Add a Config class to simplify adding configuration settings

The typical use is to inherit Config and add Setting<T> members:

  class MyClass : private Config
  {
    Setting<int> foo{this, 123, "foo", "the number of foos to use"};
    Setting<std::string> bar{this, "blabla", "bar", "the name of the bar"};

    MyClass() : Config(readConfigFile("/etc/my-app.conf"))
    {
      std::cout << foo << "\n"; // will print 123 unless overriden
    }
  };

Currently, this is used by Store and its subclasses for store
parameters. You now get a warning if you specify a non-existant store
parameter in a store URI.

src/libutil/config.cc

@@ -0,0 +1,112 @@
+#include "config.hh"
+#include "args.hh"
+
+namespace nix {
+
+void Config::set(const std::string & name, const std::string & value)
+{
+    auto i = _settings.find(name);
+    if (i == _settings.end())
+        throw UsageError("unknown setting '%s'", name);
+    i->second.setting->set(value);
+}
+
+void Config::add(AbstractSetting * setting)
+{
+    _settings.emplace(setting->name, Config::SettingData{false, setting});
+    for (auto & alias : setting->aliases)
+        _settings.emplace(alias, Config::SettingData{true, setting});
+
+    bool set = false;
+
+    auto i = initials.find(setting->name);
+    if (i != initials.end()) {
+        setting->set(i->second);
+        initials.erase(i);
+        set = true;
+    }
+
+    for (auto & alias : setting->aliases) {
+        auto i = initials.find(alias);
+        if (i != initials.end()) {
+            if (set)
+                warn("setting '%s' is set, but it's an alias of '%s' which is also set",
+                    alias, setting->name);
+            else {
+                setting->set(i->second);
+                initials.erase(i);
+                set = true;
+            }
+        }
+    }
+}
+
+void Config::warnUnused()
+{
+    for (auto & i : initials)
+        warn("unknown setting '%s'", i.first);
+}
+
+std::string Config::dump()
+{
+    std::string res;
+    for (auto & opt : _settings)
+        if (!opt.second.isAlias)
+            res += opt.first + " = " + opt.second.setting->to_string() + "\n";
+    return res;
+}
+
+AbstractSetting::AbstractSetting(
+    const std::string & name,
+    const std::string & description,
+    const std::set<std::string> & aliases)
+    : name(name), description(description), aliases(aliases)
+{
+}
+
+template<> void Setting<std::string>::set(const std::string & str)
+{
+    value = str;
+}
+
+template<> std::string Setting<std::string>::to_string()
+{
+    return value;
+}
+
+template<> void Setting<int>::set(const std::string & str)
+{
+    try {
+        value = std::stoi(str);
+    } catch (...) {
+        throw UsageError("setting '%s' has invalid value '%s'", name, str);
+    }
+}
+
+template<> std::string Setting<int>::to_string()
+{
+    return std::to_string(value);
+}
+
+template<> void Setting<bool>::set(const std::string & str)
+{
+    value = str == "true" || str == "1";
+}
+
+template<> std::string Setting<bool>::to_string()
+{
+    return value ? "true" : "false";
+}
+
+void PathSetting::set(const std::string & str)
+{
+    if (str == "") {
+        if (allowEmpty)
+            value = "";
+        else
+            throw UsageError("setting '%s' cannot be empty", name);
+    } else
+        value = canonPath(str);
+}
+
+}