libstore: Deduplicate LocalFSStoreConfig::rootDir initializers

Co-authored-by: John Ericson <John.Ericson@Obsidian.Systems>

src/libstore/include/nix/store/local-fs-store.hh

@@ -9,6 +9,18 @@ namespace nix {
 
 struct LocalFSStoreConfig : virtual StoreConfig
 {
+private:
+    static OptionalPathSetting makeRootDirSetting(LocalFSStoreConfig & self, std::optional<Path> defaultValue)
+    {
+        return {
+            &self,
+            std::move(defaultValue),
+            "root",
+            "Directory prefixed to all other paths.",
+        };
+    }
+
+public:
     using StoreConfig::StoreConfig;
 
     /**
@@ -20,7 +32,7 @@ struct LocalFSStoreConfig : virtual StoreConfig
      */
     LocalFSStoreConfig(PathView path, const Params & params);
 
-    OptionalPathSetting rootDir{this, std::nullopt, "root", "Directory prefixed to all other paths."};
+    OptionalPathSetting rootDir = makeRootDirSetting(*this, std::nullopt);
 
 private:
 

src/libstore/local-fs-store.cc

@@ -20,13 +20,17 @@ Path LocalFSStoreConfig::getDefaultLogDir()
 
 LocalFSStoreConfig::LocalFSStoreConfig(PathView rootDir, const Params & params)
     : StoreConfig(params)
-    // Default `?root` from `rootDir` if non set
+    /* Default `?root` from `rootDir` if non set
-    // FIXME don't duplicate description once we don't have root setting
+     * NOTE: We would like to just do rootDir.set(...), which would take care of
-    , rootDir{
+     * all normalization and error checking for us. Unfortunately we cannot do
-          this,
+     * that because of the complicated initialization order of other fields with
-          !rootDir.empty() && params.count("root") == 0 ? (std::optional{canonPath(rootDir)}) : std::nullopt,
+     * the virtual class hierarchy of nix store configs, and the design of the
-          "root",
+     * settings system. As such, we have no choice but to redefine the field and
-          "Directory prefixed to all other paths."}
+     * manually repeat the same normalization logic.
+     */
+    , rootDir{makeRootDirSetting(
+          *this,
+          !rootDir.empty() && params.count("root") == 0 ? std::optional<Path>{canonPath(rootDir)} : std::nullopt)}
 {
 }