init: Allow -proxy="" setting values

This drops the `No proxy server specified. Use -proxy=<ip> or -proxy=<ip:port>`
error when a empty `-proxy=` command line argument, `bitcoin.conf` value, or
`settings.json` value is specified, and just makes bitcoin connect and listen
normally in these cases.

The error was originally added in https://github.com/bitcoin/bitcoin/pull/20003
to prevent a bare `-proxy` command line argument with no assignment from
clearing proxy settings. But it was implemented in an overbroad way breaking
empty `-proxy=` assignments as well.

The motivation for this change is to prevent a GUI bug that happens with
https://github.com/bitcoin/bitcoin/pull/15936, reported in
https://github.com/bitcoin/bitcoin/pull/15936#pullrequestreview-937685759 by
vasild, that happens after a proxy setting is enabled and disabled in the GUI.
But this change also makes sense on its own to remove a potentially confusing
error message.

src/init.cpp

@@ -470,7 +470,7 @@ void SetupServerArgs(ArgsManager& argsman)
     // TODO: remove the sentence "Nodes not using ... incoming connections." once the changes from
     // https://github.com/bitcoin/bitcoin/pull/23542 have become widespread.
     argsman.AddArg("-port=<port>", strprintf("Listen for connections on <port>. Nodes not using the default ports (default: %u, testnet: %u, signet: %u, regtest: %u) are unlikely to get incoming connections. Not relevant for I2P (see doc/i2p.md).", defaultChainParams->GetDefaultPort(), testnetChainParams->GetDefaultPort(), signetChainParams->GetDefaultPort(), regtestChainParams->GetDefaultPort()), ArgsManager::ALLOW_ANY | ArgsManager::NETWORK_ONLY, OptionsCategory::CONNECTION);
-    argsman.AddArg("-proxy=<ip:port>", "Connect through SOCKS5 proxy, set -noproxy to disable (default: disabled)", ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
+    argsman.AddArg("-proxy=<ip:port>", "Connect through SOCKS5 proxy, set -noproxy to disable (default: disabled)", ArgsManager::ALLOW_ANY | ArgsManager::DISALLOW_ELISION, OptionsCategory::CONNECTION);
     argsman.AddArg("-proxyrandomize", strprintf("Randomize credentials for every proxy connection. This enables Tor stream isolation (default: %u)", DEFAULT_PROXYRANDOMIZE), ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
     argsman.AddArg("-seednode=<ip>", "Connect to a node to retrieve peer addresses, and disconnect. This option can be specified multiple times to connect to multiple nodes.", ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
     argsman.AddArg("-networkactive", "Enable all P2P network activity (default: 1). Can be changed by the setnetworkactive RPC command", ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
@@ -1029,10 +1029,6 @@ bool AppInitParameterInteraction(const ArgsManager& args, bool use_syscall_sandb
 
     nMaxTipAge = args.GetIntArg("-maxtipage", DEFAULT_MAX_TIP_AGE);
 
-    if (args.IsArgSet("-proxy") && args.GetArg("-proxy", "").empty()) {
-        return InitError(_("No proxy server specified. Use -proxy=<ip> or -proxy=<ip:port>."));
-    }
-
     if (args.GetBoolArg("-reindex-chainstate", false)) {
         // indexes that must be deactivated to prevent index corruption, see #24630
         if (args.GetBoolArg("-coinstatsindex", DEFAULT_COINSTATSINDEX)) {

src/util/system.cpp

@@ -236,7 +236,7 @@ KeyInfo InterpretKey(std::string key)
  * @return parsed settings value if it is valid, otherwise nullopt accompanied
  * by a descriptive error string
  */
-static std::optional<util::SettingsValue> InterpretValue(const KeyInfo& key, const std::string& value,
+static std::optional<util::SettingsValue> InterpretValue(const KeyInfo& key, const std::string* value,
                                                          unsigned int flags, std::string& error)
 {
     // Return negated settings as false values.
@@ -246,13 +246,17 @@ static std::optional<util::SettingsValue> InterpretValue(const KeyInfo& key, con
             return std::nullopt;
         }
         // Double negatives like -nofoo=0 are supported (but discouraged)
-        if (!InterpretBool(value)) {
+        if (value && !InterpretBool(*value)) {
-            LogPrintf("Warning: parsed potentially confusing double-negative -%s=%s\n", key.name, value);
+            LogPrintf("Warning: parsed potentially confusing double-negative -%s=%s\n", key.name, *value);
             return true;
         }
         return false;
     }
-    return value;
+    if (!value && (flags & ArgsManager::DISALLOW_ELISION)) {
+        error = strprintf("Can not set -%s with no value. Please specify value with -%s=value.", key.name, key.name);
+        return std::nullopt;
+    }
+    return value ? *value : "";
 }
 
 // Define default constructor and destructor that are not inline, so code instantiating this class doesn't need to
@@ -320,7 +324,7 @@ bool ArgsManager::ParseParameters(int argc, const char* const argv[], std::strin
 #endif
 
         if (key == "-") break; //bitcoin-tx using stdin
-        std::string val;
+        std::optional<std::string> val;
         size_t is_index = key.find('=');
         if (is_index != std::string::npos) {
             val = key.substr(is_index + 1);
@@ -366,7 +370,7 @@ bool ArgsManager::ParseParameters(int argc, const char* const argv[], std::strin
             return false;
         }
 
-        std::optional<util::SettingsValue> value = InterpretValue(keyinfo, val, *flags, error);
+        std::optional<util::SettingsValue> value = InterpretValue(keyinfo, val ? &*val : nullptr, *flags, error);
         if (!value) return false;
 
         m_settings.command_line_options[keyinfo.name].push_back(*value);
@@ -887,7 +891,7 @@ bool ArgsManager::ReadConfigStream(std::istream& stream, const std::string& file
         KeyInfo key = InterpretKey(option.first);
         std::optional<unsigned int> flags = GetArgFlags('-' + key.name);
         if (flags) {
-            std::optional<util::SettingsValue> value = InterpretValue(key, option.second, *flags, error);
+            std::optional<util::SettingsValue> value = InterpretValue(key, &option.second, *flags, error);
             if (!value) {
                 return false;
             }

src/util/system.h

@@ -175,6 +175,7 @@ public:
         // ALLOW_STRING = 0x08,   //!< unimplemented, draft implementation in #16545
         // ALLOW_LIST = 0x10,     //!< unimplemented, draft implementation in #16545
         DISALLOW_NEGATION = 0x20, //!< disallow -nofoo syntax
+        DISALLOW_ELISION = 0x40,  //!< disallow -foo syntax that doesn't assign any value
 
         DEBUG_ONLY = 0x100,
         /* Some options would cause cross-contamination if values for

test/functional/feature_config_args.py

@@ -85,7 +85,7 @@ class ConfArgsTest(BitcoinTestFramework):
 
     def test_invalid_command_line_options(self):
         self.nodes[0].assert_start_raises_init_error(
-            expected_msg='Error: No proxy server specified. Use -proxy=<ip> or -proxy=<ip:port>.',
+            expected_msg='Error: Error parsing command line arguments: Can not set -proxy with no value. Please specify value with -proxy=value.',
             extra_args=['-proxy'],
         )