monero/src/common/thread_group.cpp

// Copyright (c) 2014-2016, The Monero Project
//
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//    conditions and the following disclaimer.
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//    of conditions and the following disclaimer in the documentation and/or other
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#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->ptr == 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(); 
}
}