Consensus: Minimal way to move dust out of consensus

2 files changed, 43 insertions, 1 deletions

diff --git a/src/policy/policy.cpp b/src/policy/policy.cpp
index 6f8b6c2953..2b19a6714b 100644
--- a/src/policy/policy.cpp
+++ b/src/policy/policy.cpp
@@ -15,6 +15,43 @@
 
 #include <boost/foreach.hpp>
 
+CAmount GetDustThreshold(const CTxOut& txout, const CFeeRate& dustRelayFee)
+{
+    // "Dust" is defined in terms of dustRelayFee,
+    // 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 spendable non-segwit txout is 34 bytes big, and will
+    // need a CTxIn of at least 148 bytes to spend:
+    // so dust is a spendable txout less than
+    // 546*dustRelayFee/1000 (in satoshis).
+    // A typical spendable segwit txout is 31 bytes big, and will
+    // need a CTxIn of at least 67 bytes to spend:
+    // so dust is a spendable txout less than
+    // 294*dustRelayFee/1000 (in satoshis).
+    if (txout.scriptPubKey.IsUnspendable())
+        return 0;
+
+    size_t nSize = GetSerializeSize(txout, SER_DISK, 0);
+    int witnessversion = 0;
+    std::vector<unsigned char> witnessprogram;
+
+    if (txout.scriptPubKey.IsWitnessProgram(witnessversion, witnessprogram)) {
+        // sum the sizes of the parts of a transaction input
+        // with 75% segwit discount applied to the script size.
+        nSize += (32 + 4 + 1 + (107 / WITNESS_SCALE_FACTOR) + 4);
+    } else {
+        nSize += (32 + 4 + 1 + 107 + 4); // the 148 mentioned above
+    }
+
+    return 3 * dustRelayFee.GetFee(nSize);
+}
+
+bool IsDust(const CTxOut& txout, const CFeeRate& dustRelayFee)
+{
+    return (txout.nValue < GetDustThreshold(txout, dustRelayFee));
+}
+
     /**
      * Check transaction inputs to mitigate two
      * potential denial-of-service attacks:
@@ -106,7 +143,7 @@ bool IsStandardTx(const CTransaction& tx, std::string& reason, const bool witnes
         else if ((whichType == TX_MULTISIG) && (!fIsBareMultisigStd)) {
             reason = "bare-multisig";
             return false;
-        } else if (txout.IsDust(dustRelayFee)) {
+        } else if (IsDust(txout, ::dustRelayFee)) {
             reason = "dust";
             return false;
         }
diff --git a/src/policy/policy.h b/src/policy/policy.h
index 6df541bc0f..24bf3fa5d7 100644
--- a/src/policy/policy.h
+++ b/src/policy/policy.h
@@ -13,6 +13,7 @@
 #include <string>
 
 class CCoinsViewCache;
+class CTxOut;
 
 /** Default for -blockmaxsize, which controls the maximum size of block the mining code will create **/
 static const unsigned int DEFAULT_BLOCK_MAX_SIZE = 750000;
@@ -72,6 +73,10 @@ static const unsigned int STANDARD_NOT_MANDATORY_VERIFY_FLAGS = STANDARD_SCRIPT_
 static const unsigned int STANDARD_LOCKTIME_VERIFY_FLAGS = LOCKTIME_VERIFY_SEQUENCE |
                                                            LOCKTIME_MEDIAN_TIME_PAST;
 
+CAmount GetDustThreshold(const CTxOut& txout, const CFeeRate& dustRelayFee);
+
+bool IsDust(const CTxOut& txout, const CFeeRate& dustRelayFee);
+
 bool IsStandard(const CScript& scriptPubKey, txnouttype& whichType, const bool witnessEnabled = false);
     /**
      * Check for standard transaction types