update estimatesmartfee rpc to return max of estimateSmartFee, mempoolMinFee and minRelayTxFee.

This will provide better estimates which would be closer to fee paid in actual
transactions.
The test has also been changed such that when the node is restarted with a
high mempoolMinFee, the estimatesmartfee still returns a feeRate greater
than or equal to the mempoolMinFee, minRelayTxFee.(just like the feeRate of actual transactions)

src/rpc/mining.cpp

@@ -1103,6 +1103,8 @@ static RPCHelpMan estimatesmartfee()
     RPCTypeCheckArgument(request.params[0], UniValue::VNUM);
 
     CBlockPolicyEstimator& fee_estimator = EnsureAnyFeeEstimator(request.context);
+    const NodeContext& node = EnsureAnyNodeContext(request.context);
+    const CTxMemPool& mempool = EnsureMemPool(node);
 
     unsigned int max_target = fee_estimator.HighestTargetTracked(FeeEstimateHorizon::LONG_HALFLIFE);
     unsigned int conf_target = ParseConfirmTarget(request.params[0], max_target);
@@ -1118,7 +1120,10 @@ static RPCHelpMan estimatesmartfee()
     UniValue result(UniValue::VOBJ);
     UniValue errors(UniValue::VARR);
     FeeCalculation feeCalc;
-    CFeeRate feeRate = fee_estimator.estimateSmartFee(conf_target, &feeCalc, conservative);
+    CFeeRate feeRate{fee_estimator.estimateSmartFee(conf_target, &feeCalc, conservative)};
+    CFeeRate min_mempool_feerate{mempool.GetMinFee(gArgs.GetIntArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000)};
+    CFeeRate min_relay_feerate{::minRelayTxFee};
+    feeRate = std::max({feeRate, min_mempool_feerate, min_relay_feerate});
     if (feeRate != CFeeRate(0)) {
         result.pushKV("feerate", ValueFromAmount(feeRate.GetFeePerK()));
     } else {

test/functional/feature_fee_estimation.py

@@ -132,9 +132,13 @@ def check_smart_estimates(node, fees_seen):
     delta = 1.0e-6  # account for rounding error
     last_feerate = float(max(fees_seen))
     all_smart_estimates = [node.estimatesmartfee(i) for i in range(1, 26)]
+    mempoolMinFee = node.getmempoolinfo()['mempoolminfee']
+    minRelaytxFee = node.getmempoolinfo()['minrelaytxfee']
     for i, e in enumerate(all_smart_estimates):  # estimate is for i+1
         feerate = float(e["feerate"])
         assert_greater_than(feerate, 0)
+        assert_greater_than_or_equal(feerate, float(mempoolMinFee))
+        assert_greater_than_or_equal(feerate, float(minRelaytxFee))
 
         if feerate + delta < min(fees_seen) or feerate - delta > max(fees_seen):
             raise AssertionError(f"Estimated fee ({feerate}) out of range ({min(fees_seen)},{max(fees_seen)})")
@@ -275,6 +279,12 @@ class EstimateFeeTest(BitcoinTestFramework):
         self.log.info("Final estimates after emptying mempools")
         check_estimates(self.nodes[1], self.fees_per_kb)
 
+        # check that the effective feerate is greater than or equal to the mempoolminfee even for high mempoolminfee
+        self.log.info("Test fee rate estimation after restarting node with high MempoolMinFee")
+        high_val = 3*self.nodes[1].estimatesmartfee(1)['feerate']
+        self.restart_node(1, extra_args=[f'-minrelaytxfee={high_val}'])
+        check_estimates(self.nodes[1], self.fees_per_kb)
+
         self.log.info("Testing that fee estimation is disabled in blocksonly.")
         self.restart_node(0, ["-blocksonly"])
         assert_raises_rpc_error(-32603, "Fee estimation disabled",