src/hci/async_manager.cc - platform/tools/netsim - Git at Google

 /*
  * Copyright 2016 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  *      http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
 #include "model/setup/async_manager.h"  // for AsyncManager

 #include <errno.h>                              // for errno
 #include <atomic>                               // for atomic_bool, atomic_e...
 #include <condition_variable>                   // for condition_variable
 #include <cstring>                              // for strerror
 #include <limits>                               // for numeric_limits
 #include <map>                                  // for map<>::value_type, map
 #include <mutex>                                // for unique_lock, mutex
 #include <ratio>                                // for ratio
 #include <set>                                  // for set
 #include <thread>                               // for thread
 #include <type_traits>                          // for remove_extent_t
 #include <utility>                              // for pair, make_pair, oper...
 #include <vector>                               // for vector

 #include "aemu/base/EintrWrapper.h"          // for HANDLE_EINTR
 #include "aemu/base/Log.h"                   // for LogStreamVoidify, Log...
 #include "aemu/base/sockets/SocketUtils.h"   // for socketRecv, socketSet...
 #include "aemu/base/sockets/SocketWaiter.h"  // for SocketWaiter, SocketW...
 #include "aemu/base/logging/CLog.h"

 namespace rootcanal {
 // Implementation of AsyncManager is divided between two classes, three if
 // AsyncManager itself is taken into account, but its only responsability
 // besides being a proxy for the other two classes is to provide a global
 // synchronization mechanism for callbacks and client code to use.

 // The watching of file descriptors is done through AsyncFdWatcher. Several
 // objects of this class may coexist simultaneosly as they share no state.
 // After construction of this objects nothing happens beyond some very simple
 // member initialization. When the first FD is set up for watching the object
 // starts a new thread which watches the given (and later provided) FDs using
 // select() inside a loop. A special FD (a pipe) is also watched which is
 // used to notify the thread of internal changes on the object state (like
 // the addition of new FDs to watch on). Every access to internal state is
 // synchronized using a single internal mutex. The thread is only stopped on
 // destruction of the object, by modifying a flag, which is the only member
 // variable accessed without acquiring the lock (because the notification to
 // the thread is done later by writing to a pipe which means the thread will
 // be notified regardless of what phase of the loop it is in that moment)

 // The scheduling of asynchronous tasks, periodic or not, is handled by the
 // AsyncTaskManager class. Like the one for FDs, this class shares no internal
 // state between different instances so it is safe to use several objects of
 // this class, also nothing interesting happens upon construction, but only
 // after a Task has been scheduled and access to internal state is synchronized
 // using a single internal mutex. When the first task is scheduled a thread
 // is started which monitors a queue of tasks. The queue is peeked to see
 // when the next task should be carried out and then the thread performs a
 // (absolute) timed wait on a condition variable. The wait ends because of a
 // time out or a notify on the cond var, the former means a task is due
 // for execution while the later means there has been a change in internal
 // state, like a task has been scheduled/canceled or the flag to stop has
 // been set. Setting and querying the stop flag or modifying the task queue
 // and subsequent notification on the cond var is done atomically (e.g while
 // holding the lock on the internal mutex) to ensure that the thread never
 // misses the notification, since notifying a cond var is not persistent as
 // writing on a pipe (if not done this way, the thread could query the
 // stopping flag and be put aside by the OS scheduler right after, then the
 // 'stop thread' procedure could run, setting the flag, notifying a cond
 // var that no one is waiting on and joining the thread, the thread then
 // resumes execution believing that it needs to continue and waits on the
 // cond var possibly forever if there are no tasks scheduled, efectively
 // causing a deadlock).

 // This number also states the maximum number of scheduled tasks we can handle
 // at a given time
 static const uint16_t kMaxTaskId = -1; /* 2^16 - 1, permisible ids are {1..2^16-1}*/
 static inline AsyncTaskId NextAsyncTaskId(const AsyncTaskId id) {
   return (id == kMaxTaskId) ? 1 : id + 1;
 }
 // The buffer is only 10 bytes because the expected number of bytes
 // written on this socket is 1. It is possible that the thread is notified
 // more than once but highly unlikely, so a buffer of size 10 seems enough
 // and the reads are performed inside a while just in case it isn't. From
 // the thread routine's point of view it is the same to have been notified
 // just once or 100 times so it just tries to consume the entire buffer.
 // In the cases where an interrupt would cause read to return without
 // having read everything that was available a new iteration of the thread
 // loop will bring execution to this point almost immediately, so there is
 // no need to treat that case.
 static const int kNotificationBufferSize = 10;


 using android::base::SocketWaiter;

 // Async File Descriptor Watcher Implementation:
 class AsyncManager::AsyncFdWatcher {

  public:
   int WatchFdForNonBlockingReads(int file_descriptor, const ReadCallback& on_read_fd_ready_callback) {
     // add file descriptor and callback
     {
       std::unique_lock<std::recursive_mutex> guard(internal_mutex_);
       watched_shared_fds_[file_descriptor] = on_read_fd_ready_callback;
     }

     // start the thread if not started yet
     int started = tryStartThread();
     if (started != 0) {
       derror("%s: Unable to start thread", __func__);
       return started;
     }

