BIP141: Other consensus critical limits, and BIP145

Includes changes by Suhas Daftuar, Luke-jr, and mruddy.

4 files changed, 42 insertions, 5 deletions

diff --git a/src/primitives/block.cpp b/src/primitives/block.cpp
index 6fb33230a5..df900388f2 100644
--- a/src/primitives/block.cpp
+++ b/src/primitives/block.cpp
@@ -31,3 +31,12 @@ std::string CBlock::ToString() const
     }
     return s.str();
 }
+
+int64_t GetBlockCost(const CBlock& block)
+{
+    // This implements the cost = (stripped_size * 4) + witness_size formula,
+    // using only serialization with and without witness data. As witness_size
+    // is equal to total_size - stripped_size, this formula is identical to:
+    // cost = (stripped_size * 3) + total_size.
+    return ::GetSerializeSize(block, SER_NETWORK, PROTOCOL_VERSION | SERIALIZE_TRANSACTION_NO_WITNESS) * (WITNESS_SCALE_FACTOR - 1) + ::GetSerializeSize(block, SER_NETWORK, PROTOCOL_VERSION);
+}
diff --git a/src/primitives/block.h b/src/primitives/block.h
index 29307aed5b..e2a309e63d 100644
--- a/src/primitives/block.h
+++ b/src/primitives/block.h
@@ -154,4 +154,7 @@ struct CBlockLocator
     }
 };
 
+/** Compute the consensus-critical block cost (see BIP 141). */
+int64_t GetBlockCost(const CBlock& tx);
+
 #endif // BITCOIN_PRIMITIVES_BLOCK_H
diff --git a/src/primitives/transaction.cpp b/src/primitives/transaction.cpp
index b0230530e8..7f10409c05 100644
--- a/src/primitives/transaction.cpp
+++ b/src/primitives/transaction.cpp
@@ -121,7 +121,7 @@ unsigned int CTransaction::CalculateModifiedSize(unsigned int nTxSize) const
     // Providing any more cleanup incentive than making additional inputs free would
     // risk encouraging people to create junk outputs to redeem later.
     if (nTxSize == 0)
-        nTxSize = ::GetSerializeSize(*this, SER_NETWORK, PROTOCOL_VERSION);
+        nTxSize = (GetTransactionCost(*this) + WITNESS_SCALE_FACTOR - 1) / WITNESS_SCALE_FACTOR;
     for (std::vector<CTxIn>::const_iterator it(vin.begin()); it != vin.end(); ++it)
     {
         unsigned int offset = 41U + std::min(110U, (unsigned int)it->scriptSig.size());
@@ -148,3 +148,8 @@ std::string CTransaction::ToString() const
         str += "    " + vout[i].ToString() + "\n";
     return str;
 }
+
+int64_t GetTransactionCost(const CTransaction& tx)
+{
+    return ::GetSerializeSize(tx, SER_NETWORK, PROTOCOL_VERSION | SERIALIZE_TRANSACTION_NO_WITNESS) * (WITNESS_SCALE_FACTOR -1) + ::GetSerializeSize(tx, SER_NETWORK, PROTOCOL_VERSION);
+}
diff --git a/src/primitives/transaction.h b/src/primitives/transaction.h
index 5a58241347..e87ad90f0d 100644
--- a/src/primitives/transaction.h
+++ b/src/primitives/transaction.h
@@ -13,6 +13,8 @@
 
 static const int SERIALIZE_TRANSACTION_NO_WITNESS = 0x40000000;
 
+static const int WITNESS_SCALE_FACTOR = 4;
+
 /** An outpoint - a combination of a transaction hash and an index n into its vout */
 class COutPoint
 {
@@ -166,15 +168,30 @@ public:
         // 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 txout is 34 bytes big, and will
+        // 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*minRelayTxFee/1000 (in satoshis)
+        // 546*minRelayTxFee/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*minRelayTxFee/1000 (in satoshis).
         if (scriptPubKey.IsUnspendable())
             return 0;
 
-        size_t nSize = GetSerializeSize(SER_DISK,0)+148u;
-        return 3*minRelayTxFee.GetFee(nSize);
+        size_t nSize = GetSerializeSize(SER_DISK, 0);
+        int witnessversion = 0;
+        std::vector<unsigned char> witnessprogram;
+
+        if (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 * minRelayTxFee.GetFee(nSize);
     }
 
     bool IsDust(const CFeeRate &minRelayTxFee) const
@@ -442,4 +459,7 @@ struct CMutableTransaction
     uint256 GetHash() const;
 };
 
+/** Compute the cost of a transaction, as defined by BIP 141 */
+int64_t GetTransactionCost(const CTransaction &tx);
+
 #endif // BITCOIN_PRIMITIVES_TRANSACTION_H