adding thread_group for managing async tasks

3 files changed, 301 insertions, 2 deletions

diff --git a/src/common/CMakeLists.txt b/src/common/CMakeLists.txt
index 4b6149cbd..d5d22bca6 100644
--- a/src/common/CMakeLists.txt
+++ b/src/common/CMakeLists.txt
@@ -32,7 +32,8 @@ set(common_sources
   dns_utils.cpp
   util.cpp
   i18n.cpp
-  perf_timer.cpp)
+  perf_timer.cpp
+  thread_group.cpp)
 
 if (STACK_TRACE)
   list(APPEND common_sources stack_trace.cpp)
@@ -55,7 +56,8 @@ set(common_private_headers
   varint.h
   i18n.h
   perf_timer.h
-  stack_trace.h)
+  stack_trace.h
+  thread_group.h)
 
 monero_private_headers(common
   ${common_private_headers})
diff --git a/src/common/thread_group.cpp b/src/common/thread_group.cpp
new file mode 100644
index 000000000..ece268ab7
--- /dev/null
+++ b/src/common/thread_group.cpp
@@ -0,0 +1,164 @@
+// 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 "common/thread_group.h"
+
+#include <cassert>
+#include <limits>
+#include <stdexcept>
+
+#include "common/util.h"
+
+namespace tools
+{
+thread_group::thread_group(std::size_t count) : internal() {
+  static_assert(
+    std::numeric_limits<unsigned>::max() <= std::numeric_limits<std::size_t>::max(), 
+    "unexpected truncation"
+  );
+  count = std::min<std::size_t>(count, get_max_concurrency());
+  count = count ? count - 1 : 0;
+
+  if (count) {
+    internal.emplace(count);
+  }
+}
+
+thread_group::data::data(std::size_t count)
+  : threads()
+  , head{nullptr}
+  , last(std::addressof(head))
+  , pending(count)
+  , mutex()
+  , has_work()
+  , finished_work()
+  , stop(false) {
+  threads.reserve(count);
+  while (count--) {
+    threads.push_back(std::thread(&thread_group::data::run, this));
+  }
+}
+
+thread_group::data::~data() noexcept {
+  {
+    const std::unique_lock<std::mutex> lock(mutex);
+    stop = true;
+  }
+  has_work.notify_all();
+  finished_work.notify_all();
+  for (auto& worker : threads) {
+    try {
+      worker.join();
+    }
+    catch(...) {}
+  }
+}
+
+
+void thread_group::data::sync() noexcept {
+  /* This function and `run()` can both throw when acquiring the lock, or in
+  the dispatched function. It is tough to recover from either, particularly the
+  lock case. These functions are marked as noexcept so that if either call
+  throws, the entire process is terminated. Users of the `dispatch` call are
+  expected to make their functions noexcept, or use std::packaged_task to copy
+  exceptions so that the process will continue in all but the most pessimistic
+  cases (std::bad_alloc). This was the existing behavior;
+  `asio::io_service::run` propogates errors from dispatched calls, and uncaught
+  exceptions on threads result in process termination. */
+  assert(!threads.empty());
+  bool not_first = false;
+  while (true) {
+    std::unique_ptr<work> next = nullptr;
+    {
+      std::unique_lock<std::mutex> lock(mutex);
+      pending -= std::size_t(not_first);
+      not_first = true;
+      finished_work.notify_all();
+
+      if (stop) {
+        return;
+      }
+
+      next = get_next();
+      if (next == nullptr) {
+        finished_work.wait(lock, [this] { return pending == 0 || stop; });
+        return;
+      }
+    }
+    assert(next->f);
+    next->f();
+  }
+}
+
+std::unique_ptr<thread_group::data::work> thread_group::data::get_next() noexcept {
+  std::unique_ptr<work> rc = std::move(head.ptr);
+  if (rc != nullptr) {
+    head.ptr = std::move(rc->next.ptr);
+    if (head.ptr == nullptr) {
+      last = std::addressof(head);
+    }
+  }
+  return rc;
+}
+
+void thread_group::data::run() noexcept {
+  // see `sync()` source for additional information
+  while (true) {
+    std::unique_ptr<work> next = nullptr;
+    {
+      std::unique_lock<std::mutex> lock(mutex);
+      --pending;
+      finished_work.notify_all();
+      has_work.wait(lock, [this] { return head.ptr != nullptr || stop; });
+      if (stop) {
+        return;
+      }
+      next = get_next();
+    }
+    assert(next != nullptr);
+    assert(next->f);
+    next->f();
+  }
+}
+
+void thread_group::data::dispatch(std::function<void()> f) {
+  std::unique_ptr<work> latest(new work{std::move(f), node{nullptr}});
+  node* const latest_node = std::addressof(latest->next);
+  {
+    const std::unique_lock<std::mutex> lock(mutex);
+    assert(last != nullptr);
+    assert(last->next == nullptr);
+    if (pending == std::numeric_limits<std::size_t>::max()) {
+      throw std::overflow_error("thread_group exceeded max queue depth");
+    }
+    last->ptr = std::move(latest);
+    last = latest_node;
+    ++pending;
+  }
+  has_work.notify_one(); 
+}
+}
diff --git a/src/common/thread_group.h b/src/common/thread_group.h
new file mode 100644
index 000000000..d8461d49a
--- /dev/null
+++ b/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;
+};
+}