Define dust transaction outputs, and make them non-standard

Author: gavinandresen

src/main.cpp

@@ -384,7 +384,7 @@ bool CTransaction::IsStandard() const
     BOOST_FOREACH(const CTxOut& txout, vout) {
         if (!::IsStandard(txout.scriptPubKey))
             return false;
-        if (txout.nValue == 0)
+        if (txout.IsDust())
             return false;
     }
     return true;

src/main.h

@@ -439,6 +439,24 @@ class CTxOut
         return !(a == b);
     }
 
+    size_t size() const
+    {
+        return sizeof(nValue)+scriptPubKey.size();
+    }
+
+    bool IsDust() const
+    {
+        // "Dust" is defined in terms of MIN_RELAY_TX_FEE, which
+        // has units satoshis-per-kilobyte.
+        // If you'd pay more than 1/3 in fees
+        // to spend something, then we consider it dust.
+        // A typical txout is 32 bytes big, and will
+        // need a CTxIn of at least 148 bytes to spend, 
+        // so dust is a txout less than 54 uBTC
+        // (5400 satoshis)
+        return ((nValue*1000)/(3*(size()+148)) < MIN_RELAY_TX_FEE);
+    }
+
     std::string ToString() const
     {
         if (scriptPubKey.size() < 6)

src/test/script_P2SH_tests.cpp

@@ -78,7 +78,9 @@ BOOST_AUTO_TEST_CASE(sign)
     for (int i = 0; i < 4; i++)
     {
         txFrom.vout[i].scriptPubKey = evalScripts[i];
+        txFrom.vout[i].nValue = COIN;
         txFrom.vout[i+4].scriptPubKey = standardScripts[i];
+        txFrom.vout[i+4].nValue = COIN;
     }
     BOOST_CHECK(txFrom.IsStandard());
 
@@ -169,6 +171,7 @@ BOOST_AUTO_TEST_CASE(set)
     for (int i = 0; i < 4; i++)
     {
         txFrom.vout[i].scriptPubKey = outer[i];
+        txFrom.vout[i].nValue = CENT;
     }
     BOOST_CHECK(txFrom.IsStandard());
 
@@ -179,7 +182,7 @@ BOOST_AUTO_TEST_CASE(set)
         txTo[i].vout.resize(1);
         txTo[i].vin[0].prevout.n = i;
         txTo[i].vin[0].prevout.hash = txFrom.GetHash();
-        txTo[i].vout[0].nValue = 1;
+        txTo[i].vout[0].nValue = 1*CENT;
         txTo[i].vout[0].scriptPubKey = inner[i];
         BOOST_CHECK_MESSAGE(IsMine(keystore, txFrom.vout[i].scriptPubKey), strprintf("IsMine %d", i));
     }

src/test/transaction_tests.cpp

@@ -242,24 +242,34 @@ BOOST_AUTO_TEST_CASE(test_Get)
     BOOST_CHECK(!t1.AreInputsStandard(coins));
 }
 
-BOOST_AUTO_TEST_CASE(test_GetThrow)
+BOOST_AUTO_TEST_CASE(test_IsStandard)
 {
     CBasicKeyStore keystore;
     CCoinsView coinsDummy;
     CCoinsViewCache coins(coinsDummy);
     std::vector<CTransaction> dummyTransactions = SetupDummyInputs(keystore, coins);
 
