Modularize config settings

Allow global config settings to be defined in multiple Config
classes. For example, this means that libutil can have settings and
evaluator settings can be moved out of libstore. The Config classes
are registered in a new GlobalConfig class to which config files
etc. are applied.

Relevant to https://github.com/NixOS/nix/issues/2009 in that it
removes the need for ad hoc handling of useCaseHack, which was the
underlying cause of that issue.

4 files changed, 150 insertions, 49 deletions

diff --git a/src/libutil/archive.cc b/src/libutil/archive.cc
index 154e2d20430c..1be8934a2eba 100644
--- a/src/libutil/archive.cc
+++ b/src/libutil/archive.cc
@@ -13,17 +13,25 @@
 
 #include "archive.hh"
 #include "util.hh"
-
+#include "config.hh"
 
 namespace nix {
 
+struct ArchiveSettings : Config
+{
+    Setting<bool> useCaseHack{this,
+        #if __APPLE__
+            true,
+        #else
+            false,
+        #endif
+        "use-case-hack",
+        "Whether to enable a Darwin-specific hack for dealing with file name collisions."};
+};
 
-bool useCaseHack =
-#if __APPLE__
-    true;
-#else
-    false;
-#endif
+static ArchiveSettings archiveSettings;
+
+static GlobalConfig::Register r1(&archiveSettings);
 
 const std::string narVersionMagic1 = "nix-archive-1";
 
@@ -78,7 +86,7 @@ static void dump(const Path & path, Sink & sink, PathFilter & filter)
            the case hack applied by restorePath(). */
         std::map<string, string> unhacked;
         for (auto & i : readDirectory(path))
-            if (useCaseHack) {
+            if (archiveSettings.useCaseHack) {
                 string name(i.name);
                 size_t pos = i.name.find(caseHackSuffix);
                 if (pos != string::npos) {
@@ -243,7 +251,7 @@ static void parse(ParseSink & sink, Source & source, const Path & path)
                     if (name <= prevName)
                         throw Error("NAR directory is not sorted");
                     prevName = name;
-                    if (useCaseHack) {
+                    if (archiveSettings.useCaseHack) {
                         auto i = names.find(name);
                         if (i != names.end()) {
                             debug(format("case collision between '%1%' and '%2%'") % i->first % name);
diff --git a/src/libutil/archive.hh b/src/libutil/archive.hh
index 7a0e688e4201..25be426c1a4d 100644
--- a/src/libutil/archive.hh
+++ b/src/libutil/archive.hh
@@ -78,10 +78,6 @@ void restorePath(const Path & path, Source & source);
 void copyNAR(Source & source, Sink & sink);
 
 
-// FIXME: global variables are bad m'kay.
-extern bool useCaseHack;
-
-
 extern const std::string narVersionMagic1;
 
 
diff --git a/src/libutil/config.cc b/src/libutil/config.cc
index ce6858f0d65a..9023cb1bb6de 100644
--- a/src/libutil/config.cc
+++ b/src/libutil/config.cc
@@ -4,15 +4,13 @@
 
 namespace nix {
 
-void Config::set(const std::string & name, const std::string & value)
+bool Config::set(const std::string & name, const std::string & value)
 {
     auto i = _settings.find(name);
-    if (i == _settings.end()) {
-        extras.emplace(name, value);
-    } else {
-        i->second.setting->set(value);
-        i->second.setting->overriden = true;
-    }
+    if (i == _settings.end()) return false;
+    i->second.setting->set(value);
+    i->second.setting->overriden = true;
+    return true;
 }
 
 void Config::addSetting(AbstractSetting * setting)
@@ -23,46 +21,51 @@ void Config::addSetting(AbstractSetting * setting)
 
     bool set = false;
 
-    auto i = extras.find(setting->name);
-    if (i != extras.end()) {
+    auto i = unknownSettings.find(setting->name);
+    if (i != unknownSettings.end()) {
         setting->set(i->second);
         setting->overriden = true;
-        extras.erase(i);
+        unknownSettings.erase(i);
         set = true;
     }
 
     for (auto & alias : setting->aliases) {
-        auto i = extras.find(alias);
-        if (i != extras.end()) {
+        auto i = unknownSettings.find(alias);
+        if (i != unknownSettings.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);
                 setting->overriden = true;
-                extras.erase(i);
+                unknownSettings.erase(i);
                 set = true;
             }
         }
     }
 }
 
-void Config::handleUnknownSettings()
+void AbstractConfig::warnUnknownSettings()
 {
-    for (auto & s : extras)
+    for (auto & s : unknownSettings)
         warn("unknown setting '%s'", s.first);
 }
 
-StringMap Config::getSettings(bool overridenOnly)
+void AbstractConfig::reapplyUnknownSettings()
+{
+    auto unknownSettings2 = std::move(unknownSettings);
+    for (auto & s : unknownSettings2)
+        set(s.first, s.second);
+}
+
+void Config::getSettings(std::map<std::string, SettingInfo> & res, bool overridenOnly)
 {
-    StringMap res;
     for (auto & opt : _settings)
         if (!opt.second.isAlias && (!overridenOnly || opt.second.setting->overriden))
-            res.emplace(opt.first, opt.second.setting->to_string());
-    return res;
+            res.emplace(opt.first, SettingInfo{opt.second.setting->to_string(), opt.second.setting->description});
 }
 
