1 files changed, 84 insertions, 0 deletions
diff --git a/third_party/nix/src/libutil/sync.hh b/third_party/nix/src/libutil/sync.hh
new file mode 100644
index 000000000000..ef640d5b56ef
--- /dev/null
+++ b/third_party/nix/src/libutil/sync.hh
@@ -0,0 +1,84 @@
+#pragma once
+
+#include <cassert>
+#include <condition_variable>
+#include <cstdlib>
+#include <mutex>
+
+namespace nix {
+
+/* This template class ensures synchronized access to a value of type
+   T. It is used as follows:
+
+     struct Data { int x; ... };
+
+     Sync<Data> data;
+
+     {
+       auto data_(data.lock());
+       data_->x = 123;
+     }
+
+   Here, "data" is automatically unlocked when "data_" goes out of
+   scope.
+*/
+
+template <class T, class M = std::mutex>
+class Sync {
+ private:
+  M mutex;
+  T data;
+
+ public:
+  Sync() {}
+  explicit Sync(const T& data) : data(data) {}
+  explicit Sync(T&& data) noexcept : data(std::move(data)) {}
+
+  class Lock {
+   private:
+    Sync* s;
+    std::unique_lock<M> lk;
+    friend Sync;
+    explicit Lock(Sync* s) : s(s), lk(s->mutex) {}
+
+   public:
+    Lock(Lock&& l) : s(l.s) { abort(); }
+    Lock(const Lock& l) = delete;
+    ~Lock() {}
+    T* operator->() { return &s->data; }
+    T& operator*() { return s->data; }
+
+    void wait(std::condition_variable& cv) {
+      assert(s);
+      cv.wait(lk);
+    }
+
+    template <class Rep, class Period>
+    std::cv_status wait_for(
+        std::condition_variable& cv,
+        const std::chrono::duration<Rep, Period>& duration) {
+      assert(s);
+      return cv.wait_for(lk, duration);
+    }
+
+    template <class Rep, class Period, class Predicate>
+    bool wait_for(std::condition_variable& cv,
+                  const std::chrono::duration<Rep, Period>& duration,
+                  Predicate pred) {
+      assert(s);
+      return cv.wait_for(lk, duration, pred);
+    }
+
+    template <class Clock, class Duration>
+    std::cv_status wait_until(
+        std::condition_variable& cv,
+        const std::chrono::time_point<Clock, Duration>& duration) {
+      assert(s);
+      return cv.wait_until(lk, duration);
+    }
+  };
+
+  Lock lock() { return Lock(this); }
+};
+
+}  // namespace nix