     // notify the thread so that it knows of the new FD
     notifyThread();

     return 0;
   }

   void StopWatchingFileDescriptor(int file_descriptor) {
     std::unique_lock<std::recursive_mutex> guard(internal_mutex_);
     watched_shared_fds_.erase(file_descriptor);
   }

   AsyncFdWatcher() = default;
   AsyncFdWatcher(const AsyncFdWatcher&) = delete;
   AsyncFdWatcher& operator=(const AsyncFdWatcher&) = delete;

   ~AsyncFdWatcher() = default;

   int stopThread() {
     if (!std::atomic_exchange(&running_, false)) {
       return 0;  // if not running already
     }

     notifyThread();

     if (std::this_thread::get_id() != thread_.get_id()) {
       thread_.join();
     } else {
       dwarning("%s: Starting thread stop from inside the reading thread itself", __func__);
     }

     {
       std::unique_lock<std::recursive_mutex> guard(internal_mutex_);
       watched_shared_fds_.clear();
     }

     return 0;
   }

  private:
   // Make sure to call this with at least one file descriptor ready to be
   // watched upon or the thread routine will return immediately
   int tryStartThread() {
     if (std::atomic_exchange(&running_, true)) {
       return 0;  // if already running
     }
     // set up the communication channel
     if (android::base::socketCreatePair(&notification_listen_fd_, &notification_write_fd_)) {
       derror(
           "%s:Unable to establish a communication channel to the reading "
           "thread",
           __func__);
       return -1;
     }
     android::base::socketSetNonBlocking(notification_listen_fd_);
     android::base::socketSetNonBlocking(notification_write_fd_);

     thread_ = std::thread([this]() { ThreadRoutine(); });
     if (!thread_.joinable()) {
       derror("%s: Unable to start reading thread", __func__);
       return -1;
     }
     return 0;
   }

   int notifyThread() {
     char buffer = '0';
     if (android::base::socketSend(notification_write_fd_, &buffer, 1) < 0) {
       derror("%s: Unable to send message to reading thread", __func__);
       return -1;
     }
     return 0;
   }

   void setUpFileDescriptorSet(SocketWaiter* read_fds) {
     // add comm channel to the set
     read_fds->update(notification_listen_fd_, SocketWaiter::Event::kEventRead);

     // add watched FDs to the set
     {
       std::unique_lock<std::recursive_mutex> guard(internal_mutex_);
       for (auto& fdp : watched_shared_fds_) {
         read_fds->update(fdp.first, SocketWaiter::Event::kEventRead);
       }
     }
   }

   // check the comm channel and read everything there
   bool consumeThreadNotifications(SocketWaiter* read_fds) {
     if (read_fds->pendingEventsFor(notification_listen_fd_)) {
       char buffer[kNotificationBufferSize];
       while (HANDLE_EINTR(android::base::socketRecv(notification_listen_fd_, buffer, kNotificationBufferSize)) ==
              kNotificationBufferSize) {
       }
       return true;
     }
     return false;
   }

   // check all file descriptors and call callbacks if necesary
   void runAppropriateCallbacks(SocketWaiter* read_fds) {
     // not a good idea to call a callback while holding the FD lock,
     // nor to release the lock while traversing the map
     std::vector<decltype(watched_shared_fds_)::value_type> fds;
     std::unique_lock<std::recursive_mutex> guard(internal_mutex_);
     for (auto& fdc : watched_shared_fds_) {
       auto pending = read_fds->pendingEventsFor(fdc.first);
       if (pending == SocketWaiter::kEventRead) {
         fds.push_back(fdc);
       }
     }

     for (auto& p : fds) {
       p.second(p.first);
     }

   }

   void ThreadRoutine() {
     auto read_fds = std::unique_ptr<SocketWaiter>(SocketWaiter::create());
     while (running_) {
       read_fds->reset();
       setUpFileDescriptorSet(read_fds.get());

       // wait until there is data available to read on some FD
       int retval = read_fds->wait(std::numeric_limits<int64_t>::max());
       if (retval <= 0) {  // there was some error or a timeout
         derror(
             "%s: There was an error while waiting for data on the file "
             "descriptors: %s",
             __func__, strerror(errno));
         continue;
       }

       consumeThreadNotifications(read_fds.get());

       // Do not read if there was a call to stop running
       if (!running_) {
         break;
       }

       runAppropriateCallbacks(read_fds.get());
     }
   }

   std::atomic_bool running_{false};
   std::thread thread_;
   std::recursive_mutex internal_mutex_;


   //android::base::SocketWaiter socket_waiter_;
   std::map<int, ReadCallback> watched_shared_fds_;

   // A pair of FD to send information to the reading thread
   int notification_listen_fd_{};
   int notification_write_fd_{};
 };

 // Async task manager implementation
 class AsyncManager::AsyncTaskManager {
  public:
   AsyncUserId GetNextUserId() { return lastUserId_++; }

   AsyncTaskId ExecAsync(AsyncUserId user_id, std::chrono::milliseconds delay,
                         const TaskCallback& callback) {
     return scheduleTask(std::make_shared<Task>(
         std::chrono::steady_clock::now() + delay, callback, user_id));
   }

