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);
+}
+
+}

src/libutil/config.hh

@@ -0,0 +1,151 @@
+#include <map>
+#include <set>
+
+#include "types.hh"
+
+#pragma once
+
+namespace nix {
+
+class Args;
+class AbstractSetting;
+
+/* A class to simplify providing 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
+     }
+   };
+*/
+
+class Config
+{
+    friend class AbstractSetting;
+
+    struct SettingData
+    {
+        bool isAlias = false;
+        AbstractSetting * setting;
+    };
+
+    std::map<std::string, SettingData> _settings;
+
+    StringMap initials;
+
+public:
+
+    Config(const StringMap & initials)
+        : initials(initials)
+    { }
+
+    void set(const std::string & name, const std::string & value);
+
+    void add(AbstractSetting * setting);
+
+    void warnUnused();
+
+    std::string dump();
+};
+
+class AbstractSetting
+{
+    friend class Config;
+
+public:
+
+    const std::string name;
+    const std::string description;
+    const std::set<std::string> aliases;
+
+    int created = 123;
+
+protected:
+
+    AbstractSetting(
+        const std::string & name,
+        const std::string & description,
+        const std::set<std::string> & aliases);
+
+    virtual ~AbstractSetting()
+    {
+        // Check against a gcc miscompilation causing our constructor
+        // not to run.
+        assert(created == 123);
+    }
+
+    virtual void set(const std::string & value) = 0;
+
+    virtual std::string to_string() = 0;
+};
+
+/* A setting of type T. */
+template<typename T>
+class Setting : public AbstractSetting
+{
+protected:
+
+    T value;
+
+public:
+
+    Setting(Config * options,
+        const T & def,
+        const std::string & name,
+        const std::string & description,
+        const std::set<std::string> & aliases = {})
+        : AbstractSetting(name, description, aliases)
+        , value(def)
+    {
+        options->add(this);
+    }
+
+    operator const T &() const { return value; }
+    bool operator ==(const T & v2) const { return value == v2; }
+    bool operator !=(const T & v2) const { return value != v2; }
+    void operator =(const T & v) { value = v; }
+
+    void set(const std::string & str) override;
+
+    std::string to_string() override;
+};
+
+template<typename T>
+std::ostream & operator <<(std::ostream & str, const Setting<T> & opt)
+{
+    str << (const T &) opt;
+    return str;
+}
+
+/* A special setting for Paths. These are automatically canonicalised
+   (e.g. "/foo//bar/" becomes "/foo/bar"). */
+class PathSetting : public Setting<Path>
+{
+    bool allowEmpty;
+
+public:
+
+    PathSetting(Config * options,
+        bool allowEmpty,
+        const Path & def,
+        const std::string & name,
+        const std::string & description,
+        const std::set<std::string> & aliases = {})
+        : Setting<Path>(options, def, name, description, aliases)
+        , allowEmpty(allowEmpty)
+    {
+        set(value);
+    }
+
+    void set(const std::string & str) override;
+
+    Path operator +(const char * p) const { return value + p; }
+};
+
+}

src/libutil/types.hh

@@ -7,6 +7,7 @@
 #include <list>
 #include <set>
 #include <memory>
+#include <map>
 
 #include <boost/format.hpp>
 
@@ -141,6 +142,7 @@ private:
 
 typedef list<string> Strings;
 typedef set<string> StringSet;
+typedef std::map<std::string, std::string> StringMap;
 
 
 /* Paths are just strings. */