Get rid of ambiguous OutputType::NONE value

Based on suggestion by Pieter Wuille <pieter.wuille@gmail.com> at
https://github.com/bitcoin/bitcoin/pull/12119#issuecomment-357982763

After #12119, the NONE output type was overloaded to refer to either an output
type that couldn't be parsed, or to an automatic change output mode.  This
change drops the NONE enum and uses a simple bool indicate parse failure, and a
new CHANGE_AUTO enum to refer the change output type.

This change is almost a pure refactoring except it makes RPCs reject empty
string ("") address types instead of treating them like they were unset. This
simplifies the parsing code a little bit and could prevent RPC usage mistakes.
It's noted in the release notes.

4 files changed, 31 insertions, 29 deletions

diff --git a/src/qt/paymentserver.cpp b/src/qt/paymentserver.cpp
index 926a699754..275dba0b15 100644
--- a/src/qt/paymentserver.cpp
+++ b/src/qt/paymentserver.cpp
@@ -648,7 +648,7 @@ void PaymentServer::fetchPaymentACK(WalletModel* walletModel, const SendCoinsRec
         // use for change. Despite an actual payment and not change, this is a close match:
         // it's the output type we use subject to privacy issues, but not restricted by what
         // other software supports.
-        const OutputType change_type = walletModel->wallet().getDefaultChangeType() != OutputType::NONE ? walletModel->wallet().getDefaultChangeType() : walletModel->wallet().getDefaultAddressType();
+        const OutputType change_type = walletModel->wallet().getDefaultChangeType() != OutputType::CHANGE_AUTO ? walletModel->wallet().getDefaultChangeType() : walletModel->wallet().getDefaultAddressType();
         walletModel->wallet().learnRelatedScripts(newKey, change_type);
         CTxDestination dest = GetDestinationForKey(newKey, change_type);
         std::string label = tr("Refund from %1").arg(recipient.authenticatedMerchant).toStdString();
diff --git a/src/wallet/rpcwallet.cpp b/src/wallet/rpcwallet.cpp
index c34b166a41..f669921750 100644
--- a/src/wallet/rpcwallet.cpp
+++ b/src/wallet/rpcwallet.cpp
@@ -164,8 +164,7 @@ UniValue getnewaddress(const JSONRPCRequest& request)
 
     OutputType output_type = pwallet->m_default_address_type;
     if (!request.params[1].isNull()) {
-        output_type = ParseOutputType(request.params[1].get_str(), pwallet->m_default_address_type);
-        if (output_type == OutputType::NONE) {
+        if (!ParseOutputType(request.params[1].get_str(), output_type)) {
             throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, strprintf("Unknown address type '%s'", request.params[1].get_str()));
         }
     }
@@ -259,10 +258,9 @@ UniValue getrawchangeaddress(const JSONRPCRequest& request)
         pwallet->TopUpKeyPool();
     }
 
-    OutputType output_type = pwallet->m_default_change_type != OutputType::NONE ? pwallet->m_default_change_type : pwallet->m_default_address_type;
+    OutputType output_type = pwallet->m_default_change_type != OutputType::CHANGE_AUTO ? pwallet->m_default_change_type : pwallet->m_default_address_type;
     if (!request.params[0].isNull()) {
-        output_type = ParseOutputType(request.params[0].get_str(), output_type);
-        if (output_type == OutputType::NONE) {
+        if (!ParseOutputType(request.params[0].get_str(), output_type)) {
             throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, strprintf("Unknown address type '%s'", request.params[0].get_str()));
         }
     }
@@ -1226,8 +1224,7 @@ UniValue addmultisigaddress(const JSONRPCRequest& request)
 
     OutputType output_type = pwallet->m_default_address_type;
     if (!request.params[3].isNull()) {
-        output_type = ParseOutputType(request.params[3].get_str(), output_type);
-        if (output_type == OutputType::NONE) {
+        if (!ParseOutputType(request.params[3].get_str(), output_type)) {
             throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, strprintf("Unknown address type '%s'", request.params[3].get_str()));
         }
     }
@@ -3180,8 +3177,8 @@ UniValue fundrawtransaction(const JSONRPCRequest& request)
             if (options.exists("changeAddress")) {
                 throw JSONRPCError(RPC_INVALID_PARAMETER, "Cannot specify both changeAddress and address_type options");
             }
-            coinControl.m_change_type = ParseOutputType(options["change_type"].get_str(), pwallet->m_default_change_type);
-            if (coinControl.m_change_type == OutputType::NONE) {
+            coinControl.m_change_type = pwallet->m_default_change_type;
+            if (!ParseOutputType(options["change_type"].get_str(), *coinControl.m_change_type)) {
                 throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, strprintf("Unknown change type '%s'", options["change_type"].get_str()));
             }
         }
diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index dbc1760c80..965eb067cc 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -2648,7 +2648,7 @@ bool CWallet::FundTransaction(CMutableTransaction& tx, CAmount& nFeeRet, int& nC
 OutputType CWallet::TransactionChangeType(OutputType change_type, const std::vector<CRecipient>& vecSend)
 {
     // If -changetype is specified, always use that change type.
-    if (change_type != OutputType::NONE) {
+    if (change_type != OutputType::CHANGE_AUTO) {
         return change_type;
     }
 
