
                             },
                         },
-                        {"locktime", RPCArg::Type::NUM, RPCArg::Default{0}, "Raw locktime. Non-0 value also locktime-activates inputs"},
+                        {"locktime", RPCArg::Type::NUM, RPCArg::DefaultHint{"locktime close to block height to prevent fee sniping"}, "Raw locktime. Non-0 value also locktime-activates inputs"},
                         {"lock_unspents", RPCArg::Type::BOOL, RPCArg::Default{false}, "Lock selected unspent outputs"},
                         {"psbt", RPCArg::Type::BOOL,  RPCArg::DefaultHint{"automatic"}, "Always return a PSBT, implies add_to_wallet=false."},
                         {"send_max", RPCArg::Type::BOOL, RPCArg::Default{false}, "When true, only use UTXOs that can pay for their own fees to maximize the output amount. When 'false' (default), no UTXO is left behind. send_max is incompatible with providing specific inputs."},

src/wallet/spend.cpp

@@ -936,11 +936,7 @@ static bool IsCurrentForAntiFeeSniping(interfaces::Chain& chain, const uint256&
     return true;
 }
 
-/**
- * Set a height-based locktime for new transactions (uses the height of the
- * current chain tip unless we are not synced with the current chain
- */
-static void DiscourageFeeSniping(CMutableTransaction& tx, FastRandomContext& rng_fast,
+void DiscourageFeeSniping(CMutableTransaction& tx, FastRandomContext& rng_fast,
                                  interfaces::Chain& chain, const uint256& block_hash, int block_height)
 {
     // All inputs must be added by now

src/wallet/spend.h

@@ -218,6 +218,12 @@ struct CreatedTransactionResult
         : tx(_tx), fee(_fee), fee_calc(_fee_calc), change_pos(_change_pos) {}
 };
 
+/**
+ * Set a height-based locktime for new transactions (uses the height of the
+ * current chain tip unless we are not synced with the current chain
+ */
+void DiscourageFeeSniping(CMutableTransaction& tx, FastRandomContext& rng_fast, interfaces::Chain& chain, const uint256& block_hash, int block_height);
+
 /**
  * Create a new transaction paying the recipients with a set of coins
  * selected by SelectCoins(); Also create the change output, when needed

test/functional/wallet_rescan_unconfirmed.py

@@ -53,7 +53,7 @@ def run_test(self):
         # The only UTXO available to spend is tx_parent_to_reorg.
         assert_equal(len(w0_utxos), 1)
         assert_equal(w0_utxos[0]["txid"], tx_parent_to_reorg["txid"])
-        tx_child_unconfirmed_sweep = w0.sendall([ADDRESS_BCRT1_UNSPENDABLE])
+        tx_child_unconfirmed_sweep = w0.sendall(recipients=[ADDRESS_BCRT1_UNSPENDABLE], options={"locktime":0})
         assert tx_child_unconfirmed_sweep["txid"] in node.getrawmempool()
         node.syncwithvalidationinterfacequeue()