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

3 files changed, 48 insertions, 19 deletions

diff --git a/src/init.cpp b/src/init.cpp
index 5d42786038..3cdd42e2be 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -287,6 +287,7 @@ std::string HelpMessage(HelpMessageMode mode)
     strUsage += "  -sendfreetransactions  " + strprintf(_("Send transactions as zero-fee transactions if possible (default: %u)"), 0) + "\n";
     strUsage += "  -spendzeroconfchange   " + strprintf(_("Spend unconfirmed change when sending transactions (default: %u)"), 1) + "\n";
     strUsage += "  -txconfirmtarget=<n>   " + strprintf(_("If paytxfee is not set, include enough fee so transactions are confirmed on average within n blocks (default: %u)"), 1) + "\n";
+    strUsage += "  -maxtxfee=<amt>        " + strprintf(_("Maximum total fees to use in a single wallet transaction, setting too low may abort large transactions (default: %s)"), FormatMoney(maxTxFee)) + "\n";
     strUsage += "  -upgradewallet         " + _("Upgrade wallet to latest format") + " " + _("on startup") + "\n";
     strUsage += "  -wallet=<file>         " + _("Specify wallet file (within data directory)") + " " + strprintf(_("(default: %s)"), "wallet.dat") + "\n";
     strUsage += "  -walletnotify=<cmd>    " + _("Execute command when a wallet transaction changes (%s in cmd is replaced by TxID)") + "\n";
@@ -696,6 +697,20 @@ bool AppInit2(boost::thread_group& threadGroup)
                                        mapArgs["-paytxfee"], ::minRelayTxFee.ToString()));
         }
     }
+    if (mapArgs.count("-maxtxfee"))
+    {
+        CAmount nMaxFee = 0;
+        if (!ParseMoney(mapArgs["-maxtxfee"], nMaxFee))
+            return InitError(strprintf(_("Invalid amount for -maxtxfee=<amount>: '%s'"), mapArgs["-maptxfee"]));
+        if (nMaxFee > nHighTransactionMaxFeeWarning)
+            InitWarning(_("Warning: -maxtxfee is set very high! Fees this large could be paid on a single transaction."));
+        maxTxFee = nMaxFee;
+        if (CFeeRate(maxTxFee, 1000) < ::minRelayTxFee)
+        {
+            return InitError(strprintf(_("Invalid amount for -maxtxfee=<amount>: '%s' (must be at least the minrelay fee of %s to prevent stuck transactions)"),
+                                       mapArgs["-maxtxfee"], ::minRelayTxFee.ToString()));
+        }
+    }
     nTxConfirmTarget = GetArg("-txconfirmtarget", 1);
     bSpendZeroConfChange = GetArg("-spendzeroconfchange", true);
     fSendFreeTransactions = GetArg("-sendfreetransactions", false);
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;
 }
 
diff --git a/src/wallet.h b/src/wallet.h
index 2dd2146e8c..058107edf1 100644
--- a/src/wallet.h
+++ b/src/wallet.h
@@ -30,6 +30,7 @@
  * Settings
  */
 extern CFeeRate payTxFee;
+extern CAmount maxTxFee;
 extern unsigned int nTxConfirmTarget;
 extern bool bSpendZeroConfChange;
 extern bool fSendFreeTransactions;
@@ -39,6 +40,10 @@ extern bool fPayAtLeastCustomFee;
 static const CAmount DEFAULT_TRANSACTION_FEE = 0;
 //! -paytxfee will warn if called with a higher fee than this amount (in satoshis) per KB
 static const CAmount nHighTransactionFeeWarning = 0.01 * COIN;
+//! -maxtxfee default
+static const CAmount DEFAULT_TRANSACTION_MAXFEE = 0.1 * COIN;
+//! -maxtxfee will warn if called with a higher fee than this amount (in satoshis)
+static const CAmount nHighTransactionMaxFeeWarning = 100 * nHighTransactionFeeWarning;
 //! Largest (in bytes) free transaction we're willing to create
 static const unsigned int MAX_FREE_TRANSACTION_CREATE_SIZE = 1000;