libutil/config: unify path setting types

There have been multiple setting types for paths that are supposed to be
canonicalised, depending on whether zero or one, one, or any number of paths is
to be specified. Naturally, they behaved in slightly different ways in the
code. Simplify things by unifying them and removing special behaviour (mainly
the "multiple paths type can coerce to boolean" thing).

Change-Id: I7c1ce95e9c8e1829a866fb37d679e167811e9705

src/libcmd/repl.cc

@@ -926,7 +926,7 @@ void NixRepl::loadFiles()
 
 void NixRepl::loadReplOverlays()
 {
-    if (!evalSettings.replOverlays) {
+    if (evalSettings.replOverlays.get().empty()) {
         return;
     }
 

src/libexpr/eval-settings.hh

@@ -151,7 +151,7 @@ struct EvalSettings : Config
           This is useful for debugging warnings in third-party Nix code.
         )"};
 
-    PathsSetting replOverlays{this, Paths(), "repl-overlays",
+    PathsSetting<Paths> replOverlays{this, Paths(), "repl-overlays",
         R"(
           A list of files containing Nix expressions that can be used to add
           default bindings to [`nix

src/libstore/globals.hh

@@ -634,7 +634,7 @@ public:
           line.
         )"};
 
-    OptionalPathSetting diffHook{
+    PathsSetting<std::optional<Path>> diffHook{
         this, std::nullopt, "diff-hook",
         R"(
           Absolute path to an executable capable of diffing build

src/libstore/local-fs-store.hh

@@ -11,21 +11,21 @@ struct LocalFSStoreConfig : virtual StoreConfig
 {
     using StoreConfig::StoreConfig;
 
-    const OptionalPathSetting rootDir{this, std::nullopt,
+    const PathsSetting<std::optional<Path>>  rootDir{this, std::nullopt,
         "root",
         "Directory prefixed to all other paths."};
 
-    const PathSetting stateDir{this,
+    const PathsSetting<Path> stateDir{this,
         rootDir.get() ? *rootDir.get() + "/nix/var/nix" : settings.nixStateDir,
         "state",
         "Directory where Lix will store state."};
 
-    const PathSetting logDir{this,
+    const PathsSetting<Path> logDir{this,
         rootDir.get() ? *rootDir.get() + "/nix/var/log/nix" : settings.nixLogDir,
         "log",
         "directory where Lix will store log files."};
 
-    const PathSetting realStoreDir{this,
+    const PathsSetting<Path> realStoreDir{this,
         rootDir.get() ? *rootDir.get() + "/nix/store" : storeDir, "real",
         "Physical path of the Nix store."};
 };

src/libstore/store-api.hh

@@ -144,7 +144,7 @@ struct StoreConfig : public Config
         return std::nullopt;
     }
 
-    const PathSetting storeDir_{this, settings.nixStore,
+    const PathsSetting<Path> storeDir_{this, settings.nixStore,
         "store",
         R"(
           Logical location of the Nix store, usually

src/libutil/config.cc

@@ -434,34 +434,12 @@ static Path parsePath(const AbstractSetting & s, const std::string & str)
         return canonPath(str);
 }
 
-PathSetting::PathSetting(Config * options,
-    const Path & def,
-    const std::string & name,
-    const std::string & description,
-    const std::set<std::string> & aliases)
-    : BaseSetting<Path>(def, true, name, description, aliases)
-{
-    options->addSetting(this);
-}
-
-Path PathSetting::parse(const std::string & str) const
+template<> Path PathsSetting<Path>::parse(const std::string & str) const
 {
     return parsePath(*this, str);
 }
 
-
-OptionalPathSetting::OptionalPathSetting(Config * options,
-    const std::optional<Path> & def,
-    const std::string & name,
-    const std::string & description,
-    const std::set<std::string> & aliases)
-    : BaseSetting<std::optional<Path>>(def, true, name, description, aliases)
-{
-    options->addSetting(this);
-}
-
-
-std::optional<Path> OptionalPathSetting::parse(const std::string & str) const
+template<> std::optional<Path> PathsSetting<std::optional<Path>>::parse(const std::string & str) const
 {
     if (str == "")
         return std::nullopt;
@@ -469,23 +447,7 @@ std::optional<Path> OptionalPathSetting::parse(const std::string & str) const
         return parsePath(*this, str);
 }
 
