[rpc, fees]: add more detail on the fee estimation modes

- Add description that indicates the fee estimation modes behaviour.
- This description will be returned in the RPC's help texts.

src/common/messages.cpp

@@ -53,6 +53,34 @@ const std::vector<std::pair<std::string, FeeEstimateMode>>& FeeModeMap()
     return FEE_MODES;
 }
 
+std::string FeeModeInfo(const std::pair<std::string, FeeEstimateMode>& mode, std::string& default_info)
+{
+    switch (mode.second) {
+        case FeeEstimateMode::UNSET:
+            return strprintf("%s means no mode set (%s). \n", mode.first, default_info);
+        case FeeEstimateMode::ECONOMICAL:
+            return strprintf("%s estimates use a shorter time horizon, making them more\n"
+                   "responsive to short-term drops in the prevailing fee market. This mode\n"
+                   "potentially returns a lower fee rate estimate.\n", mode.first);
+        case FeeEstimateMode::CONSERVATIVE:
+            return strprintf("%s estimates use a longer time horizon, making them\n"
+                   "less responsive to short-term drops in the prevailing fee market. This mode\n"
+                   "potentially returns a higher fee rate estimate.\n", mode.first);
+        default:
+            // Other modes apart from the ones handled are fee rate units; they should not be clarified.
+            assert(false);
+    }
+}
+
+std::string FeeModesDetail(std::string default_info)
+{
+    std::string info;
+    for (const auto& fee_mode : FeeModeMap()) {
+        info += FeeModeInfo(fee_mode, default_info);
+    }
+    return strprintf("%s \n%s", FeeModes(", "), info);
+}
+
 std::string FeeModes(const std::string& delimiter)
 {
     return Join(FeeModeMap(), delimiter, [&](const std::pair<std::string, FeeEstimateMode>& i) { return i.first; });

src/common/messages.h

@@ -26,6 +26,8 @@ enum class PSBTError;
 bool FeeModeFromString(const std::string& mode_string, FeeEstimateMode& fee_estimate_mode);
 std::string StringForFeeReason(FeeReason reason);
 std::string FeeModes(const std::string& delimiter);
+std::string FeeModeInfo(std::pair<std::string, FeeEstimateMode>& mode);
+std::string FeeModesDetail(std::string default_info);
 std::string InvalidEstimateModeErrorMessage();
 bilingual_str PSBTErrorString(PSBTError error);
 bilingual_str TransactionErrorString(const node::TransactionError error);

src/rpc/fees.cpp

@@ -23,7 +23,7 @@
 #include <string>
 
 using common::FeeModeFromString;
-using common::FeeModes;
+using common::FeeModesDetail;
 using common::InvalidEstimateModeErrorMessage;
 using node::NodeContext;
 
@@ -37,12 +37,7 @@ static RPCHelpMan estimatesmartfee()
         {
             {"conf_target", RPCArg::Type::NUM, RPCArg::Optional::NO, "Confirmation target in blocks (1 - 1008)"},
             {"estimate_mode", RPCArg::Type::STR, RPCArg::Default{"economical"}, "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 (case insensitive):\n"
-             "\"" + FeeModes("\"\n\"") + "\""},
+              + FeeModesDetail(std::string("default mode will be used"))},
         },
         RPCResult{
             RPCResult::Type::OBJ, "", "",

