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/pool.hh

@@ -0,0 +1,102 @@
+#pragma once
+
+#include <memory>
+#include <list>
+#include <functional>
+
+#include "sync.hh"
+#include "ref.hh"
+
+namespace nix {
+
+/* This template class implements a simple pool manager of resources
+   of some type R, such as database connections. It is used as
+   follows:
+
+     class Connection { ... };
+
+     Pool<Connection> pool;
+
+     {
+       auto conn(pool.get());
+       conn->exec("select ...");
+     }
+
+   Here, the Connection object referenced by ‘conn’ is automatically
+   returned to the pool when ‘conn’ goes out of scope.
+*/
+
+template <class R>
+class Pool
+{
+public:
+
+    typedef std::function<ref<R>()> Factory;
+
+private:
+
+    Factory factory;
+
+    struct State
+    {
+        unsigned int count = 0;
+        std::list<ref<R>> idle;
+    };
+
+    Sync<State> state;
+
+public:
+
+    Pool(const Factory & factory = []() { return make_ref<R>(); })
+        : factory(factory)
+    { }
+
+    class Handle
+    {
+    private:
+        Pool & pool;
+        ref<R> r;
+
+        friend Pool;
+
+        Handle(Pool & pool, std::shared_ptr<R> r) : pool(pool), r(r) { }
+
+    public:
+        Handle(Handle && h) : pool(h.pool), r(h.r) { abort(); }
+
+        Handle(const Handle & l) = delete;
+
+        ~Handle()
+        {
+            auto state_(pool.state.lock());
+            state_->idle.push_back(r);
+        }
+
+        R * operator -> () { return &*r; }
+        R & operator * () { return *r; }
+    };
+
+    Handle get()
+    {
+        {
+            auto state_(state.lock());
+            if (!state_->idle.empty()) {
+                auto p = state_->idle.back();
+                state_->idle.pop_back();
+                return Handle(*this, p);
+            }
+            state_->count++;
+        }
+        /* Note: we don't hold the lock while creating a new instance,
+           because creation might take a long time. */
+        return Handle(*this, factory());
+    }
+
+    unsigned int count()
+    {
+        auto state_(state.lock());
+        return state_->count;
+    }
+};
+
+}