RemoteStore: Make thread-safe

This allows a RemoteStore object to be used safely from multiple
threads concurrently. It will make multiple daemon connections if
necessary.

Note: pool.hh and sync.hh have been copied from the Hydra source tree.

src/libutil/sync.hh

@@ -0,0 +1,78 @@
+#pragma once
+
+#include <mutex>
+#include <condition_variable>
+#include <cassert>
+
+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 Sync
+{
+private:
+    std::mutex mutex;
+    T data;
+
+public:
+
+    Sync() { }
+    Sync(const T & data) : data(data) { }
+
+    class Lock
+    {
+    private:
+        Sync * s;
+        friend Sync;
+        Lock(Sync * s) : s(s) { s->mutex.lock(); }
+    public:
+        Lock(Lock && l) : s(l.s) { l.s = 0; }
+        Lock(const Lock & l) = delete;
+        ~Lock() { if (s) s->mutex.unlock(); }
+        T * operator -> () { return &s->data; }
+        T & operator * () { return s->data; }
+
+        /* FIXME: performance impact of condition_variable_any? */
+        void wait(std::condition_variable_any & cv)
+        {
+            assert(s);
+            cv.wait(s->mutex);
+        }
+
+        template<class Rep, class Period, class Predicate>
+        bool wait_for(std::condition_variable_any & cv,
+            const std::chrono::duration<Rep, Period> & duration,
+            Predicate pred)
+        {
+            assert(s);
+            return cv.wait_for(s->mutex, duration, pred);
+        }
+
+        template<class Clock, class Duration>
+        std::cv_status wait_until(std::condition_variable_any & cv,
+            const std::chrono::time_point<Clock, Duration> & duration)
+        {
+            assert(s);
+            return cv.wait_until(s->mutex, duration);
+        }
+    };
+
+    Lock lock() { return Lock(this); }
+};
+
+}