Improve api to estimatesmartfee

Change parameter for conservative estimates to be an estimate_mode string.
Change to never return a -1 for failure but to instead omit the feerate and
return an error string.  Throw JSONRPC error on invalid nblocks parameter.

src/policy/fees.cpp

@@ -839,20 +839,20 @@ CFeeRate CBlockPolicyEstimator::estimateSmartFee(int confTarget, FeeCalculation
     EstimationResult tempResult;
 
     // Return failure if trying to analyze a target we're not tracking
-    if (confTarget <= 0 || (unsigned int)confTarget > longStats->GetMaxConfirms())
-        return CFeeRate(0);
+    if (confTarget <= 0 || (unsigned int)confTarget > longStats->GetMaxConfirms()) {
+        return CFeeRate(0);  // error conditon
+    }
 
     // It's not possible to get reasonable estimates for confTarget of 1
-    if (confTarget == 1)
-        confTarget = 2;
+    if (confTarget == 1) confTarget = 2;
 
     unsigned int maxUsableEstimate = MaxUsableEstimate();
-    if (maxUsableEstimate <= 1)
-        return CFeeRate(0);
-
     if ((unsigned int)confTarget > maxUsableEstimate) {
         confTarget = maxUsableEstimate;
     }
+    if (feeCalc) feeCalc->returnedTarget = confTarget;
+
+    if (confTarget <= 1) return CFeeRate(0); // error conditon
 
     assert(confTarget > 0); //estimateCombinedFee and estimateConservativeFee take unsigned ints
     /** true is passed to estimateCombined fee for target/2 and target so
@@ -899,10 +899,7 @@ CFeeRate CBlockPolicyEstimator::estimateSmartFee(int confTarget, FeeCalculation
         }
     }
 
-    if (feeCalc) feeCalc->returnedTarget = confTarget;
-
-    if (median < 0)
-        return CFeeRate(0);
+    if (median < 0) return CFeeRate(0); // error conditon
 
     return CFeeRate(median);
 }

src/rpc/client.cpp

@@ -114,7 +114,6 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "getrawmempool", 0, "verbose" },
     { "estimatefee", 0, "nblocks" },
     { "estimatesmartfee", 0, "nblocks" },
-    { "estimatesmartfee", 1, "conservative" },
     { "estimaterawfee", 0, "nblocks" },
     { "estimaterawfee", 1, "threshold" },
     { "prioritisetransaction", 1, "dummy" },

src/rpc/mining.cpp

@@ -806,42 +806,62 @@ UniValue estimatesmartfee(const JSONRPCRequest& request)
 {
     if (request.fHelp || request.params.size() < 1 || request.params.size() > 2)
         throw std::runtime_error(
-            "estimatesmartfee nblocks (conservative)\n"
+            "estimatesmartfee nblocks (\"estimate_mode\")\n"
             "\nEstimates the approximate fee per kilobyte needed for a transaction to begin\n"
             "confirmation within nblocks blocks if possible and return the number of blocks\n"
             "for which the estimate is valid. Uses virtual transaction size as defined\n"
             "in BIP 141 (witness data is discounted).\n"
             "\nArguments:\n"
-            "1. nblocks       (numeric)\n"
-            "2. conservative  (bool, optional, default=true) Whether to return a more conservative estimate which\n"
-            "                 also satisfies a longer history. A conservative estimate potentially returns a higher\n"
-            "                 feerate and is more likely to be sufficient for the desired target, but is not as\n"
-            "                 responsive to short term drops in the prevailing fee market\n"
+            "1. nblocks         (numeric) Confirmation target in blocks (1 - 1008)\n"
+            "2. \"estimate_mode\" (string, optional, default=CONSERVATIVE) The fee estimate mode.\n"
+            "                   Whether to return a more conservative estimate which also satisfies\n"
+            "                   a longer history. A conservative estimate potentially returns a\n"
+            "                   higher feerate and is more likely to be sufficient for the desired\n"
+            "                   target, but is not as responsive to short term drops in the\n"
+            "                   prevailing fee market.  Must be one of:\n"
+            "       \"UNSET\" (defaults to CONSERVATIVE)\n"
+            "       \"ECONOMICAL\"\n"
+            "       \"CONSERVATIVE\"\n"
             "\nResult:\n"
             "{\n"
-            "  \"feerate\" : x.x,     (numeric) estimate fee-per-kilobyte (in BTC)\n"
+            "  \"feerate\" : x.x,     (numeric, optional) estimate fee-per-kilobyte (in BTC)\n"
+            "  \"errors\": [ str... ] (json array of strings, optional) Errors encountered during processing\n"
             "  \"blocks\" : n         (numeric) block number where estimate was found\n"
             "}\n"
             "\n"
-            "A negative value is returned if not enough transactions and blocks\n"
+            "The request target will be clamped between 2 and the highest target\n"
+            "fee estimation is able to return based on how long it has been running.\n"
+            "An error is returned if not enough transactions and blocks\n"
             "have been observed to make an estimate for any number of blocks.\n"
             "\nExample:\n"
             + HelpExampleCli("estimatesmartfee", "6")
             );
 
