1 files changed, 45 insertions, 0 deletions
diff --git a/third_party/nix/src/libutil/lazy.hh b/third_party/nix/src/libutil/lazy.hh
new file mode 100644
index 000000000000..5c6ff5d8df6a
--- /dev/null
+++ b/third_party/nix/src/libutil/lazy.hh
@@ -0,0 +1,45 @@
+#include <exception>
+#include <functional>
+#include <mutex>
+
+namespace nix {
+
+/* A helper class for lazily-initialized variables.
+
+     Lazy<T> var([]() { return value; });
+
+   declares a variable of type T that is initialized to 'value' (in a
+   thread-safe way) on first use, that is, when var() is first
+   called. If the initialiser code throws an exception, then all
+   subsequent calls to var() will rethrow that exception. */
+template <typename T>
+class Lazy {
+  typedef std::function<T()> Init;
+
+  Init init;
+
+  std::once_flag done;
+
+  T value;
+
+  std::exception_ptr ex;
+
+ public:
+  explicit Lazy(Init init) : init(init) {}
+
+  const T& operator()() {
+    std::call_once(done, [&]() {
+      try {
+        value = init();
+      } catch (...) {
+        ex = std::current_exception();
+      }
+    });
+    if (ex) {
+      std::rethrow_exception(ex);
+    }
+    return value;
+  }
+};
+
+}  // namespace nix