Enforce minRelayTxFee on wallet created tx and add a maxtxfee option.

Previously the minRelayTxFee was only enforced on user specified values.

It was possible for smartfee to produce a fee below minRelayTxFee which
 would just result in the transaction getting stuck because it can't be
 relayed.

This also introduces a maxtxfee option which sets an absolute maximum
 for any fee created by the wallet, with an intention of increasing
 user confidence that the automatic fees won't burn them. This was
 frequently a concern even before smartfees.

If the configured fee policy won't even allow the wallet to meet the relay
 fee the transaction creation may be aborted.

Rebased-From: aa279d613152e87ea25edfdf76c86779c0632f18
Github-Pull: #5485

1 files changed, 28 insertions, 19 deletions

diff --git a/src/wallet.cpp b/src/wallet.cpp
index 32a64daac0..dd87fab44b 100644
--- a/src/wallet.cpp
+++ b/src/wallet.cpp
@@ -26,6 +26,7 @@ using namespace std;
  * Settings
  */
 CFeeRate payTxFee(DEFAULT_TRANSACTION_FEE);
+CAmount maxTxFee = DEFAULT_TRANSACTION_MAXFEE;
 unsigned int nTxConfirmTarget = 1;
 bool bSpendZeroConfChange = true;
 bool fSendFreeTransactions = false;
@@ -1499,27 +1500,32 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, CAmount> >& vecSend,
                 }
                 dPriority = wtxNew.ComputePriority(dPriority, nBytes);
 
-                CAmount nFeeNeeded = GetMinimumFee(nBytes, nTxConfirmTarget, mempool);
+                // Can we complete this as a free transaction?
+                if (fSendFreeTransactions && nBytes <= MAX_FREE_TRANSACTION_CREATE_SIZE)
+                {
+                    // 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;
+
+                    // Small enough, and priority high enough, to send for free
+                    if (dPriorityNeeded > 0 && dPriority >= dPriorityNeeded)
+                        break;
+                }
 
-                if (nFeeRet >= nFeeNeeded)
-                    break; // Done, enough fee included.
+                CAmount nFeeNeeded = GetMinimumFee(nBytes, nTxConfirmTarget, mempool);
 
-                // Too big to send for free? Include more fee and try again:
-                if (!fSendFreeTransactions || nBytes > MAX_FREE_TRANSACTION_CREATE_SIZE)
+                // If we made it here and we aren't even able to meet the relay fee on the next pass, give up
+                // because we must be at the maximum allowed fee.
+                if (nFeeNeeded < ::minRelayTxFee.GetFee(nBytes))
                 {
-                    nFeeRet = nFeeNeeded;
-                    continue;
+                    strFailReason = _("Transaction too large for fee policy");
+                    return false;
                 }
 
-                // 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;
-
-                // Small enough, and priority high enough, to send for free
-                if (dPriorityNeeded > 0 && dPriority >= dPriorityNeeded)
-                    break;
+                if (nFeeRet >= nFeeNeeded)
+                    break; // Done, enough fee included.
 
                 // Include more fee and try again.
                 nFeeRet = nFeeNeeded;
@@ -1591,9 +1597,6 @@ CAmount CWallet::GetMinimumFee(unsigned int nTxBytes, unsigned int nConfirmTarge
 {
     // payTxFee is user-set "I want to pay this much"
     CAmount nFeeNeeded = payTxFee.GetFee(nTxBytes);
-    // prevent user from paying a non-sense fee (like 1 satoshi): 0 < fee < minRelayFee
-    if (nFeeNeeded > 0 && nFeeNeeded < ::minRelayTxFee.GetFee(nTxBytes))
-        nFeeNeeded = ::minRelayTxFee.GetFee(nTxBytes);
     // user selected total at least (default=true)
     if (fPayAtLeastCustomFee && nFeeNeeded > 0 && nFeeNeeded < payTxFee.GetFeePerK())
         nFeeNeeded = payTxFee.GetFeePerK();
@@ -1604,6 +1607,12 @@ CAmount CWallet::GetMinimumFee(unsigned int nTxBytes, unsigned int nConfirmTarge
     // back to a hard-coded fee
     if (nFeeNeeded == 0)
         nFeeNeeded = minTxFee.GetFee(nTxBytes);
+    // prevent user from paying a non-sense fee (like 1 satoshi): 0 < fee < minRelayFee
+    if (nFeeNeeded < ::minRelayTxFee.GetFee(nTxBytes))
+        nFeeNeeded = ::minRelayTxFee.GetFee(nTxBytes);
+    // But always obey the maximum
+    if (nFeeNeeded > maxTxFee)
+        nFeeNeeded = maxTxFee;
     return nFeeNeeded;
 }