Add sane fallback for fee estimation

- Always respect GetRequiredFee for wallet txs
- Add sane fallback for fee estimation
- SQUASHME: Fix rpc tests that assumed fallback to minRelayTxFee

Add new commandline option "-fallbackfee" to use when fee estimation does not have sufficient data.

Github-Pull: #7296
Rebased-From: 995b9f3 e420a1b bebe58b

6 files changed, 41 insertions, 10 deletions

diff --git a/qa/rpc-tests/fundrawtransaction.py b/qa/rpc-tests/fundrawtransaction.py
index d6493dbb8a..dda9166151 100755
--- a/qa/rpc-tests/fundrawtransaction.py
+++ b/qa/rpc-tests/fundrawtransaction.py
@@ -30,6 +30,11 @@ class RawTransactionsTest(BitcoinTestFramework):
         print "Mining blocks..."
 
         min_relay_tx_fee = self.nodes[0].getnetworkinfo()['relayfee']
+        # This test is not meant to test fee estimation and we'd like
+        # to be sure all txs are sent at a consistent desired feerate
+        for node in self.nodes:
+            node.settxfee(min_relay_tx_fee)
+
         # if the fee's positive delta is higher than this value tests will fail,
         # neg. delta always fail the tests.
         # The size of the signature of every input may be at most 2 bytes larger
@@ -447,6 +452,10 @@ class RawTransactionsTest(BitcoinTestFramework):
         wait_bitcoinds()
 
         self.nodes = start_nodes(4, self.options.tmpdir)
+        # This test is not meant to test fee estimation and we'd like
+        # to be sure all txs are sent at a consistent desired feerate
+        for node in self.nodes:
+            node.settxfee(min_relay_tx_fee)
 
         connect_nodes_bi(self.nodes,0,1)
         connect_nodes_bi(self.nodes,1,2)
diff --git a/qa/rpc-tests/mempool_limit.py b/qa/rpc-tests/mempool_limit.py
index 3ba17ac4f1..a8cf6360ee 100755
--- a/qa/rpc-tests/mempool_limit.py
+++ b/qa/rpc-tests/mempool_limit.py
@@ -33,7 +33,9 @@ class MempoolLimitTest(BitcoinTestFramework):
         inputs = [{ "txid" : us0["txid"], "vout" : us0["vout"]}]
         outputs = {self.nodes[0].getnewaddress() : 0.0001}
         tx = self.nodes[0].createrawtransaction(inputs, outputs)
+        self.nodes[0].settxfee(self.relayfee) # specifically fund this tx with low fee
         txF = self.nodes[0].fundrawtransaction(tx)
+        self.nodes[0].settxfee(0) # return to automatic fee selection
         txFS = self.nodes[0].signrawtransaction(txF['hex'])
         txid = self.nodes[0].sendrawtransaction(txFS['hex'])
         self.nodes[0].lockunspent(True, [us0])
diff --git a/src/init.cpp b/src/init.cpp
index 60b6194b3c..47d1271248 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -393,6 +393,8 @@ std::string HelpMessage(HelpMessageMode mode)
     strUsage += HelpMessageGroup(_("Wallet options:"));
     strUsage += HelpMessageOpt("-disablewallet", _("Do not load the wallet and disable wallet RPC calls"));
     strUsage += HelpMessageOpt("-keypool=<n>", strprintf(_("Set key pool size to <n> (default: %u)"), DEFAULT_KEYPOOL_SIZE));
+    strUsage += HelpMessageOpt("-fallbackfee=<amt>", strprintf(_("A fee rate (in %s/kB) that will be used when fee estimation has insufficient data (default: %s)"),
+        CURRENCY_UNIT, FormatMoney(DEFAULT_FALLBACK_FEE)));
     strUsage += HelpMessageOpt("-mintxfee=<amt>", strprintf(_("Fees (in %s/kB) smaller than this are considered zero fee for transaction creation (default: %s)"),
             CURRENCY_UNIT, FormatMoney(DEFAULT_TRANSACTION_MINFEE)));
     strUsage += HelpMessageOpt("-paytxfee=<amt>", strprintf(_("Fee (in %s/kB) to add to transactions you send (default: %s)"),
@@ -949,6 +951,15 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
         else
             return InitError(strprintf(_("Invalid amount for -mintxfee=<amount>: '%s'"), mapArgs["-mintxfee"]));
     }
+    if (mapArgs.count("-fallbackfee"))
+    {
+        CAmount nFeePerK = 0;
+        if (!ParseMoney(mapArgs["-fallbackfee"], nFeePerK))
+            return InitError(strprintf(_("Invalid amount for -fallbackfee=<amount>: '%s'"), mapArgs["-fallbackfee"]));
+        if (nFeePerK > nHighTransactionFeeWarning)
+            InitWarning(_("-fallbackfee is set very high! This is the transaction fee you may pay when fee estimates are not available."));
+        CWallet::fallbackFee = CFeeRate(nFeePerK);
+    }
     if (mapArgs.count("-paytxfee"))
     {
         CAmount nFeePerK = 0;
diff --git a/src/qt/sendcoinsdialog.cpp b/src/qt/sendcoinsdialog.cpp
index 31c9028c4b..c834c3b564 100644
--- a/src/qt/sendcoinsdialog.cpp
+++ b/src/qt/sendcoinsdialog.cpp
@@ -640,13 +640,15 @@ void SendCoinsDialog::updateSmartFeeLabel()
     CFeeRate feeRate = mempool.estimateSmartFee(nBlocksToConfirm, &estimateFoundAtBlocks);
     if (feeRate <= CFeeRate(0)) // not enough data => minfee
     {
-        ui->labelSmartFee->setText(BitcoinUnits::formatWithUnit(model->getOptionsModel()->getDisplayUnit(), CWallet::GetRequiredFee(1000)) + "/kB");
+        ui->labelSmartFee->setText(BitcoinUnits::formatWithUnit(model->getOptionsModel()->getDisplayUnit(),
+                                                                std::max(CWallet::fallbackFee.GetFeePerK(), CWallet::GetRequiredFee(1000))) + "/kB");
         ui->labelSmartFee2->show(); // (Smart fee not initialized yet. This usually takes a few blocks...)
         ui->labelFeeEstimation->setText("");
     }
     else
     {
-        ui->labelSmartFee->setText(BitcoinUnits::formatWithUnit(model->getOptionsModel()->getDisplayUnit(), feeRate.GetFeePerK()) + "/kB");
+        ui->labelSmartFee->setText(BitcoinUnits::formatWithUnit(model->getOptionsModel()->getDisplayUnit(),
+                                                                std::max(feeRate.GetFeePerK(), CWallet::GetRequiredFee(1000))) + "/kB");
         ui->labelSmartFee2->hide();
         ui->labelFeeEstimation->setText(tr("Estimated to begin confirmation within %n block(s).", "", estimateFoundAtBlocks));
     }
diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index 5d03ce3283..610d99dc8d 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -47,6 +47,12 @@ bool fSendFreeTransactions = DEFAULT_SEND_FREE_TRANSACTIONS;
  * Override with -mintxfee
  */
 CFeeRate CWallet::minTxFee = CFeeRate(DEFAULT_TRANSACTION_MINFEE);
+/**
+ * If fee estimation does not have enough data to provide estimates, use this fee instead.
+ * Has no effect if not using fee estimation
+ * Override with -fallbackfee
+ */
+CFeeRate CWallet::fallbackFee = CFeeRate(DEFAULT_FALLBACK_FEE);
 