@@ -4022,16 +4022,12 @@ CWallet* CWallet::CreateWalletFromFile(const std::string& name, const fs::path&
         }
     }
 
-    walletInstance->m_default_address_type = ParseOutputType(gArgs.GetArg("-addresstype", ""), DEFAULT_ADDRESS_TYPE);
-    if (walletInstance->m_default_address_type == OutputType::NONE) {
+    if (!gArgs.GetArg("-addresstype", "").empty() && !ParseOutputType(gArgs.GetArg("-addresstype", ""), walletInstance->m_default_address_type)) {
         InitError(strprintf("Unknown address type '%s'", gArgs.GetArg("-addresstype", "")));
         return nullptr;
     }
 
-    // If changetype is set in config file or parameter, check that it's valid.
-    // Default to OutputType::NONE if not set.
-    walletInstance->m_default_change_type = ParseOutputType(gArgs.GetArg("-changetype", ""), OutputType::NONE);
-    if (walletInstance->m_default_change_type == OutputType::NONE && !gArgs.GetArg("-changetype", "").empty()) {
+    if (!gArgs.GetArg("-changetype", "").empty() && !ParseOutputType(gArgs.GetArg("-changetype", ""), walletInstance->m_default_change_type)) {
         InitError(strprintf("Unknown change type '%s'", gArgs.GetArg("-changetype", "")));
         return nullptr;
     }
@@ -4219,19 +4215,19 @@ static const std::string OUTPUT_TYPE_STRING_LEGACY = "legacy";
 static const std::string OUTPUT_TYPE_STRING_P2SH_SEGWIT = "p2sh-segwit";
 static const std::string OUTPUT_TYPE_STRING_BECH32 = "bech32";
 
-OutputType ParseOutputType(const std::string& type, OutputType default_type)
+bool ParseOutputType(const std::string& type, OutputType& output_type)
 {
-    if (type.empty()) {
-        return default_type;
-    } else if (type == OUTPUT_TYPE_STRING_LEGACY) {
-        return OutputType::LEGACY;
+    if (type == OUTPUT_TYPE_STRING_LEGACY) {
+        output_type = OutputType::LEGACY;
+        return true;
     } else if (type == OUTPUT_TYPE_STRING_P2SH_SEGWIT) {
-        return OutputType::P2SH_SEGWIT;
+        output_type = OutputType::P2SH_SEGWIT;
+        return true;
     } else if (type == OUTPUT_TYPE_STRING_BECH32) {
-        return OutputType::BECH32;
-    } else {
-        return OutputType::NONE;
+        output_type = OutputType::BECH32;
+        return true;
     }
+    return false;
 }
 
 const std::string& FormatOutputType(OutputType type)
diff --git a/src/wallet/wallet.h b/src/wallet/wallet.h
index 898c32c708..1d85fbcea1 100644
--- a/src/wallet/wallet.h
+++ b/src/wallet/wallet.h
@@ -99,15 +99,24 @@ enum WalletFeature
 };
 
 enum class OutputType {
-    NONE,
     LEGACY,
     P2SH_SEGWIT,
     BECH32,
+
+    /**
+     * Special output type for change outputs only. Automatically choose type
+     * based on address type setting and the types other of non-change outputs
+     * (see -changetype option documentation and implementation in
+     * CWallet::TransactionChangeType for details).
+     */
+    CHANGE_AUTO,
 };
 
 //! Default for -addresstype
 constexpr OutputType DEFAULT_ADDRESS_TYPE{OutputType::P2SH_SEGWIT};
 
+//! Default for -changetype
+constexpr OutputType DEFAULT_CHANGE_TYPE{OutputType::CHANGE_AUTO};
 
 /** A key pool entry */
 class CKeyPool
@@ -988,7 +997,7 @@ public:
     static CFeeRate fallbackFee;
     static CFeeRate m_discard_rate;
     OutputType m_default_address_type{DEFAULT_ADDRESS_TYPE};
-    OutputType m_default_change_type{OutputType::NONE}; // Default to OutputType::NONE if not set by -changetype
+    OutputType m_default_change_type{DEFAULT_CHANGE_TYPE};
 
     bool NewKeyPool();
     size_t KeypoolCountExternalKeys();
@@ -1232,7 +1241,7 @@ public:
     }
 };
 
-OutputType ParseOutputType(const std::string& str, OutputType default_type);
+bool ParseOutputType(const std::string& str, OutputType& output_type);
 const std::string& FormatOutputType(OutputType type);
 
 /**