adding thread_group for managing async tasks

src/common/thread_group.h

@@ -0,0 +1,133 @@
+// Copyright (c) 2014-2016, The Monero Project
+// 
+// All rights reserved.
+// 
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+// 
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+// 
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+// 
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+// 
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#include <boost/optional/optional.hpp>
+#include <condition_variable>
+#include <cstddef>
+#include <functional>
+#include <thread>
+#include <utility>
+#include <vector>
+
+namespace tools 
+{
+//! Manages zero or more threads for work dispatching
+class thread_group
+{
+public:
+  //! Create `min(count, get_max_concurrency()) - 1`  threads
+  explicit thread_group(std::size_t count);
+
+  thread_group(thread_group const&) = delete;
+  thread_group(thread_group&&) = delete;
+
+  //! Joins threads, but does not necessarily run all dispatched functions.
+  ~thread_group() = default;
+
+  thread_group& operator=(thread_group const&) = delete;
+  thread_group& operator=(thread_group&&) = delete;
+
+  /*! Blocks until all functions provided to `dispatch` complete. Does not
+  destroy threads. If a dispatched function calls `this->dispatch(...)`,
+  `this->sync()` will continue to block until that new function completes. */
+  void sync() noexcept {
+    if (internal) {
+      internal->sync();
+    }
+  }
+
+  /*! Example usage:
+    std::unique_ptr<thread_group, thread_group::lazy_sync> sync(std::addressof(group));
+  which guarantees synchronization before the unique_ptr destructor returns. */
+  struct lazy_sync {
+    void operator()(thread_group* group) const noexcept {
+      if (group != nullptr) {
+        group->sync();
+      }
+    }
+  };
+
+  /*! `f` is invoked immediately if the thread_group is empty, otherwise
+  execution of `f` is queued for next available thread. If `f` is queued, any
+  exception leaving that function will result in process termination. Use
+  std::packaged_task if exceptions need to be handled. */
+  template<typename F>
+  void dispatch(F&& f) {
+    if (internal) {
+      internal->dispatch(std::forward<F>(f));
+    }
+    else {
+      f();
+    }
+  }
+
+private:
+  class data {
+  public:
+    data(std::size_t count);
+    ~data() noexcept;
+
+    void sync() noexcept;
+
+    void dispatch(std::function<void()> f);
+
+  private:
+    struct work;
+
+    struct node {
+      node() = delete;
+      std::unique_ptr<work> ptr;
+    };
+
+    struct work {
+      work() = delete;
+      std::function<void()> f;
+      node next;
+    };
+
+    //! Requires lock on `mutex`.
+    std::unique_ptr<work> get_next() noexcept;
+
+    //! Blocks until destructor is invoked, only call from thread.
+    void run() noexcept;
+
+  private:
+    std::vector<std::thread> threads;
+    node head;
+    node* last;
+    std::size_t pending;
+    std::condition_variable has_work;
+    std::condition_variable finished_work;
+    std::mutex mutex;
+    bool stop;
+  };
+
+private:
+  // optionally construct elements, without separate heap allocation
+  boost::optional<data> internal;
+};
+}