Policy to reject extremely small transactions

A transaction with 1 segwit input and 1 P2WPHK output has non-witness size of 82 bytes. Anything smaller than this have unnecessary malloc overhead and are not relayed/mined.

Github-Pull: bitcoin#11423
Rebased-From: 7485488

src/policy/policy.h

@@ -22,6 +22,8 @@ static const unsigned int DEFAULT_BLOCK_MAX_WEIGHT = MAX_BLOCK_WEIGHT - 4000;
 static const unsigned int DEFAULT_BLOCK_MIN_TX_FEE = 1000;
 /** The maximum weight for transactions we're willing to relay/mine */
 static const unsigned int MAX_STANDARD_TX_WEIGHT = 400000;
+/** The minimum non-witness size for transactions we're willing to relay/mine (1 segwit input + 1 P2WPKH output = 82 bytes) */
+static const unsigned int MIN_STANDARD_TX_NONWITNESS_SIZE = 82;
 /** Maximum number of signature check operations in an IsStandard() P2SH script */
 static const unsigned int MAX_P2SH_SIGOPS = 15;
 /** The maximum number of sigops we're willing to relay/mine in a single tx */

src/validation.cpp

@@ -576,6 +576,12 @@ static bool AcceptToMemoryPoolWorker(const CChainParams& chainparams, CTxMemPool
     if (fRequireStandard && !IsStandardTx(tx, reason, witnessEnabled))
         return state.DoS(0, false, REJECT_NONSTANDARD, reason);
 
+    // Do not work on transactions that are too small.
+    // A transaction with 1 segwit input and 1 P2WPHK output has non-witness size of 82 bytes.
+    // Transactions smaller than this are not relayed to reduce unnecessary malloc overhead.
+    if (::GetSerializeSize(tx, SER_NETWORK, PROTOCOL_VERSION | SERIALIZE_TRANSACTION_NO_WITNESS) < MIN_STANDARD_TX_NONWITNESS_SIZE)
+        return state.DoS(0, false, REJECT_NONSTANDARD, "tx-size-small");
+
     // Only accept nLockTime-using transactions that can be mined in the next
     // block; we don't want our mempool filled up with transactions that can't
     // be mined yet.