Add sane fallback for fee estimation

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

2 files changed, 12 insertions, 0 deletions

diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index 244bc37e76..581c688fc7 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
  *
@@ -2223,6 +2229,9 @@ 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 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));
diff --git a/src/wallet/wallet.h b/src/wallet/wallet.h
index 53a2b96690..ce57f4dae1 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
@@ -666,6 +668,7 @@ 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