Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libutil/sync.hh

@@ -0,0 +1,118 @@
+#pragma once
+///@file
+
+#include <cstdlib>
+#include <mutex>
+#include <shared_mutex>
+#include <condition_variable>
+#include <cassert>
+
+#include "error.hh"
+
+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, class WL, class RL>
+class SyncBase
+{
+private:
+    M mutex;
+    T data;
+
+public:
+
+    SyncBase() { }
+    SyncBase(const T & data) : data(data) { }
+    SyncBase(T && data) noexcept : data(std::move(data)) { }
+
+    template<class L>
+    class Lock
+    {
+    protected:
+        SyncBase * s;
+        L lk;
+        friend SyncBase;
+        Lock(SyncBase * s) : s(s), lk(s->mutex) { }
+    public:
+        Lock(Lock && l) : s(l.s) { unreachable(); }
+        Lock(const Lock & l) = delete;
+        ~Lock() { }
+
+        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);
+        }
+    };
+
+    struct WriteLock : Lock<WL>
+    {
+        T * operator -> () { return &WriteLock::s->data; }
+        T & operator * () { return WriteLock::s->data; }
+    };
+
+    /**
+     * Acquire write (exclusive) access to the inner value.
+     */
+    WriteLock lock() { return WriteLock(this); }
+
+    struct ReadLock : Lock<RL>
+    {
+        const T * operator -> () { return &ReadLock::s->data; }
+        const T & operator * () { return ReadLock::s->data; }
+    };
+
+    /**
+     * Acquire read access to the inner value. When using
+     * `std::shared_mutex`, this will use a shared lock.
+     */
+    ReadLock readLock() const { return ReadLock(const_cast<SyncBase *>(this)); }
+};
+
+template<class T>
+using Sync = SyncBase<T, std::mutex, std::unique_lock<std::mutex>, std::unique_lock<std::mutex>>;
+
+template<class T>
+using SharedSync = SyncBase<T, std::shared_mutex, std::unique_lock<std::shared_mutex>, std::shared_lock<std::shared_mutex>>;
+
+}