 /** @defgroup mapWallet
  *
@@ -2225,14 +2231,12 @@ CAmount CWallet::GetMinimumFee(unsigned int nTxBytes, unsigned int nConfirmTarge
     if (nFeeNeeded == 0) {
         int estimateFoundTarget = nConfirmTarget;
         nFeeNeeded = pool.estimateSmartFee(nConfirmTarget, &estimateFoundTarget).GetFee(nTxBytes);
-        // ... unless we don't have enough mempool data for our desired target
-        // so we make sure we're paying at least minTxFee
-        if (nFeeNeeded == 0 || (unsigned int)estimateFoundTarget > nConfirmTarget)
-            nFeeNeeded = std::max(nFeeNeeded, GetRequiredFee(nTxBytes));
-    }
-    // prevent user from paying a non-sense fee (like 1 satoshi): 0 < fee < minRelayFee
-    if (nFeeNeeded < ::minRelayTxFee.GetFee(nTxBytes))
-        nFeeNeeded = ::minRelayTxFee.GetFee(nTxBytes);
+        // ... unless we don't have enough mempool data for estimatefee, then use fallbackFee
+        if (nFeeNeeded == 0)
+            nFeeNeeded = fallbackFee.GetFee(nTxBytes);
+    }
+    // prevent user from paying a fee below minRelayTxFee or minTxFee
+    nFeeNeeded = std::max(nFeeNeeded, GetRequiredFee(nTxBytes));
     // But always obey the maximum
     if (nFeeNeeded > maxTxFee)
         nFeeNeeded = maxTxFee;
diff --git a/src/wallet/wallet.h b/src/wallet/wallet.h
index 33c339bba1..89fc17bc74 100644
--- a/src/wallet/wallet.h
+++ b/src/wallet/wallet.h
@@ -41,6 +41,8 @@ static const unsigned int DEFAULT_KEYPOOL_SIZE = 100;
 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;
+//! -fallbackfee default
+static const CAmount DEFAULT_FALLBACK_FEE = 20000;
 //! -mintxfee default
 static const CAmount DEFAULT_TRANSACTION_MINFEE = 1000;
 //! -maxtxfee default
@@ -665,6 +667,7 @@ public:
     bool AddAccountingEntry(const CAccountingEntry&, CWalletDB & pwalletdb);
 
     static CFeeRate minTxFee;
+    static CFeeRate fallbackFee;
     /**
      * Estimate the minimum fee considering user set parameters
      * and the required fee