   AsyncTaskId ExecAsyncPeriodically(AsyncUserId user_id,
                                     std::chrono::milliseconds delay,
                                     std::chrono::milliseconds period,
                                     const TaskCallback& callback) {
     return scheduleTask(std::make_shared<Task>(
         std::chrono::steady_clock::now() + delay, period, callback, user_id));
   }

   bool CancelAsyncTask(AsyncTaskId async_task_id) {
     // remove task from queue (and task id association) while holding lock
     std::unique_lock<std::mutex> guard(internal_mutex_);
     return cancel_task_with_lock_held(async_task_id);
   }

   bool CancelAsyncTasksFromUser(AsyncUserId user_id) {
     // remove task from queue (and task id association) while holding lock
     std::unique_lock<std::mutex> guard(internal_mutex_);
     if (tasks_by_user_id_.count(user_id) == 0) {
       return false;
     }
     for (auto task : tasks_by_user_id_[user_id]) {
       cancel_task_with_lock_held(task);
     }
     tasks_by_user_id_.erase(user_id);
     return true;
   }

   void Synchronize(const CriticalCallback& critical) {
     std::unique_lock<std::mutex> guard(synchronization_mutex_);
     critical();
   }

   AsyncTaskManager() = default;
   AsyncTaskManager(const AsyncTaskManager&) = delete;
   AsyncTaskManager& operator=(const AsyncTaskManager&) = delete;

   ~AsyncTaskManager() = default;

   int stopThread() {
     {
       std::unique_lock<std::mutex> guard(internal_mutex_);
       tasks_by_id_.clear();
       task_queue_.clear();
       if (!running_) {
         return 0;
       }
       running_ = false;
       // notify the thread
       internal_cond_var_.notify_one();
     }  // release the lock before joining a thread that is likely waiting for it
     if (std::this_thread::get_id() != thread_.get_id()) {
       thread_.join();
     } else {
       dwarning("%s: Starting thread stop from inside the task thread itself", __func__);
     }
     return 0;
   }

  private:
   // Holds the data for each task
   class Task {
    public:
     Task(std::chrono::steady_clock::time_point time,
          std::chrono::milliseconds period, const TaskCallback& callback,
          AsyncUserId user)
         : time(time),
           periodic(true),
           period(period),
           callback(callback),
           task_id(kInvalidTaskId),
           user_id(user) {}
     Task(std::chrono::steady_clock::time_point time,
          const TaskCallback& callback, AsyncUserId user)
         : time(time),
           periodic(false),
           callback(callback),
           task_id(kInvalidTaskId),
           user_id(user) {}

     // Operators needed to be in a collection
     bool operator<(const Task& another) const {
       return std::make_pair(time, task_id) < std::make_pair(another.time, another.task_id);
     }

     bool isPeriodic() const {
       return periodic;
     }

     // These fields should no longer be public if the class ever becomes
     // public or gets more complex
     std::chrono::steady_clock::time_point time;
     bool periodic;
     std::chrono::milliseconds period{};
     std::mutex in_callback; // Taken when the callback is active
     TaskCallback callback;
     AsyncTaskId task_id;
     AsyncUserId user_id;
   };

   // A comparator class to put shared pointers to tasks in an ordered set
   struct task_p_comparator {
     bool operator()(const std::shared_ptr<Task>& t1, const std::shared_ptr<Task>& t2) const {
       return *t1 < *t2;
     }
   };

   bool cancel_task_with_lock_held(AsyncTaskId async_task_id) {
     if (tasks_by_id_.count(async_task_id) == 0) {
       return false;
     }

     // Now make sure we are not running this task.
     // 2 cases
     // - This is called from thread_, this means a scheduled task is actually
     //   unregistering.
     // - Another thread is calling us, let's make sure the task is not active.
     if (thread_.get_id() != std::this_thread::get_id()) {
       auto task = tasks_by_id_[async_task_id];
       const std::lock_guard<std::mutex> lock(task->in_callback);
       task_queue_.erase(task);
       tasks_by_id_.erase(async_task_id);
     } else {
       task_queue_.erase(tasks_by_id_[async_task_id]);
       tasks_by_id_.erase(async_task_id);
     }

     return true;
   }

   AsyncTaskId scheduleTask(const std::shared_ptr<Task>& task) {
     {
       std::unique_lock<std::mutex> guard(internal_mutex_);
       // no more room for new tasks, we need a larger type for IDs
       if (tasks_by_id_.size() == kMaxTaskId)  // TODO potentially type unsafe
         return kInvalidTaskId;
       do {
         lastTaskId_ = NextAsyncTaskId(lastTaskId_);
       } while (isTaskIdInUse(lastTaskId_));
       task->task_id = lastTaskId_;
       // add task to the queue and map
       tasks_by_id_[lastTaskId_] = task;
       tasks_by_user_id_[task->user_id].insert(task->task_id);
       task_queue_.insert(task);
     }
     // start thread if necessary
     int started = tryStartThread();
     if (started != 0) {
       derror("%s: Unable to start thread", __func__);
       return kInvalidTaskId;
     }
     // notify the thread so that it knows of the new task
     internal_cond_var_.notify_one();
     // return task id
     return task->task_id;
   }

