text: //------------------------------------------------------------------------------
  /*
      This file is part of rippled: https://github.com/ripple/rippled
      Copyright (c) 2012, 2013 Ripple Labs Inc.
  
      Permission to use, copy, modify, and/or distribute this software for any
      purpose  with  or without fee is hereby granted, provided that the above
      copyright notice and this permission notice appear in all copies.
  
      THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
      WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
      MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
      ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
      WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
      ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
      OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
  //==============================================================================
  
  #include <ripple/basics/Log.h>
  #include <ripple/basics/ResolverAsio.h>
  #include <ripple/beast/net/IPAddressConversion.h>
  #include <ripple/beast/net/IPEndpoint.h>
  #include <boost/asio.hpp>
  #include <atomic>
  #include <cassert>
  #include <condition_variable>
  #include <deque>
  #include <locale>
  #include <memory>
  #include <mutex>
  
  namespace ripple {
  
  /** Mix-in to track when all pending I/O is complete.
      Derived classes must be callable with this signature:
          void asyncHandlersComplete()
  */
  template <class Derived>
  class AsyncObject
  {
  protected:
      AsyncObject() : m_pending(0)
      {
      }
  
  public:
      ~AsyncObject()
      {
          // Destroying the object with I/O pending? Not a clean exit!
          assert(m_pending.load() == 0);
      }
  
      /** RAII container that maintains the count of pending I/O.
          Bind this into the argument list of every handler passed
          to an initiating function.
      */
      class CompletionCounter
      {
      public:
          explicit CompletionCounter(Derived* owner) : m_owner(owner)
          {
              ++m_owner->m_pending;
          }
  
          CompletionCounter(CompletionCounter const& other)
              : m_owner(other.m_owner)
          {
              ++m_owner->m_pending;
          }
  
          ~CompletionCounter()
          {
              if (--m_owner->m_pending == 0)
                  m_owner->asyncHandlersComplete();
          }
  
          CompletionCounter&
          operator=(CompletionCounter const&) = delete;
  
      private:
          Derived* m_owner;
      };
  
      void
      addReference()
      {
          ++m_pending;
      }
  
      void
      removeReference()
      {
          if (--m_pending == 0)
              (static_cast<Derived*>(this))->asyncHandlersComplete();
      }
  
  private:
      // The number of handlers pending.
      std::atomic<int> m_pending;
  };
  
  class ResolverAsioImpl : public ResolverAsio,
                           public AsyncObject<ResolverAsioImpl>
  {
  public:
      using HostAndPort = std::pair<std::string, std::string>;
  
      beast::Journal m_journal;
  
      boost::asio::io_service& m_io_service;
      boost::asio::io_service::strand m_strand;
      boost::asio::ip::tcp::resolver m_resolver;
  
      std::condition_variable m_cv;
      std::mutex m_mut;
      bool m_asyncHandlersCompleted;
  
      std::atomic<bool> m_stop_called;
      std::atomic<bool> m_stopped;
  
      // Represents a unit of work for the resolver to do
      struct Work
      {
          std::vector<std::string> names;
          HandlerType handler;
  
          template <class StringSequence>
          Work(StringSequence const& names_, HandlerType const& handler_)
              : handler(handler_)
          {
              names.reserve(names_.size());
  
              std::reverse_copy(
                  names_.begin(), names_.end(), std::back_inserter(names));
          }
      };
  
      std::deque<Work> m_work;
  
      ResolverAsioImpl(
          boost::asio::io_service& io_service,
          beast::Journal journal)
          : m_journal(journal)
          , m_io_service(io_service)
          , m_strand(io_service)
          , m_resolver(io_service)
          , m_asyncHandlersCompleted(true)
          , m_stop_called(false)
          , m_stopped(true)
      {
      }
  
      ~ResolverAsioImpl() override
      {
          assert(m_work.empty());
          assert(m_stopped);
      }
  
      //-------------------------------------------------------------------------
      // AsyncObject
      void
      asyncHandlersComplete()
      {
          std::unique_lock<std::mutex> lk{m_mut};
          m_asyncHandlersCompleted = true;
          m_cv.notify_all();
      }
  
      //--------------------------------------------------------------------------
      //
      // Resolver
      //
      //--------------------------------------------------------------------------
  
      void
      start() override
      {
          assert(m_stopped == true);
          assert(m_stop_called == false);
  
          if (m_stopped.exchange(false) == true)
          {
              {
                  std::lock_guard lk{m_mut};
                  m_asyncHandlersCompleted = false;
              }
              addReference();
          }
      }
  
      void
      stop_async() override
      {
          if (m_stop_called.exchange(true) == false)
          {
              m_io_service.dispatch(m_strand.wrap(std::bind(
                  &ResolverAsioImpl::do_stop, this, CompletionCounter(this))));
  
              JLOG(m_journal.debug()) << "Queued a stop request";
          }
      }
  
      void
      stop() override
      {
          stop_async();
  
          JLOG(m_journal.debug()) << "Waiting to stop";
          std::unique_lock<std::mutex> lk{m_mut};
          m_cv.wait(lk, [this] { return m_asyncHandlersCompleted; });
          lk.unlock();
          JLOG(m_journal.debug()) << "Stopped";
      }
  
      void
      resolve(std::vector<std::string> const& names, HandlerType const& handler)
          override
      {
          assert(m_stop_called == false);
          assert(m_stopped == true);
          assert(!names.empty());
  