-    RPCTypeCheck(request.params, {UniValue::VNUM});
-
+    RPCTypeCheck(request.params, {UniValue::VNUM, UniValue::VSTR});
+    RPCTypeCheckArgument(request.params[0], UniValue::VNUM);
     int nBlocks = request.params[0].get_int();
+    if (nBlocks < 1 || (unsigned int)nBlocks > ::feeEstimator.HighestTargetTracked(FeeEstimateHorizon::LONG_HALFLIFE)) {
+        throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid nblocks");
+    }
     bool conservative = true;
     if (request.params.size() > 1 && !request.params[1].isNull()) {
-        RPCTypeCheckArgument(request.params[1], UniValue::VBOOL);
-        conservative = request.params[1].get_bool();
+        FeeEstimateMode fee_mode;
+        if (!FeeModeFromString(request.params[1].get_str(), fee_mode)) {
+            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid estimate_mode parameter");
+        }
+        if (fee_mode == FeeEstimateMode::ECONOMICAL) conservative = false;
     }
 
     UniValue result(UniValue::VOBJ);
+    UniValue errors(UniValue::VARR);
     FeeCalculation feeCalc;
     CFeeRate feeRate = ::feeEstimator.estimateSmartFee(nBlocks, &feeCalc, conservative);
-    result.push_back(Pair("feerate", feeRate == CFeeRate(0) ? -1.0 : ValueFromAmount(feeRate.GetFeePerK())));
+    if (feeRate != CFeeRate(0)) {
+        result.push_back(Pair("feerate", ValueFromAmount(feeRate.GetFeePerK())));
+    } else {
+        errors.push_back("Insufficient data or no feerate found");
+        result.push_back(Pair("errors", errors));
+    }
     result.push_back(Pair("blocks", feeCalc.returnedTarget));
     return result;
 }
@@ -889,7 +909,7 @@ UniValue estimaterawfee(const JSONRPCRequest& request)
             + HelpExampleCli("estimaterawfee", "6 0.9")
             );
 
-    RPCTypeCheck(request.params, {UniValue::VNUM, UniValue::VNUM, UniValue::VNUM}, true);
+    RPCTypeCheck(request.params, {UniValue::VNUM, UniValue::VNUM}, true);
     RPCTypeCheckArgument(request.params[0], UniValue::VNUM);
     int nBlocks = request.params[0].get_int();
     if (nBlocks < 1 || (unsigned int)nBlocks > ::feeEstimator.HighestTargetTracked(FeeEstimateHorizon::LONG_HALFLIFE)) {
@@ -963,7 +983,7 @@ static const CRPCCommand commands[] =
     { "generating",         "generatetoaddress",      &generatetoaddress,      true,  {"nblocks","address","maxtries"} },
 
     { "util",               "estimatefee",            &estimatefee,            true,  {"nblocks"} },
-    { "util",               "estimatesmartfee",       &estimatesmartfee,       true,  {"nblocks", "conservative"} },
+    { "util",               "estimatesmartfee",       &estimatesmartfee,       true,  {"nblocks", "estimate_mode"} },
 
     { "hidden",             "estimaterawfee",         &estimaterawfee,         true,  {"nblocks", "threshold"} },
 };