   bool isTaskIdInUse(const AsyncTaskId& task_id) const {
     return tasks_by_id_.count(task_id) != 0;
   }

   int tryStartThread() {
     // need the lock because of the running flag and the cond var
     std::unique_lock<std::mutex> guard(internal_mutex_);
     // check that the thread is not yet running
     if (running_) {
       return 0;
     }
     // start the thread
     running_ = true;
     thread_ = std::thread([this]() { ThreadRoutine(); });
     if (!thread_.joinable()) {
       derror("%s: Unable to start task thread", __func__);
       return -1;
     }
     return 0;
   }

   void ThreadRoutine() {
     while (running_) {
       TaskCallback callback;
       std::shared_ptr<Task> task_p;
       bool run_it = false;
       {
         std::unique_lock<std::mutex> guard(internal_mutex_);
         if (!task_queue_.empty()) {
           task_p = *(task_queue_.begin());
           if (task_p->time < std::chrono::steady_clock::now()) {
             run_it = true;
             callback = task_p->callback;
             task_queue_.erase(task_p);  // need to remove and add again if
                                         // periodic to update order
             if (task_p->isPeriodic()) {
               task_p->time += task_p->period;
               task_queue_.insert(task_p);
             } else {
               tasks_by_user_id_[task_p->user_id].erase(task_p->task_id);
               tasks_by_id_.erase(task_p->task_id);
             }
           }
         }
       }
       if (run_it) {
         const std::lock_guard<std::mutex> lock(task_p->in_callback);
         Synchronize(callback);
       }
       {
         std::unique_lock<std::mutex> guard(internal_mutex_);
         // check for termination right before waiting
         if (!running_) break;
         // wait until time for the next task (if any)
         if (task_queue_.size() > 0) {
           // Make a copy of the time_point because wait_until takes a reference
           // to it and may read it after waiting, by which time the task may
           // have been freed (e.g. via CancelAsyncTask).
           std::chrono::steady_clock::time_point time =
               (*task_queue_.begin())->time;
           internal_cond_var_.wait_until(guard, time);
         } else {
           internal_cond_var_.wait(guard);
         }
       }
     }
   }

   bool running_ = false;
   std::thread thread_;
   std::mutex internal_mutex_;
   std::mutex synchronization_mutex_;
   std::condition_variable internal_cond_var_;

   AsyncTaskId lastTaskId_ = kInvalidTaskId;
   AsyncUserId lastUserId_{1};
   std::map<AsyncTaskId, std::shared_ptr<Task> > tasks_by_id_;
   std::map<AsyncUserId, std::set<AsyncTaskId>> tasks_by_user_id_;
   std::set<std::shared_ptr<Task>, task_p_comparator> task_queue_;
 };

 // Async Manager Implementation:
 AsyncManager::AsyncManager() : fdWatcher_p_(new AsyncFdWatcher()), taskManager_p_(new AsyncTaskManager()) {}

 AsyncManager::~AsyncManager() {
   // Make sure the threads are stopped before destroying the object.
   // The threads need to be stopped here and not in each internal class'
   // destructor because unique_ptr's reset() first assigns nullptr to the
   // pointer and only then calls the destructor, so any callback running
   // on these threads would dereference a null pointer if they called a member
   // function of this class.
   fdWatcher_p_->stopThread();
   taskManager_p_->stopThread();
 }

 int AsyncManager::WatchFdForNonBlockingReads(int file_descriptor, const ReadCallback& on_read_fd_ready_callback) {
   return fdWatcher_p_->WatchFdForNonBlockingReads(file_descriptor, on_read_fd_ready_callback);
 }

 void AsyncManager::StopWatchingFileDescriptor(int file_descriptor) {
   fdWatcher_p_->StopWatchingFileDescriptor(file_descriptor);
 }

 AsyncUserId AsyncManager::GetNextUserId() {
   return taskManager_p_->GetNextUserId();
 }

 AsyncTaskId AsyncManager::ExecAsync(AsyncUserId user_id,
                                     std::chrono::milliseconds delay,
                                     const TaskCallback& callback) {
   return taskManager_p_->ExecAsync(user_id, delay, callback);
 }

 AsyncTaskId AsyncManager::ExecAsyncPeriodically(
     AsyncUserId user_id, std::chrono::milliseconds delay,
     std::chrono::milliseconds period, const TaskCallback& callback) {
   return taskManager_p_->ExecAsyncPeriodically(user_id, delay, period,
                                                callback);
 }

 bool AsyncManager::CancelAsyncTask(AsyncTaskId async_task_id) {
   return taskManager_p_->CancelAsyncTask(async_task_id);
 }

 bool AsyncManager::CancelAsyncTasksFromUser(
     rootcanal::AsyncUserId user_id) {
   return taskManager_p_->CancelAsyncTasksFromUser(user_id);
 }

 void AsyncManager::Synchronize(const CriticalCallback& critical) {
   taskManager_p_->Synchronize(critical);
 }
 }  // namespace rootcanal
	/*
	* Copyright 2016 The Android Open Source Project
	*
	* Licensed under the Apache License, Version 2.0 (the "License");
	* you may not use this file except in compliance with the License.
	* You may obtain a copy of the License at
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/
	#include "model/setup/async_manager.h" // for AsyncManager