src/wallet/rpc/spend.cpp

@@ -24,7 +24,7 @@
 #include <univalue.h>
 
 using common::FeeModeFromString;
-using common::FeeModes;
+using common::FeeModesDetail;
 using common::InvalidEstimateModeErrorMessage;
 using common::StringForFeeReason;
 using common::TransactionErrorString;
@@ -245,7 +245,7 @@ RPCHelpMan sendtoaddress()
                     {"replaceable", RPCArg::Type::BOOL, RPCArg::DefaultHint{"wallet default"}, "Signal that this transaction can be replaced by a transaction (BIP 125)"},
                     {"conf_target", RPCArg::Type::NUM, RPCArg::DefaultHint{"wallet -txconfirmtarget"}, "Confirmation target in blocks"},
                     {"estimate_mode", RPCArg::Type::STR, RPCArg::Default{"unset"}, "The fee estimate mode, must be one of (case insensitive):\n"
-                     "\"" + FeeModes("\"\n\"") + "\""},
+                      + FeeModesDetail(std::string("economical mode is used if the transaction is replaceable;\notherwise, conservative mode is used"))},
                     {"avoid_reuse", RPCArg::Type::BOOL, RPCArg::Default{true}, "(only available if avoid_reuse wallet flag is set) Avoid spending from dirty addresses; addresses are considered\n"
                                          "dirty if they have previously been used in a transaction. If true, this also activates avoidpartialspends, grouping outputs by their addresses."},
                     {"fee_rate", RPCArg::Type::AMOUNT, RPCArg::DefaultHint{"not set, fall back to wallet fee estimation"}, "Specify a fee rate in " + CURRENCY_ATOM + "/vB."},
@@ -349,7 +349,7 @@ RPCHelpMan sendmany()
                     {"replaceable", RPCArg::Type::BOOL, RPCArg::DefaultHint{"wallet default"}, "Signal that this transaction can be replaced by a transaction (BIP 125)"},
                     {"conf_target", RPCArg::Type::NUM, RPCArg::DefaultHint{"wallet -txconfirmtarget"}, "Confirmation target in blocks"},
                     {"estimate_mode", RPCArg::Type::STR, RPCArg::Default{"unset"}, "The fee estimate mode, must be one of (case insensitive):\n"
-                     "\"" + FeeModes("\"\n\"") + "\""},
+                      + FeeModesDetail(std::string("economical mode is used if the transaction is replaceable;\notherwise, conservative mode is used"))},
                     {"fee_rate", RPCArg::Type::AMOUNT, RPCArg::DefaultHint{"not set, fall back to wallet fee estimation"}, "Specify a fee rate in " + CURRENCY_ATOM + "/vB."},
                     {"verbose", RPCArg::Type::BOOL, RPCArg::Default{false}, "If true, return extra information about the transaction."},
                 },
@@ -463,7 +463,7 @@ static std::vector<RPCArg> FundTxDoc(bool solving_data = true)
     std::vector<RPCArg> args = {
         {"conf_target", RPCArg::Type::NUM, RPCArg::DefaultHint{"wallet -txconfirmtarget"}, "Confirmation target in blocks", RPCArgOptions{.also_positional = true}},
         {"estimate_mode", RPCArg::Type::STR, RPCArg::Default{"unset"}, "The fee estimate mode, must be one of (case insensitive):\n"
-         "\"" + FeeModes("\"\n\"") + "\"", RPCArgOptions{.also_positional = true}},
+          + FeeModesDetail(std::string("economical mode is used if the transaction is replaceable;\notherwise, conservative mode is used")), RPCArgOptions{.also_positional = true}},
         {
             "replaceable", RPCArg::Type::BOOL, RPCArg::DefaultHint{"wallet default"}, "Marks this transaction as BIP125-replaceable.\n"
             "Allows this transaction to be replaced by a transaction with higher fees"
@@ -1018,7 +1018,7 @@ static RPCHelpMan bumpfee_helper(std::string method_name)
                              "still be replaceable in practice, for example if it has unconfirmed ancestors which\n"
                              "are replaceable).\n"},
                     {"estimate_mode", RPCArg::Type::STR, RPCArg::Default{"unset"}, "The fee estimate mode, must be one of (case insensitive):\n"
-                             "\"" + FeeModes("\"\n\"") + "\""},
+                              + FeeModesDetail(std::string("economical mode is used if the transaction is replaceable;\notherwise, conservative mode is used"))},
                     {"outputs", RPCArg::Type::ARR, RPCArg::Default{UniValue::VARR}, "The outputs specified as key-value pairs.\n"
                              "Each key may only appear once, i.e. there can only be one 'data' output, and no address may be duplicated.\n"
                              "At least one output of either type must be specified.\n"
@@ -1205,7 +1205,7 @@ RPCHelpMan send()
                 RPCArgOptions{.skip_type_check = true}},
             {"conf_target", RPCArg::Type::NUM, RPCArg::DefaultHint{"wallet -txconfirmtarget"}, "Confirmation target in blocks"},
             {"estimate_mode", RPCArg::Type::STR, RPCArg::Default{"unset"}, "The fee estimate mode, must be one of (case insensitive):\n"
-             "\"" + FeeModes("\"\n\"") + "\""},
+              + FeeModesDetail(std::string("economical mode is used if the transaction is replaceable;\notherwise, conservative mode is used"))},
             {"fee_rate", RPCArg::Type::AMOUNT, RPCArg::DefaultHint{"not set, fall back to wallet fee estimation"}, "Specify a fee rate in " + CURRENCY_ATOM + "/vB."},
             {"options", RPCArg::Type::OBJ_NAMED_PARAMS, RPCArg::Optional::OMITTED, "",
                 Cat<std::vector<RPCArg>>(
@@ -1331,7 +1331,7 @@ RPCHelpMan sendall()
             },
             {"conf_target", RPCArg::Type::NUM, RPCArg::DefaultHint{"wallet -txconfirmtarget"}, "Confirmation target in blocks"},
             {"estimate_mode", RPCArg::Type::STR, RPCArg::Default{"unset"}, "The fee estimate mode, must be one of (case insensitive):\n"
-             "\"" + FeeModes("\"\n\"") + "\""},
+              + FeeModesDetail(std::string("economical mode is used if the transaction is replaceable;\notherwise, conservative mode is used"))},
             {"fee_rate", RPCArg::Type::AMOUNT, RPCArg::DefaultHint{"not set, fall back to wallet fee estimation"}, "Specify a fee rate in " + CURRENCY_ATOM + "/vB."},
             {
                 "options", RPCArg::Type::OBJ_NAMED_PARAMS, RPCArg::Optional::OMITTED, "",