util: disallow setting conf in bitcoin.conf

Help from `bitcoind -h` states that conf can only be used from the commandline.
However, if conf is set in a bitcoin.conf file, it is ignored but there is no error.

Show an error to user if conf is set in a .conf file and prompt them to use
`includeconf` if they wish to specify additional config files.

Adds `IsConfSupported` function to allow for easily adding conf options
to disallow or throw warnings for.

Author: josibake

src/util/system.cpp

@@ -935,6 +935,14 @@ static bool GetConfigOptions(std::istream& stream, const std::string& filepath,
     return true;
 }
 
+bool IsConfSupported(KeyInfo& key, std::string& error) {
+    if (key.name == "conf") {
+        error = "conf cannot be set in the configuration file; use includeconf= if you want to include additional config files";
+        return false;
+    }
+    return true;
+}
+
 bool ArgsManager::ReadConfigStream(std::istream& stream, const std::string& filepath, std::string& error, bool ignore_invalid_keys)
 {
     LOCK(cs_args);
@@ -945,6 +953,7 @@ bool ArgsManager::ReadConfigStream(std::istream& stream, const std::string& file
     for (const std::pair<std::string, std::string>& option : options) {
         KeyInfo key = InterpretKey(option.first);
         std::optional<unsigned int> flags = GetArgFlags('-' + key.name);
+        if (!IsConfSupported(key, error)) return false;
         if (flags) {
             std::optional<util::SettingsValue> value = InterpretValue(key, &option.second, *flags, error);
             if (!value) {