	#include <errno.h> // for errno
	#include <atomic> // for atomic_bool, atomic_e...
	#include <condition_variable> // for condition_variable
	#include <cstring> // for strerror
	#include <limits> // for numeric_limits
	#include <map> // for map<>::value_type, map
	#include <mutex> // for unique_lock, mutex
	#include <ratio> // for ratio
	#include <set> // for set
	#include <thread> // for thread
	#include <type_traits> // for remove_extent_t
	#include <utility> // for pair, make_pair, oper...
	#include <vector> // for vector

	#include "aemu/base/EintrWrapper.h" // for HANDLE_EINTR
	#include "aemu/base/Log.h" // for LogStreamVoidify, Log...
	#include "aemu/base/sockets/SocketUtils.h" // for socketRecv, socketSet...
	#include "aemu/base/sockets/SocketWaiter.h" // for SocketWaiter, SocketW...
	#include "aemu/base/logging/CLog.h"

	namespace rootcanal {
	// Implementation of AsyncManager is divided between two classes, three if
	// AsyncManager itself is taken into account, but its only responsability
	// besides being a proxy for the other two classes is to provide a global
	// synchronization mechanism for callbacks and client code to use.

	// The watching of file descriptors is done through AsyncFdWatcher. Several
	// objects of this class may coexist simultaneosly as they share no state.
	// After construction of this objects nothing happens beyond some very simple
	// member initialization. When the first FD is set up for watching the object
	// starts a new thread which watches the given (and later provided) FDs using
	// select() inside a loop. A special FD (a pipe) is also watched which is
	// used to notify the thread of internal changes on the object state (like
	// the addition of new FDs to watch on). Every access to internal state is
	// synchronized using a single internal mutex. The thread is only stopped on
	// destruction of the object, by modifying a flag, which is the only member
	// variable accessed without acquiring the lock (because the notification to
	// the thread is done later by writing to a pipe which means the thread will
	// be notified regardless of what phase of the loop it is in that moment)

	// The scheduling of asynchronous tasks, periodic or not, is handled by the
	// AsyncTaskManager class. Like the one for FDs, this class shares no internal
	// state between different instances so it is safe to use several objects of
	// this class, also nothing interesting happens upon construction, but only
	// after a Task has been scheduled and access to internal state is synchronized
	// using a single internal mutex. When the first task is scheduled a thread
	// is started which monitors a queue of tasks. The queue is peeked to see
	// when the next task should be carried out and then the thread performs a
	// (absolute) timed wait on a condition variable. The wait ends because of a
	// time out or a notify on the cond var, the former means a task is due
	// for execution while the later means there has been a change in internal
	// state, like a task has been scheduled/canceled or the flag to stop has
	// been set. Setting and querying the stop flag or modifying the task queue
	// and subsequent notification on the cond var is done atomically (e.g while
	// holding the lock on the internal mutex) to ensure that the thread never
	// misses the notification, since notifying a cond var is not persistent as
	// writing on a pipe (if not done this way, the thread could query the
	// stopping flag and be put aside by the OS scheduler right after, then the
	// 'stop thread' procedure could run, setting the flag, notifying a cond
	// var that no one is waiting on and joining the thread, the thread then
	// resumes execution believing that it needs to continue and waits on the
	// cond var possibly forever if there are no tasks scheduled, efectively
	// causing a deadlock).

	// This number also states the maximum number of scheduled tasks we can handle
	// at a given time
	static const uint16_t kMaxTaskId = -1; /* 2^16 - 1, permisible ids are {1..2^16-1}*/
	static inline AsyncTaskId NextAsyncTaskId(const AsyncTaskId id) {
	return (id == kMaxTaskId) ? 1 : id + 1;
	}
	// The buffer is only 10 bytes because the expected number of bytes
	// written on this socket is 1. It is possible that the thread is notified
	// more than once but highly unlikely, so a buffer of size 10 seems enough
	// and the reads are performed inside a while just in case it isn't. From
	// the thread routine's point of view it is the same to have been notified
	// just once or 100 times so it just tries to consume the entire buffer.
	// In the cases where an interrupt would cause read to return without
	// having read everything that was available a new iteration of the thread
	// loop will bring execution to this point almost immediately, so there is
	// no need to treat that case.
	static const int kNotificationBufferSize = 10;


	using android::base::SocketWaiter;

	// Async File Descriptor Watcher Implementation:
	class AsyncManager::AsyncFdWatcher {

	public:
	int WatchFdForNonBlockingReads(int file_descriptor, const ReadCallback& on_read_fd_ready_callback) {
	// add file descriptor and callback
	{
	std::unique_lock<std::recursive_mutex> guard(internal_mutex_);
	watched_shared_fds_[file_descriptor] = on_read_fd_ready_callback;
	}