-void Config::applyConfigFile(const Path & path)
+void AbstractConfig::applyConfigFile(const Path & path)
 {
     try {
         string contents = readFile(path);
@@ -287,4 +290,49 @@ void PathSetting::set(const std::string & str)
         value = canonPath(str);
 }
 
+bool GlobalConfig::set(const std::string & name, const std::string & value)
+{
+    for (auto & config : *configRegistrations)
+        if (config->set(name, value)) return true;
+
+    unknownSettings.emplace(name, value);
+
+    return false;
+}
+
+void GlobalConfig::getSettings(std::map<std::string, SettingInfo> & res, bool overridenOnly)
+{
+    for (auto & config : *configRegistrations)
+        config->getSettings(res, overridenOnly);
+}
+
+void GlobalConfig::resetOverriden()
+{
+    for (auto & config : *configRegistrations)
+        config->resetOverriden();
+}
+
+void GlobalConfig::toJSON(JSONObject & out)
+{
+    for (auto & config : *configRegistrations)
+        config->toJSON(out);
+}
+
+void GlobalConfig::convertToArgs(Args & args, const std::string & category)
+{
+    for (auto & config : *configRegistrations)
+        config->convertToArgs(args, category);
+}
+
+GlobalConfig globalConfig;
+
+GlobalConfig::ConfigRegistrations * GlobalConfig::configRegistrations;
+
+GlobalConfig::Register::Register(Config * config)
+{
+    if (!configRegistrations)
+        configRegistrations = new ConfigRegistrations;
+    configRegistrations->emplace_back(config);
+}
+
 }
diff --git a/src/libutil/config.hh b/src/libutil/config.hh
index d2e7faf17434..d86c65ff033a 100644
--- a/src/libutil/config.hh
+++ b/src/libutil/config.hh
@@ -12,6 +12,40 @@ class AbstractSetting;
 class JSONPlaceholder;
 class JSONObject;
 
+class AbstractConfig
+{
+protected:
+    StringMap unknownSettings;
+
+    AbstractConfig(const StringMap & initials = {})
+        : unknownSettings(initials)
+    { }
+
+public:
+
+    virtual bool set(const std::string & name, const std::string & value) = 0;
+
+    struct SettingInfo
+    {
+        std::string value;
+        std::string description;
+    };
+
+    virtual void getSettings(std::map<std::string, SettingInfo> & res, bool overridenOnly = false) = 0;
+
+    void applyConfigFile(const Path & path);
+
+    virtual void resetOverriden() = 0;
+
+    virtual void toJSON(JSONObject & out) = 0;
+
+    virtual void convertToArgs(Args & args, const std::string & category) = 0;
+
+    void warnUnknownSettings();
+
+    void reapplyUnknownSettings();
+};
+
 /* A class to simplify providing configuration settings. The typical
    use is to inherit Config and add Setting<T> members:
 
@@ -27,7 +61,7 @@ class JSONObject;
    };
 */
 
-class Config
+class Config : public AbstractConfig
 {
     friend class AbstractSetting;
 
@@ -48,31 +82,23 @@ private:
 
     Settings _settings;
 
-    StringMap extras;
-
 public:
 
-    Config(const StringMap & initials)
-        : extras(initials)
+    Config(const StringMap & initials = {})
+        : AbstractConfig(initials)
     { }
 
-    void set(const std::string & name, const std::string & value);
+    bool set(const std::string & name, const std::string & value) override;
 
     void addSetting(AbstractSetting * setting);
 
-    void handleUnknownSettings();
-
-    StringMap getSettings(bool overridenOnly = false);
+    void getSettings(std::map<std::string, SettingInfo> & res, bool overridenOnly = false) override;
 
-    const Settings & _getSettings() { return _settings; }
-
-    void applyConfigFile(const Path & path);
+    void resetOverriden() override;
 
-    void resetOverriden();
+    void toJSON(JSONObject & out) override;
 
-    void toJSON(JSONObject & out);
-
-    void convertToArgs(Args & args, const std::string & category);
+    void convertToArgs(Args & args, const std::string & category) override;
 };
 
 class AbstractSetting
@@ -209,4 +235,27 @@ public:
     void operator =(const Path & v) { this->assign(v); }
 };
 
+struct GlobalConfig : public AbstractConfig
+{
+    typedef std::vector<Config*> ConfigRegistrations;
+    static ConfigRegistrations * configRegistrations;
+
+    bool set(const std::string & name, const std::string & value) override;
+
+    void getSettings(std::map<std::string, SettingInfo> & res, bool overridenOnly = false) override;
+
+    void resetOverriden() override;
+
+    void toJSON(JSONObject & out) override;
+
+    void convertToArgs(Args & args, const std::string & category) override;
+
+    struct Register
+    {
+        Register(Config * config);
+    };
+};
+
+extern GlobalConfig globalConfig;
+
 }