Truly decouple wallet from chainparams for -fallbackfee

Before it was 0 by default for main and 20000 for test and regtest.
Now it is 0 by default for all chains, thus there's no need to call Params().

Also now the default for main is properly documented

Author: jtimon

doc/release-notes-16524.md

@@ -0,0 +1,8 @@
+
+Low-level changes
+=================
+
+Tests
+---
+
+- `-fallbackfee` was 0 (disabled) by default for the main chain, but 20000 by default for the test chains. Now it is 0 by default for all chains. Testnet and regtest users will have to add fallbackfee=20000 to their configuration if they weren't setting it and they want it to keep working like before. (#16524)

src/wallet/init.cpp

@@ -41,7 +41,8 @@ void WalletInit::AddWalletOptions() const
     gArgs.AddArg("-discardfee=<amt>", strprintf("The fee rate (in %s/kB) that indicates your tolerance for discarding change by adding it to the fee (default: %s). "
                                                                 "Note: An output is discarded if it is dust at this rate, but we will always discard up to the dust relay fee and a discard fee above that is limited by the fee estimate for the longest target",
                                                               CURRENCY_UNIT, FormatMoney(DEFAULT_DISCARD_FEE)), ArgsManager::ALLOW_ANY, OptionsCategory::WALLET);
-    gArgs.AddArg("-fallbackfee=<amt>", strprintf("A fee rate (in %s/kB) that will be used when fee estimation has insufficient data (default: %s)",
+
+    gArgs.AddArg("-fallbackfee=<amt>", strprintf("A fee rate (in %s/kB) that will be used when fee estimation has insufficient data. 0 to entirely disable the fallbackfee feature. (default: %s)",
                                                                CURRENCY_UNIT, FormatMoney(DEFAULT_FALLBACK_FEE)), ArgsManager::ALLOW_ANY, OptionsCategory::WALLET);
     gArgs.AddArg("-keypool=<n>", strprintf("Set key pool size to <n> (default: %u)", DEFAULT_KEYPOOL_SIZE), ArgsManager::ALLOW_ANY, OptionsCategory::WALLET);
     gArgs.AddArg("-maxtxfee=<amt>", strprintf("Maximum total fees (in %s) to use in a single wallet transaction; setting this too low may abort large transactions (default: %s)",

src/wallet/wallet.cpp

@@ -4420,7 +4420,6 @@ std::shared_ptr<CWallet> CWallet::CreateWalletFromFile(interfaces::Chain& chain,
         walletInstance->m_min_fee = CFeeRate(n);
     }
 
-    walletInstance->m_allow_fallback_fee = Params().IsTestChain();
     if (gArgs.IsArgSet("-fallbackfee")) {
         CAmount nFeePerK = 0;
         if (!ParseMoney(gArgs.GetArg("-fallbackfee", ""), nFeePerK)) {
@@ -4432,8 +4431,10 @@ std::shared_ptr<CWallet> CWallet::CreateWalletFromFile(interfaces::Chain& chain,
                               _("This is the transaction fee you may pay when fee estimates are not available.").translated);
         }
         walletInstance->m_fallback_fee = CFeeRate(nFeePerK);
-        walletInstance->m_allow_fallback_fee = nFeePerK != 0; //disable fallback fee in case value was set to 0, enable if non-null value
     }
+    // Disable fallback fee in case value was set to 0, enable if non-null value
+    walletInstance->m_allow_fallback_fee = walletInstance->m_fallback_fee.GetFeePerK() != 0;
+
     if (gArgs.IsArgSet("-discardfee")) {
         CAmount nFeePerK = 0;
         if (!ParseMoney(gArgs.GetArg("-discardfee", ""), nFeePerK)) {

src/wallet/wallet.h

@@ -63,7 +63,7 @@ static const unsigned int DEFAULT_KEYPOOL_SIZE = 1000;
 //! -paytxfee default
 constexpr CAmount DEFAULT_PAY_TX_FEE = 0;
 //! -fallbackfee default
-static const CAmount DEFAULT_FALLBACK_FEE = 20000;
+static const CAmount DEFAULT_FALLBACK_FEE = 0;
 //! -discardfee default
 static const CAmount DEFAULT_DISCARD_FEE = 10000;
 //! -mintxfee default
@@ -1167,7 +1167,7 @@ class CWallet final : public FillableSigningProvider, private interfaces::Chain:
     unsigned int m_confirm_target{DEFAULT_TX_CONFIRM_TARGET};
     bool m_spend_zero_conf_change{DEFAULT_SPEND_ZEROCONF_CHANGE};
     bool m_signal_rbf{DEFAULT_WALLET_RBF};
-    bool m_allow_fallback_fee{true}; //!< will be defined via chainparams
+    bool m_allow_fallback_fee{true}; //!< will be false if -fallbackfee=0
     CFeeRate m_min_fee{DEFAULT_TRANSACTION_MINFEE}; //!< Override with -mintxfee
     /**
      * If fee estimation does not have enough data to provide estimates, use this fee instead.

test/functional/test_framework/util.py

@@ -299,6 +299,7 @@ def initialize_datadir(dirname, n, chain):
         f.write("[{}]\n".format(chain_name_conf_section))
         f.write("port=" + str(p2p_port(n)) + "\n")
         f.write("rpcport=" + str(rpc_port(n)) + "\n")
+        f.write("fallbackfee=0.0002\n")
         f.write("server=1\n")
         f.write("keypool=1\n")
         f.write("discover=0\n")