	// start the thread if not started yet
	int started = tryStartThread();
	if (started != 0) {
	derror("%s: Unable to start thread", __func__);
	return started;
	}

	// notify the thread so that it knows of the new FD
	notifyThread();

	return 0;
	}

	void StopWatchingFileDescriptor(int file_descriptor) {
	std::unique_lock<std::recursive_mutex> guard(internal_mutex_);
	watched_shared_fds_.erase(file_descriptor);
	}

	AsyncFdWatcher() = default;
	AsyncFdWatcher(const AsyncFdWatcher&) = delete;
	AsyncFdWatcher& operator=(const AsyncFdWatcher&) = delete;

	~AsyncFdWatcher() = default;

	int stopThread() {
	if (!std::atomic_exchange(&running_, false)) {
	return 0; // if not running already
	}

	notifyThread();

	if (std::this_thread::get_id() != thread_.get_id()) {
	thread_.join();
	} else {
	dwarning("%s: Starting thread stop from inside the reading thread itself", __func__);
	}

	{
	std::unique_lock<std::recursive_mutex> guard(internal_mutex_);
	watched_shared_fds_.clear();
	}

	return 0;
	}

	private:
	// Make sure to call this with at least one file descriptor ready to be
	// watched upon or the thread routine will return immediately
	int tryStartThread() {
	if (std::atomic_exchange(&running_, true)) {
	return 0; // if already running
	}
	// set up the communication channel
	if (android::base::socketCreatePair(&notification_listen_fd_, &notification_write_fd_)) {
	derror(
	"%s:Unable to establish a communication channel to the reading "
	"thread",
	__func__);
	return -1;
	}
	android::base::socketSetNonBlocking(notification_listen_fd_);
	android::base::socketSetNonBlocking(notification_write_fd_);

	thread_ = std::thread([this]() { ThreadRoutine(); });
	if (!thread_.joinable()) {
	derror("%s: Unable to start reading thread", __func__);
	return -1;
	}
	return 0;
	}

	int notifyThread() {
	char buffer = '0';
	if (android::base::socketSend(notification_write_fd_, &buffer, 1) < 0) {
	derror("%s: Unable to send message to reading thread", __func__);
	return -1;
	}
	return 0;
	}

	void setUpFileDescriptorSet(SocketWaiter* read_fds) {
	// add comm channel to the set
	read_fds->update(notification_listen_fd_, SocketWaiter::Event::kEventRead);

	// add watched FDs to the set
	{
	std::unique_lock<std::recursive_mutex> guard(internal_mutex_);
	for (auto& fdp : watched_shared_fds_) {
	read_fds->update(fdp.first, SocketWaiter::Event::kEventRead);
	}
	}
	}

	// check the comm channel and read everything there
	bool consumeThreadNotifications(SocketWaiter* read_fds) {
	if (read_fds->pendingEventsFor(notification_listen_fd_)) {
	char buffer[kNotificationBufferSize];
	while (HANDLE_EINTR(android::base::socketRecv(notification_listen_fd_, buffer, kNotificationBufferSize)) ==
	kNotificationBufferSize) {
	}
	return true;
	}
	return false;
	}

	// check all file descriptors and call callbacks if necesary
	void runAppropriateCallbacks(SocketWaiter* read_fds) {
	// not a good idea to call a callback while holding the FD lock,
	// nor to release the lock while traversing the map
	std::vector<decltype(watched_shared_fds_)::value_type> fds;
	std::unique_lock<std::recursive_mutex> guard(internal_mutex_);
	for (auto& fdc : watched_shared_fds_) {
	auto pending = read_fds->pendingEventsFor(fdc.first);
	if (pending == SocketWaiter::kEventRead) {
	fds.push_back(fdc);
	}
	}

	for (auto& p : fds) {
	p.second(p.first);
	}

	}

	void ThreadRoutine() {
	auto read_fds = std::unique_ptr<SocketWaiter>(SocketWaiter::create());
	while (running_) {
	read_fds->reset();
	setUpFileDescriptorSet(read_fds.get());

	// wait until there is data available to read on some FD
	int retval = read_fds->wait(std::numeric_limits<int64_t>::max());
	if (retval <= 0) { // there was some error or a timeout
	derror(
	"%s: There was an error while waiting for data on the file "
	"descriptors: %s",
	__func__, strerror(errno));
	continue;
	}

	consumeThreadNotifications(read_fds.get());

	// Do not read if there was a call to stop running
	if (!running_) {
	break;
	}

	runAppropriateCallbacks(read_fds.get());
	}
	}

	std::atomic_bool running_{false};
	std::thread thread_;
	std::recursive_mutex internal_mutex_;


	//android::base::SocketWaiter socket_waiter_;
	std::map<int, ReadCallback> watched_shared_fds_;

	// A pair of FD to send information to the reading thread
	int notification_listen_fd_{};
	int notification_write_fd_{};
	};

	// Async task manager implementation
	class AsyncManager::AsyncTaskManager {
	public:
	AsyncUserId GetNextUserId() { return lastUserId_++; }

	AsyncTaskId ExecAsync(AsyncUserId user_id, std::chrono::milliseconds delay,
	const TaskCallback& callback) {
	return scheduleTask(std::make_shared<Task>(
	std::chrono::steady_clock::now() + delay, callback, user_id));
	}