-void OptionalPathSetting::operator =(const std::optional<Path> & v)
-{
-    this->assign(v);
-}
-
-PathsSetting::PathsSetting(Config * options,
-    const Paths & def,
-    const std::string & name,
-    const std::string & description,
-    const std::set<std::string> & aliases)
-    : BaseSetting<Paths>(def, true, name, description, aliases)
-{
-    options->addSetting(this);
-}
-
-
-Paths PathsSetting::parse(const std::string & str) const
+template<> Paths PathsSetting<Paths>::parse(const std::string & str) const
 {
     auto strings = tokenizeString<Strings>(str);
     Paths parsed;
@@ -497,10 +459,10 @@ Paths PathsSetting::parse(const std::string & str) const
     return parsed;
 }
 
-PathsSetting::operator bool() const noexcept
-{
-    return !get().empty();
-}
+template class PathsSetting<Path>;
+template class PathsSetting<std::optional<Path>>;
+template class PathsSetting<Paths>;
+
 
 bool GlobalConfig::set(const std::string & name, const std::string & value)
 {

src/libutil/config.hh

@@ -353,69 +353,32 @@ public:
 };
 
 /**
- * A special setting for Paths. These are automatically canonicalised
- * (e.g. "/foo//bar/" becomes "/foo/bar").
- *
- * It is mandatory to specify a path; i.e. the empty string is not
- * permitted.
+ * A special setting for Paths.
+ * These are automatically canonicalised (e.g. "/foo//bar/" becomes "/foo/bar").
+ * The empty string is not permitted when a path is required.
  */
-class PathSetting : public BaseSetting<Path>
+template<typename T>
+class PathsSetting : public BaseSetting<T>
 {
 public:
-
-    PathSetting(Config * options,
-        const Path & def,
-        const std::string & name,
-        const std::string & description,
-        const std::set<std::string> & aliases = {});
-
-    Path parse(const std::string & str) const override;
-
-    Path operator +(const char * p) const { return value + p; }
-
-    void operator =(const Path & v) { this->assign(v); }
-};
-
-/**
- * Like `PathSetting`, but the absence of a path is also allowed.
- *
- * `std::optional` is used instead of the empty string for clarity.
- */
-class OptionalPathSetting : public BaseSetting<std::optional<Path>>
-{
-public:
-
-    OptionalPathSetting(Config * options,
-        const std::optional<Path> & def,
-        const std::string & name,
-        const std::string & description,
-        const std::set<std::string> & aliases = {});
-
-    std::optional<Path> parse(const std::string & str) const override;
-
-    void operator =(const std::optional<Path> & v);
-};
-
-/**
- * Like `OptionalPathSetting`, but allows multiple paths.
- */
-class PathsSetting : public BaseSetting<Paths>
-{
-public:
-
     PathsSetting(Config * options,
-        const Paths & def,
+        const T & def,
         const std::string & name,
         const std::string & description,
-        const std::set<std::string> & aliases = {});
+        const std::set<std::string> & aliases = {},
+        const bool documentDefault = true,
+        std::optional<ExperimentalFeature> experimentalFeature = std::nullopt)
+        : BaseSetting<T>(def, documentDefault, name, description, aliases, std::move(experimentalFeature))
+    {
+        options->addSetting(this);
+    }
 
-    Paths parse(const std::string & str) const override;
+    T parse(const std::string & str) const override;
 
-    void operator =(const Paths & v);
-
-    operator bool() const noexcept;
+    void operator =(const T & v) { this->assign(v); }
 };
 
+
 struct GlobalConfig : public AbstractConfig
 {
     typedef std::vector<Config*> ConfigRegistrations;

tests/unit/libutil/paths-setting.cc

@@ -15,7 +15,7 @@ public:
         : Config()
     { }
 
-    PathsSetting paths{this, Paths(), "paths", "documentation"};
+    PathsSetting<Paths> paths{this, Paths(), "paths", "documentation"};
 };
 
 struct PathsSettingTest : public ::testing::Test {
@@ -55,20 +55,6 @@ TEST_F(PathsSettingTest, parse) {
     );
 }
 
-TEST_F(PathsSettingTest, bool) {
-    auto config = mkConfig();
-    // No paths:
-    ASSERT_FALSE(config.paths);
-    // Set a path:
-    config.set("paths", "/puppy.nix");
-    // Now there are paths:
-    ASSERT_TRUE(config.paths);
-
-    // Multiple paths count too:
-    config.set("paths", "/puppy.nix /doggy.nix");
-    ASSERT_TRUE(config.paths);
-}
-
 TEST_F(PathsSettingTest, append) {
     auto config = mkConfig();