inventory: unit test autoloading with a virtual fs

lib/clanTest/virtual-fs.nix

@@ -0,0 +1,51 @@
+{ lib }:
+let
+  sanitizePath =
+    rootPath: path:
+    let
+      storePrefix = builtins.unsafeDiscardStringContext ("${rootPath}");
+      pathStr = lib.removePrefix "/" (
+        lib.removePrefix storePrefix (builtins.unsafeDiscardStringContext (toString path))
+      );
+    in
+    pathStr;
+
+  mkFunctions = rootPath: passthru: virtual_fs: {
+    # Some functions to override lib functions
+    pathExists =
+      path:
+      let
+        pathStr = sanitizePath rootPath path;
+        isPassthru = builtins.any (exclude: (builtins.match exclude pathStr) != null) passthru;
+      in
+      if isPassthru then
+        builtins.pathExists path
+      else
+        let
+          res = virtual_fs ? ${pathStr};
+        in
+        lib.trace "pathExists: '${pathStr}' -> '${lib.generators.toPretty { } res}'" res;
+    readDir =
+      path:
+      let
+        pathStr = sanitizePath rootPath path;
+        base = (pathStr + "/");
+        res = lib.mapAttrs' (name: fileInfo: {
+          name = lib.removePrefix base name;
+          value = fileInfo.type;
+        }) (lib.filterAttrs (n: _: lib.hasPrefix base n) virtual_fs);
+        isPassthru = builtins.any (exclude: (builtins.match exclude pathStr) != null) passthru;
+      in
+      if isPassthru then
+        builtins.readDir path
+      else
+        lib.trace "readDir: '${pathStr}' -> '${lib.generators.toPretty { } res}'" res;
+  };
+in
+{
+  virtual_fs,
+  rootPath,
+  # Patterns
+  passthru ? [ ],
+}:
+mkFunctions rootPath passthru virtual_fs