	AsyncTaskId ExecAsyncPeriodically(AsyncUserId user_id,
	std::chrono::milliseconds delay,
	std::chrono::milliseconds period,
	const TaskCallback& callback) {
	return scheduleTask(std::make_shared<Task>(
	std::chrono::steady_clock::now() + delay, period, callback, user_id));
	}

	bool CancelAsyncTask(AsyncTaskId async_task_id) {
	// remove task from queue (and task id association) while holding lock
	std::unique_lock<std::mutex> guard(internal_mutex_);
	return cancel_task_with_lock_held(async_task_id);
	}

	bool CancelAsyncTasksFromUser(AsyncUserId user_id) {
	// remove task from queue (and task id association) while holding lock
	std::unique_lock<std::mutex> guard(internal_mutex_);
	if (tasks_by_user_id_.count(user_id) == 0) {
	return false;
	}
	for (auto task : tasks_by_user_id_[user_id]) {
	cancel_task_with_lock_held(task);
	}
	tasks_by_user_id_.erase(user_id);
	return true;
	}

	void Synchronize(const CriticalCallback& critical) {
	std::unique_lock<std::mutex> guard(synchronization_mutex_);
	critical();
	}

	AsyncTaskManager() = default;
	AsyncTaskManager(const AsyncTaskManager&) = delete;
	AsyncTaskManager& operator=(const AsyncTaskManager&) = delete;

	~AsyncTaskManager() = default;

	int stopThread() {
	{
	std::unique_lock<std::mutex> guard(internal_mutex_);
	tasks_by_id_.clear();
	task_queue_.clear();
	if (!running_) {
	return 0;
	}
	running_ = false;
	// notify the thread
	internal_cond_var_.notify_one();
	} // release the lock before joining a thread that is likely waiting for it
	if (std::this_thread::get_id() != thread_.get_id()) {
	thread_.join();
	} else {
	dwarning("%s: Starting thread stop from inside the task thread itself", __func__);
	}
	return 0;
	}

	private:
	// Holds the data for each task
	class Task {
	public:
	Task(std::chrono::steady_clock::time_point time,
	std::chrono::milliseconds period, const TaskCallback& callback,
	AsyncUserId user)
	: time(time),
	periodic(true),
	period(period),
	callback(callback),
	task_id(kInvalidTaskId),
	user_id(user) {}
	Task(std::chrono::steady_clock::time_point time,
	const TaskCallback& callback, AsyncUserId user)
	: time(time),
	periodic(false),
	callback(callback),
	task_id(kInvalidTaskId),
	user_id(user) {}

	// Operators needed to be in a collection
	bool operator<(const Task& another) const {
	return std::make_pair(time, task_id) < std::make_pair(another.time, another.task_id);
	}

	bool isPeriodic() const {
	return periodic;
	}

	// These fields should no longer be public if the class ever becomes
	// public or gets more complex
	std::chrono::steady_clock::time_point time;
	bool periodic;
	std::chrono::milliseconds period{};
	std::mutex in_callback; // Taken when the callback is active
	TaskCallback callback;
	AsyncTaskId task_id;
	AsyncUserId user_id;
	};

	// A comparator class to put shared pointers to tasks in an ordered set
	struct task_p_comparator {
	bool operator()(const std::shared_ptr<Task>& t1, const std::shared_ptr<Task>& t2) const {
	return t1 < t2;
	}
	};

	bool cancel_task_with_lock_held(AsyncTaskId async_task_id) {
	if (tasks_by_id_.count(async_task_id) == 0) {
	return false;
	}

	// Now make sure we are not running this task.
	// 2 cases
	// - This is called from thread_, this means a scheduled task is actually
	// unregistering.
	// - Another thread is calling us, let's make sure the task is not active.
	if (thread_.get_id() != std::this_thread::get_id()) {
	auto task = tasks_by_id_[async_task_id];
	const std::lock_guard<std::mutex> lock(task->in_callback);
	task_queue_.erase(task);
	tasks_by_id_.erase(async_task_id);
	} else {
	task_queue_.erase(tasks_by_id_[async_task_id]);
	tasks_by_id_.erase(async_task_id);
	}

	return true;
	}

	AsyncTaskId scheduleTask(const std::shared_ptr<Task>& task) {
	{
	std::unique_lock<std::mutex> guard(internal_mutex_);
	// no more room for new tasks, we need a larger type for IDs
	if (tasks_by_id_.size() == kMaxTaskId) // TODO potentially type unsafe
	return kInvalidTaskId;
	do {
	lastTaskId_ = NextAsyncTaskId(lastTaskId_);
	} while (isTaskIdInUse(lastTaskId_));
	task->task_id = lastTaskId_;
	// add task to the queue and map
	tasks_by_id_[lastTaskId_] = task;
	tasks_by_user_id_[task->user_id].insert(task->task_id);
	task_queue_.insert(task);
	}
	// start thread if necessary
	int started = tryStartThread();
	if (started != 0) {
	derror("%s: Unable to start thread", __func__);
	return kInvalidTaskId;
	}
	// notify the thread so that it knows of the new task
	internal_cond_var_.notify_one();
	// return task id
	return task->task_id;
	}

