add MetaEnv subsys iface

iface/CMakeLists.txt

@@ -27,6 +27,7 @@ target_sources(nf7_iface
     subsys/dealer.hh
     subsys/interface.hh
     subsys/logger.hh
+    subsys/meta_env.hh
     subsys/parallelism.hh
     env.hh
     lambda.hh

iface/subsys/meta_env.hh

@@ -0,0 +1,42 @@
+// No copyright
+#pragma once
+
+#include <cstdint>
+#include <optional>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "iface/common/exception.hh"
+#include "iface/env.hh"
+
+
+namespace nf7::subsys {
+
+class MetaEnv : public Interface {
+ public:
+  using Pair = std::pair<std::string, Env&>;
+
+ public:
+  using Interface::Interface;
+
+ public:
+  virtual Env* FindOr(std::string_view) const noexcept = 0;
+  virtual std::optional<Pair> FindOr(uint64_t) const noexcept = 0;
+  virtual std::vector<Pair> FetchAll() const = 0;
+
+ public:
+  Env& Find(std::string_view key) const {
+    auto ret = FindOr(key);
+    return ret? *ret: throw Exception {"missing file"};
+  }
+  Pair Find(uint64_t idx) const {
+    auto ret = FindOr(idx);
+    return ret? *ret: throw Exception {"missing file"};
+  }
+
+ public:
+  virtual std::shared_ptr<MetaEnv> parent() const noexcept = 0;
+};
+
+}  // namespace nf7::subsys