[Fee logic] Don't count txins for priority to encourage sweeping.

This changes the priority calculation to not include the size of per-txin
 data including up to 110 bytes of scriptsig so that transactions which
 sweep up extra UTXO don't lose priority relative to ones that don't.

I'd toyed with some other variations, but it seems like any formulation
 which results in an incentive stronger than making them not count will
 sometimes create incentives to add extra outputs so that you have
 extra inputs to consume later.  The maximum credit is limited so that
 users don't lose the disincentive to stuff random data in their
 transactions, the limit of 110 is based on the size of a P2SH
 redemption with a compressed public key.

This shouldn't need a staged deployment because the priority is not
 used as a relay criteria, only a mining criteria.

1 files changed, 14 insertions, 2 deletions

diff --git a/src/miner.cpp b/src/miner.cpp
index 3ecf1609e1..8184705ab9 100644
--- a/src/miner.cpp
+++ b/src/miner.cpp
@@ -242,9 +242,21 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
             }
             if (fMissingInputs) continue;
 
-            // Priority is sum(valuein * age) / txsize
+            // Priority is sum(valuein * age) / modified_txsize
             unsigned int nTxSize = ::GetSerializeSize(tx, SER_NETWORK, PROTOCOL_VERSION);
-            dPriority /= nTxSize;
+            unsigned int nTxSizeMod = nTxSize;
+            // In order to avoid disincentivizing cleaning up the UTXO set we don't count
+            // the constant overhead for each txin and up to 110 bytes of scriptSig (which
+            // is enough to cover a compressed pubkey p2sh redemption) for priority.
+            // Providing any more cleanup incentive than making additional inputs free would
+            // risk encouraging people to create junk outputs to redeem later.
+            BOOST_FOREACH(const CTxIn& txin, tx.vin)
+            {
+                unsigned int offset = 41U + min(110U, (unsigned int)txin.scriptSig.size());
+                if (nTxSizeMod > offset)
+                    nTxSizeMod -= offset;
+            }
+            dPriority /= nTxSizeMod;
 
             // This is a more accurate fee-per-kilobyte than is used by the client code, because the
             // client code rounds up the size to the nearest 1K. That's good, because it gives an