	bool isTaskIdInUse(const AsyncTaskId& task_id) const {
	return tasks_by_id_.count(task_id) != 0;
	}

	int tryStartThread() {
	// need the lock because of the running flag and the cond var
	std::unique_lock<std::mutex> guard(internal_mutex_);
	// check that the thread is not yet running
	if (running_) {
	return 0;
	}
	// start the thread
	running_ = true;
	thread_ = std::thread([this]() { ThreadRoutine(); });
	if (!thread_.joinable()) {
	derror("%s: Unable to start task thread", __func__);
	return -1;
	}
	return 0;
	}

	void ThreadRoutine() {
	while (running_) {
	TaskCallback callback;
	std::shared_ptr<Task> task_p;
	bool run_it = false;
	{
	std::unique_lock<std::mutex> guard(internal_mutex_);
	if (!task_queue_.empty()) {
	task_p = *(task_queue_.begin());
	if (task_p->time < std::chrono::steady_clock::now()) {
	run_it = true;
	callback = task_p->callback;
	task_queue_.erase(task_p); // need to remove and add again if
	// periodic to update order
	if (task_p->isPeriodic()) {
	task_p->time += task_p->period;
	task_queue_.insert(task_p);
	} else {
	tasks_by_user_id_[task_p->user_id].erase(task_p->task_id);
	tasks_by_id_.erase(task_p->task_id);
	}
	}
	}
	}
	if (run_it) {
	const std::lock_guard<std::mutex> lock(task_p->in_callback);
	Synchronize(callback);
	}
	{
	std::unique_lock<std::mutex> guard(internal_mutex_);
	// check for termination right before waiting
	if (!running_) break;
	// wait until time for the next task (if any)
	if (task_queue_.size() > 0) {
	// Make a copy of the time_point because wait_until takes a reference
	// to it and may read it after waiting, by which time the task may
	// have been freed (e.g. via CancelAsyncTask).
	std::chrono::steady_clock::time_point time =
	(*task_queue_.begin())->time;
	internal_cond_var_.wait_until(guard, time);
	} else {
	internal_cond_var_.wait(guard);
	}
	}
	}
	}

	bool running_ = false;
	std::thread thread_;
	std::mutex internal_mutex_;
	std::mutex synchronization_mutex_;
	std::condition_variable internal_cond_var_;

	AsyncTaskId lastTaskId_ = kInvalidTaskId;
	AsyncUserId lastUserId_{1};
	std::map<AsyncTaskId, std::shared_ptr<Task> > tasks_by_id_;
	std::map<AsyncUserId, std::set<AsyncTaskId>> tasks_by_user_id_;
	std::set<std::shared_ptr<Task>, task_p_comparator> task_queue_;
	};

	// Async Manager Implementation:
	AsyncManager::AsyncManager() : fdWatcher_p_(new AsyncFdWatcher()), taskManager_p_(new AsyncTaskManager()) {}

	AsyncManager::~AsyncManager() {
	// Make sure the threads are stopped before destroying the object.
	// The threads need to be stopped here and not in each internal class'
	// destructor because unique_ptr's reset() first assigns nullptr to the
	// pointer and only then calls the destructor, so any callback running
	// on these threads would dereference a null pointer if they called a member
	// function of this class.
	fdWatcher_p_->stopThread();
	taskManager_p_->stopThread();
	}

	int AsyncManager::WatchFdForNonBlockingReads(int file_descriptor, const ReadCallback& on_read_fd_ready_callback) {
	return fdWatcher_p_->WatchFdForNonBlockingReads(file_descriptor, on_read_fd_ready_callback);
	}

	void AsyncManager::StopWatchingFileDescriptor(int file_descriptor) {
	fdWatcher_p_->StopWatchingFileDescriptor(file_descriptor);
	}

	AsyncUserId AsyncManager::GetNextUserId() {
	return taskManager_p_->GetNextUserId();
	}

	AsyncTaskId AsyncManager::ExecAsync(AsyncUserId user_id,
	std::chrono::milliseconds delay,
	const TaskCallback& callback) {
	return taskManager_p_->ExecAsync(user_id, delay, callback);
	}

	AsyncTaskId AsyncManager::ExecAsyncPeriodically(
	AsyncUserId user_id, std::chrono::milliseconds delay,
	std::chrono::milliseconds period, const TaskCallback& callback) {
	return taskManager_p_->ExecAsyncPeriodically(user_id, delay, period,
	callback);
	}

	bool AsyncManager::CancelAsyncTask(AsyncTaskId async_task_id) {
	return taskManager_p_->CancelAsyncTask(async_task_id);
	}

	bool AsyncManager::CancelAsyncTasksFromUser(
	rootcanal::AsyncUserId user_id) {
	return taskManager_p_->CancelAsyncTasksFromUser(user_id);
	}

	void AsyncManager::Synchronize(const CriticalCallback& critical) {
	taskManager_p_->Synchronize(critical);
	}
	} // namespace rootcanal