-    CTransaction t1;
-    t1.vin.resize(3);
-    t1.vin[0].prevout.hash = dummyTransactions[0].GetHash();
-    t1.vin[0].prevout.n = 0;
-    t1.vin[1].prevout.hash = dummyTransactions[1].GetHash();;
-    t1.vin[1].prevout.n = 0;
-    t1.vin[2].prevout.hash = dummyTransactions[1].GetHash();;
-    t1.vin[2].prevout.n = 1;
-    t1.vout.resize(2);
-    t1.vout[0].nValue = 90*CENT;
-    t1.vout[0].scriptPubKey << OP_1;
+    CTransaction t;
+    t.vin.resize(1);
+    t.vin[0].prevout.hash = dummyTransactions[0].GetHash();
+    t.vin[0].prevout.n = 1;
+    t.vin[0].scriptSig << std::vector<unsigned char>(65, 0);
+    t.vout.resize(1);
+    t.vout[0].nValue = 90*CENT;
+    CKey key;
+    key.MakeNewKey(true);
+    t.vout[0].scriptPubKey.SetDestination(key.GetPubKey().GetID());
+
+    BOOST_CHECK(t.IsStandard());
+
+    t.vout[0].nValue = 5011; // dust
+    BOOST_CHECK(!t.IsStandard());
+
+    t.vout[0].nValue = 6011; // not dust
+    BOOST_CHECK(t.IsStandard());
+
+    t.vout[0].scriptPubKey = CScript() << OP_1;
+    BOOST_CHECK(!t.IsStandard());
 }
 
 BOOST_AUTO_TEST_SUITE_END()

src/test/wallet_tests.cpp

@@ -21,13 +21,12 @@ static vector<COutput> vCoins;
 
 static void add_coin(int64 nValue, int nAge = 6*24, bool fIsFromMe = false, int nInput=0)
 {
-    static int i;
-    CTransaction* tx = new CTransaction;
-    tx->nLockTime = i++;        // so all transactions get different hashes
-    tx->vout.resize(nInput+1);
-    tx->vout[nInput].nValue = nValue;
-    CWalletTx* wtx = new CWalletTx(&wallet, *tx);
-    delete tx;
+    static int nextLockTime = 0;
+    CTransaction tx;
+    tx.nLockTime = nextLockTime++;        // so all transactions get different hashes
+    tx.vout.resize(nInput+1);
+    tx.vout[nInput].nValue = nValue;
+    CWalletTx* wtx = new CWalletTx(&wallet, tx);
     if (fIsFromMe)
     {
         // IsFromMe() returns (GetDebit() > 0), and GetDebit() is 0 if vin.empty(),
@@ -55,8 +54,8 @@ static bool equal_sets(CoinSet a, CoinSet b)
 
 BOOST_AUTO_TEST_CASE(coin_selection_tests)
 {
-    static CoinSet setCoinsRet, setCoinsRet2;
-    static int64 nValueRet;
+    CoinSet setCoinsRet, setCoinsRet2;
+    int64 nValueRet;
 
     // test multiple times to allow for differences in the shuffle order
     for (int i = 0; i < RUN_TESTS; i++)

src/wallet.cpp

@@ -1162,7 +1162,12 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, int64> >& vecSend, CW
                 double dPriority = 0;
                 // vouts to the payees
                 BOOST_FOREACH (const PAIRTYPE(CScript, int64)& s, vecSend)
-                    wtxNew.vout.push_back(CTxOut(s.second, s.first));
+                {
+                    CTxOut txout(s.second, s.first);
+                    if (txout.IsDust())
+                        return false;
+                    wtxNew.vout.push_back(txout);
+                }
 
                 // Choose coins to use
                 set<pair<const CWalletTx*,unsigned int> > setCoins;
@@ -1208,9 +1213,21 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, int64> >& vecSend, CW
                     CScript scriptChange;
                     scriptChange.SetDestination(vchPubKey.GetID());
 
-                    // Insert change txn at random position:
-                    vector<CTxOut>::iterator position = wtxNew.vout.begin()+GetRandInt(wtxNew.vout.size()+1);
-                    wtxNew.vout.insert(position, CTxOut(nChange, scriptChange));
+                    CTxOut newTxOut(nChange, scriptChange);
+
+                    // Never create dust outputs; if we would, just
+                    // add the dust to the fee.
+                    if (newTxOut.IsDust())
+                    {
+                        nFeeRet += nChange;
+                        reservekey.ReturnKey();
+                    }
+                    else
+                    {
+                        // Insert change txn at random position:
+                        vector<CTxOut>::iterator position = wtxNew.vout.begin()+GetRandInt(wtxNew.vout.size()+1);
+                        wtxNew.vout.insert(position, newTxOut);
+                    }
                 }
                 else
                     reservekey.ReturnKey();