CreateTransaction: Assume minimum p2sh-p2wpkh spend size for unknown change

3 files changed, 55 insertions, 1 deletions

diff --git a/src/wallet/test/wallet_tests.cpp b/src/wallet/test/wallet_tests.cpp
index c6aac8aad5..623c5c39a2 100644
--- a/src/wallet/test/wallet_tests.cpp
+++ b/src/wallet/test/wallet_tests.cpp
@@ -17,6 +17,7 @@
 #include <validation.h>
 #include <wallet/coincontrol.h>
 #include <wallet/test/wallet_test_fixture.h>
+#include <policy/policy.h>
 
 #include <boost/test/unit_test.hpp>
 #include <univalue.h>
@@ -394,4 +395,47 @@ BOOST_FIXTURE_TEST_CASE(wallet_disableprivkeys, TestChain100Setup)
     BOOST_CHECK(!wallet->GetKeyFromPool(pubkey, false));
 }
 
+// Explicit calculation which is used to test the wallet constant
+// We get the same virtual size due to rounding(weight/4) for both use_max_sig values
+static size_t CalculateNestedKeyhashInputSize(bool use_max_sig)
+{
+    // Generate ephemeral valid pubkey
+    CKey key;
+    key.MakeNewKey(true);
+    CPubKey pubkey = key.GetPubKey();
+
+    // Generate pubkey hash
+    uint160 key_hash(Hash160(pubkey.begin(), pubkey.end()));
+
+    // Create inner-script to enter into keystore. Key hash can't be 0...
+    CScript inner_script = CScript() << OP_0 << std::vector<unsigned char>(key_hash.begin(), key_hash.end());
+
+    // Create outer P2SH script for the output
+    uint160 script_id(Hash160(inner_script.begin(), inner_script.end()));
+    CScript script_pubkey = CScript() << OP_HASH160 << std::vector<unsigned char>(script_id.begin(), script_id.end()) << OP_EQUAL;
+
+    // Add inner-script to key store and key to watchonly
+    CBasicKeyStore keystore;
+    keystore.AddCScript(inner_script);
+    keystore.AddKeyPubKey(key, pubkey);
+
+    // Fill in dummy signatures for fee calculation.
+    SignatureData sig_data;
+
+    if (!ProduceSignature(keystore, use_max_sig ? DUMMY_MAXIMUM_SIGNATURE_CREATOR : DUMMY_SIGNATURE_CREATOR, script_pubkey, sig_data)) {
+        // We're hand-feeding it correct arguments; shouldn't happen
+        assert(false);
+    }
+
+    CTxIn tx_in;
+    UpdateInput(tx_in, sig_data);
+    return (size_t)GetVirtualTransactionInputSize(tx_in);
+}
+
+BOOST_FIXTURE_TEST_CASE(dummy_input_size_test, TestChain100Setup)
+{
+    BOOST_CHECK_EQUAL(CalculateNestedKeyhashInputSize(false), DUMMY_NESTED_P2WPKH_INPUT_SIZE);
+    BOOST_CHECK_EQUAL(CalculateNestedKeyhashInputSize(true), DUMMY_NESTED_P2WPKH_INPUT_SIZE);
+}
+
 BOOST_AUTO_TEST_SUITE_END()
diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index 79e3b5b22b..eff94455db 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -2753,7 +2753,14 @@ bool CWallet::CreateTransaction(interfaces::Chain::Lock& locked_chain, const std
                 if (pick_new_inputs) {
                     nValueIn = 0;
                     setCoins.clear();
-                    coin_selection_params.change_spend_size = CalculateMaximumSignedInputSize(change_prototype_txout, this);
+                    int change_spend_size = CalculateMaximumSignedInputSize(change_prototype_txout, this);
+                    // If the wallet doesn't know how to sign change output, assume p2sh-p2wpkh
+                    // as lower-bound to allow BnB to do it's thing
+                    if (change_spend_size == -1) {
+                        coin_selection_params.change_spend_size = DUMMY_NESTED_P2WPKH_INPUT_SIZE;
+                    } else {
+                        coin_selection_params.change_spend_size = (size_t)change_spend_size;
+                    }
                     coin_selection_params.effective_fee = nFeeRateNeeded;
                     if (!SelectCoins(vAvailableCoins, nValueToSelect, setCoins, nValueIn, coin_control, coin_selection_params, bnb_used))
                     {
diff --git a/src/wallet/wallet.h b/src/wallet/wallet.h
index f96798201f..4291163bea 100644
--- a/src/wallet/wallet.h
+++ b/src/wallet/wallet.h
@@ -85,6 +85,9 @@ static const bool DEFAULT_WALLET_RBF = false;
 static const bool DEFAULT_WALLETBROADCAST = true;
 static const bool DEFAULT_DISABLE_WALLET = false;
 
+//! Pre-calculated constants for input size estimation in *virtual size*
+static constexpr size_t DUMMY_NESTED_P2WPKH_INPUT_SIZE = 91;
+
 class CBlockIndex;
 class CCoinControl;
 class COutput;