Use fee/priority estimates in wallet CreateTransaction

The wallet now uses the mempool fee estimator with a new
command-line option: -txconfirmtarget (default: 1) instead
of using hard-coded fees or priorities.

A new bitcoind that hasn't seen enough transactions to estimate
will fall back to the old hard-coded minimum priority or
transaction fee.

-paytxfee option overrides -txconfirmtarget.

Relaying and mining code isn't changed.

For Qt, the coin control dialog now uses priority estimates to
label transaction priority (instead of hard-coded constants);
unspent outputs were consistently labeled with a much higher
priority than is justified by the free transactions actually
being accepted into blocks.

I did not implement any GUI for setting -txconfirmtarget; I would
suggest getting rid of the "Pay transaction fee" GUI and replace
it with either "target number of confirmations" or maybe
a "faster confirmation <--> lower fee" slider or select box.

1 files changed, 36 insertions, 8 deletions

diff --git a/src/wallet.cpp b/src/wallet.cpp
index daca7ac04b..d9187e4be3 100644
--- a/src/wallet.cpp
+++ b/src/wallet.cpp
@@ -18,6 +18,7 @@ using namespace std;
 
 // Settings
 CFeeRate payTxFee(DEFAULT_TRANSACTION_FEE);
+unsigned int nTxConfirmTarget = 1;
 bool bSpendZeroConfChange = true;
 
 //////////////////////////////////////////////////////////////////////////////
@@ -1273,6 +1274,7 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, int64_t> >& vecSend,
         return false;
     }
 
+    wtxNew.fTimeReceivedIsTxTime = true;
     wtxNew.BindWallet(this);
     CMutableTransaction txNew;
 
@@ -1393,19 +1395,31 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, int64_t> >& vecSend,
                 }
                 dPriority = wtxNew.ComputePriority(dPriority, nBytes);
 
-                // Check that enough fee is included
-                int64_t nPayFee = payTxFee.GetFee(nBytes);
-                bool fAllowFree = AllowFree(dPriority);
-                int64_t nMinFee = GetMinFee(wtxNew, nBytes, fAllowFree, GMF_SEND);
-                if (nFeeRet < max(nPayFee, nMinFee))
+                int64_t nFeeNeeded = GetMinimumFee(nBytes, nTxConfirmTarget, mempool);
+
+                if (nFeeRet >= nFeeNeeded)
+                    break; // Done, enough fee included.
+
+                // Too big to send for free? Include more fee and try again:
+                if (nBytes > MAX_FREE_TRANSACTION_CREATE_SIZE)
                 {
-                    nFeeRet = max(nPayFee, nMinFee);
+                    nFeeRet = nFeeNeeded;
                     continue;
                 }
 
-                wtxNew.fTimeReceivedIsTxTime = true;
+                // Not enough fee: enough priority?
+                double dPriorityNeeded = mempool.estimatePriority(nTxConfirmTarget);
+                // Not enough mempool history to estimate: use hard-coded AllowFree.
+                if (dPriorityNeeded <= 0 && AllowFree(dPriority))
+                    break;
 
-                break;
+                // Small enough, and priority high enough, to send for free
+                if (dPriority >= dPriorityNeeded)
+                    break;
+
+                // Include more fee and try again.
+                nFeeRet = nFeeNeeded;
+                continue;
             }
         }
     }
@@ -1513,6 +1527,20 @@ string CWallet::SendMoneyToDestination(const CTxDestination& address, int64_t nV
     return SendMoney(scriptPubKey, nValue, wtxNew);
 }
 
+int64_t CWallet::GetMinimumFee(unsigned int nTxBytes, unsigned int nConfirmTarget, const CTxMemPool& pool)
+{
+    // payTxFee is user-set "I want to pay this much"
+    int64_t nFeeNeeded = payTxFee.GetFee(nTxBytes);
+    // User didn't set: use -txconfirmtarget to estimate...
+    if (nFeeNeeded == 0)
+        nFeeNeeded = pool.estimateFee(nConfirmTarget).GetFee(nTxBytes);
+    // ... unless we don't have enough mempool data, in which case fall
+    // back to a hard-coded fee
+    if (nFeeNeeded == 0)
+        nFeeNeeded = CTransaction::minTxFee.GetFee(nTxBytes);
+    return nFeeNeeded;
+}
+