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

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));
+        // ... 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 non-sense fee (like 1 satoshi): 0 < fee < minRelayFee
-    if (nFeeNeeded < ::minRelayTxFee.GetFee(nTxBytes))
-        nFeeNeeded = ::minRelayTxFee.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;