          // TODO NIKB use rvalue references to construct and move
          //           reducing cost.
          m_io_service.dispatch(m_strand.wrap(std::bind(
              &ResolverAsioImpl::do_resolve,
              this,
              names,
              handler,
              CompletionCounter(this))));
      }
  
      //-------------------------------------------------------------------------
      // Resolver
      void do_stop(CompletionCounter)
      {
          assert(m_stop_called == true);
  
          if (m_stopped.exchange(true) == false)
          {
              m_work.clear();
              m_resolver.cancel();
  
              removeReference();
          }
      }
  
      void
      do_finish(
          std::string name,
          boost::system::error_code const& ec,
          HandlerType handler,
          boost::asio::ip::tcp::resolver::iterator iter,
          CompletionCounter)
      {
          if (ec == boost::asio::error::operation_aborted)
              return;
  
          std::vector<beast::IP::Endpoint> addresses;
  
          // If we get an error message back, we don't return any
          // results that we may have gotten.
          if (!ec)
          {
              while (iter != boost::asio::ip::tcp::resolver::iterator())
              {
                  addresses.push_back(
                      beast::IPAddressConversion::from_asio(*iter));
                  ++iter;
              }
          }
  
          handler(name, addresses);
  
          m_io_service.post(m_strand.wrap(std::bind(
              &ResolverAsioImpl::do_work, this, CompletionCounter(this))));
      }
  
      HostAndPort
      parseName(std::string const& str)
      {
          // first attempt to parse as an endpoint (IP addr + port).
          // If that doesn't succeed, fall back to generic name + port parsing
  
          if (auto const result = beast::IP::Endpoint::from_string_checked(str))
          {
              return make_pair(
                  result->address().to_string(), std::to_string(result->port()));
          }
  
          // generic name/port parsing, which doesn't work for
          // IPv6 addresses in particular because it considers a colon
          // a port separator
  
          // Attempt to find the first and last non-whitespace
          auto const find_whitespace = std::bind(
              &std::isspace<std::string::value_type>,
              std::placeholders::_1,
              std::locale());
  
          auto host_first =
              std::find_if_not(str.begin(), str.end(), find_whitespace);
  
          auto port_last =
              std::find_if_not(str.rbegin(), str.rend(), find_whitespace).base();
  
          // This should only happen for all-whitespace strings
          if (host_first >= port_last)
              return std::make_pair(std::string(), std::string());
  
          // Attempt to find the first and last valid port separators
          auto const find_port_separator = [](char const c) -> bool {
              if (std::isspace(static_cast<unsigned char>(c)))
                  return true;
  
              if (c == ':')
                  return true;
  
              return false;
          };
  
          auto host_last =
              std::find_if(host_first, port_last, find_port_separator);
  
          auto port_first =
              std::find_if_not(host_last, port_last, find_port_separator);
  
          return make_pair(
              std::string(host_first, host_last),
              std::string(port_first, port_last));
      }
  
      void do_work(CompletionCounter)
      {
          if (m_stop_called == true)
              return;
  
          // We don't have any work to do at this time
          if (m_work.empty())
              return;
  
          std::string const name(m_work.front().names.back());
          HandlerType handler(m_work.front().handler);
  
          m_work.front().names.pop_back();
  
          if (m_work.front().names.empty())
              m_work.pop_front();
  
          auto const [host, port] = parseName(name);
  
          if (host.empty())
          {
              JLOG(m_journal.error()) << "Unable to parse '" << name << "'";
  
              m_io_service.post(m_strand.wrap(std::bind(
                  &ResolverAsioImpl::do_work, this, CompletionCounter(this))));
  
              return;
          }
  
          boost::asio::ip::tcp::resolver::query query(host, port);
  
          m_resolver.async_resolve(
              query,
              std::bind(
                  &ResolverAsioImpl::do_finish,
                  this,
                  name,
                  std::placeholders::_1,
                  handler,
                  std::placeholders::_2,
                  CompletionCounter(this)));
      }
  
      void
      do_resolve(
          std::vector<std::string> const& names,
          HandlerType const& handler,
          CompletionCounter)
      {
          assert(!names.empty());
  
          if (m_stop_called == false)
          {
              m_work.emplace_back(names, handler);
  
              JLOG(m_journal.debug())
                  << "Queued new job with " << names.size() << " tasks. "
                  << m_work.size() << " jobs outstanding.";
  
              if (m_work.size() > 0)
              {
                  m_io_service.post(m_strand.wrap(std::bind(
                      &ResolverAsioImpl::do_work,
                      this,
                      CompletionCounter(this))));
              }
          }
      }
  };
  
  //-----------------------------------------------------------------------------
  
  std::unique_ptr<ResolverAsio>
  ResolverAsio::New(boost::asio::io_service& io_service, beast::Journal journal)
  {
      return std::make_unique<ResolverAsioImpl>(io_service, journal);
  }
  
  //-----------------------------------------------------------------------------
  Resolver::~Resolver() = default;
  }  // namespace ripple
  