messaging.h

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// Copyright (c) Microsoft Corporation. All rights reserved.
// Licensed under the Apache 2.0 License.
#pragma once
 
#include "ds/internal_logger.h"
#include "ring_buffer.h"
 
#include <atomic>
#include <condition_variable>
#include <map>
#include <stdexcept>
 
namespace messaging
{
  using Handler = std::function<void(const uint8_t*, size_t)>;
 
  class no_handler : public std::logic_error
  {
    using logic_error::logic_error;
  };
 
  class already_handled : public std::logic_error
  {
    using logic_error::logic_error;
  };
 
  struct Counts
  {
    size_t messages;
    size_t bytes;
  };
 
  template <typename MessageType>
  using MessageCounts = std::unordered_map<MessageType, Counts>;
 
  template <typename MessageType>
  class Dispatcher
  {
  public:
    using MessageCounts = MessageCounts<MessageType>;
 
  private:
    // Store a name to distinguish error messages
    char const* const name;
 
    std::map<MessageType, Handler> handlers;
    std::map<MessageType, char const*> message_labels;
 
    std::string get_error_prefix()
    {
      return std::string("[") + std::string(name) + std::string("] ");
    }
 
    char const* get_message_name(MessageType m)
    {
      const auto it = message_labels.find(m);
      if (it == message_labels.end())
      {
        return "unknown";
      }
 
      return it->second;
    }
 
    static std::string decorate_message_name(MessageType m, char const* s)
    {
      return fmt::format("<{}:{}>", s, m);
    }
 
    std::string get_decorated_message_name(MessageType m)
    {
      return decorate_message_name(m, get_message_name(m));
    }
 
  public:
    Dispatcher(char const* name) : name(name), handlers() {}
 
    void set_message_handler(
      MessageType m, char const* message_label, Handler h)
    {
      // Check for presence first, so we only copy if we're actually inserting
      auto it = handlers.find(m);
      if (it != handlers.end())
      {
        throw already_handled(
          get_error_prefix() + "MessageType " + std::to_string(m) +
          " already handled by " + get_decorated_message_name(m) +
          ", cannot set handler for " +
          decorate_message_name(m, message_label));
      }
 
      LOG_DEBUG_FMT("Setting handler for {} ({})", message_label, m);
      handlers.insert(it, {m, h});
 
      if (message_label != nullptr)
      {
        message_labels.emplace(m, message_label);
      }
    }
 
    void remove_message_handler(MessageType m)
    {
      auto it = handlers.find(m);
      if (it == handlers.end())
      {
        throw no_handler(
          get_error_prefix() +
          "Can't remove non-existent handler for this message: " +
          get_decorated_message_name(m));
      }
 
      handlers.erase(it);
    }
 
    bool has_handler(MessageType m)
    {
      return handlers.find(m) != handlers.end();
    }
 
    void dispatch(MessageType m, const uint8_t* data, size_t size)
    {
      auto it = handlers.find(m);
      if (it == handlers.end())
      {
        throw no_handler(
          get_error_prefix() +
          "No handler for this message: " + get_decorated_message_name(m));
      }
 
      // Handlers may register or remove handlers, so iterator is invalidated
      try
      {
        it->second(data, size);
      }
      catch (const std::exception& e)
      {
        LOG_FAIL_FMT(
          "Exception while processing message {} of size {}",
          get_decorated_message_name(m),
          size);
        LOG_TRACE_FMT("{}", e.what());
        throw e;
      }
    }
  };
 
  using RingbufferDispatcher = Dispatcher<ringbuffer::Message>;
 
  using IdleBehaviour = std::function<void(size_t num_consecutive_idles)>;
  static inline void default_idle_behaviour(size_t /*unused*/)
  {
    std::this_thread::yield();
  }
 
  class BufferProcessor
  {
    RingbufferDispatcher dispatcher;
    std::atomic<bool> finished;
 
  public:
    BufferProcessor(char const* name = "") : dispatcher(name), finished(false)
    {}
 
    RingbufferDispatcher& get_dispatcher()
    {
      return dispatcher;
    }
 
    template <typename... Ts>
    void set_message_handler(Ts&&... ts)
    {
      dispatcher.set_message_handler(std::forward<Ts>(ts)...);
    }
 
    void set_finished(bool v = true)
    {
      finished.store(v);
    }
 
    bool get_finished()
    {
      return finished.load();
    }
 
    size_t read_n(size_t max_messages, ringbuffer::Reader& r)
    {
      size_t total_read = 0;
      size_t previous_read = -1;
 
      while (!finished.load() && total_read < max_messages)
      {
        // Read one at a time so we don't process any after being told to stop
        auto read = r.read(
          1,
          [&d = dispatcher](
            ringbuffer::Message m, const uint8_t* data, size_t size) {
            d.dispatch(m, data, size);
          });
 
        total_read += read;
 
        // Because of padding messages, a single empty read may be seen near the
        // end of the ringbuffer. Use consecutive empty reads as a sign that
        // we've actually read everything
        if (read == 0 && previous_read == 0)
        {
          break;
        }
 
        previous_read = read;
      }
 
      return total_read;
    }
 
    size_t read_all(ringbuffer::Reader& r)
    {
      size_t total_read = 0;
      while (true)
      {
        const auto read = read_n(1, r);
        total_read += read;
        if (read == 0)
        {
          break;
        }
      }
      return total_read;
    }
 
    size_t run(
      ringbuffer::Reader& r, IdleBehaviour idler = default_idle_behaviour)
    {
      size_t total_read = 0;
      size_t consecutive_idles = 0u;
 
      while (!finished.load())
      {
        auto num_read = read_all(r);
        total_read += num_read;
 
        if (num_read == 0)
        {
          idler(consecutive_idles++);
        }
        else
        {
          consecutive_idles = 0;
        }
      }
 
      return total_read;
    }
  };
 
  // The last variadic argument is expected to be the handler itself. It is
  // variadic so that you can use an inline lambda, _with commas_. The
  // preprocessor will blindly paste this as (what it thinks are) multiple
  // arguments to set_message_handler, and the real processor will recognise
  // will read it as the original lambda.
#define DISPATCHER_SET_MESSAGE_HANDLER(DISP, MSG, ...) \
  DISP.set_message_handler(MSG, #MSG, __VA_ARGS__)
}