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boost::asio::socket thread safety

由 淺唱寂寞╮ 提交于 2019-11-28 17:06:15

After restudying the code for async_write I am now convinced that any write operation is thread safe if and only if the packet size is smaller than 

default_max_transfer_size = 65536;

What happens is that as soon as an async_write is called an async_write_some is called in the same thread. Any threads in the pool calling some form of io_service::run will keep on calling async_write_some for that write operation until it completes.

These async_write_some calls can and will interleave if it has to be called more than once (the packets are larger than 65536).

ASIO does not queue writes to a socket as you would expect, one finishing after the other. In order to ensure both thread and interleave safe writes consider the following piece of code:

    void my_connection::async_serialized_write(
            boost::shared_ptr<transmission> outpacket) {
        m_tx_mutex.lock();
        bool in_progress = !m_pending_transmissions.empty();
        m_pending_transmissions.push(outpacket);
        if (!in_progress) {
            if (m_pending_transmissions.front()->scatter_buffers.size() > 0) {
                boost::asio::async_write(m_socket,
                    m_pending_transmissions.front()->scatter_buffers,
                        boost::asio::transfer_all(),
            boost::bind(&my_connection::handle_async_serialized_write,
                        shared_from_this(),
                        boost::asio::placeholders::error,
                                       boost::asio::placeholders::bytes_transferred));
            } else { // Send single buffer
                boost::asio::async_write(m_socket,
                                    boost::asio::buffer(
                                           m_pending_transmissions.front()->buffer_references.front(),                          m_pending_transmissions.front()->num_bytes_left),
                boost::asio::transfer_all(),
                boost::bind(
                        &my_connection::handle_async_serialized_write,
                        shared_from_this(),
                        boost::asio::placeholders::error,
                        boost::asio::placeholders::bytes_transferred));
            }
        }
        m_tx_mutex.unlock();
    }

    void my_connection::handle_async_serialized_write(
    const boost::system::error_code& e, size_t bytes_transferred) {
        if (!e) {
            boost::shared_ptr<transmission> transmission;
            m_tx_mutex.lock();
            transmission = m_pending_transmissions.front();
            m_pending_transmissions.pop();
            if (!m_pending_transmissions.empty()) {
                if (m_pending_transmissions.front()->scatter_buffers.size() > 0) {
            boost::asio::async_write(m_socket,
                    m_pending_transmissions.front()->scatter_buffers,
                    boost::asio::transfer_exactly(
                            m_pending_transmissions.front()->num_bytes_left),
                    boost::bind(
                            &chreosis_connection::handle_async_serialized_write,
                            shared_from_this(),
                            boost::asio::placeholders::error,
                            boost::asio::placeholders::bytes_transferred));
                } else { // Send single buffer
                    boost::asio::async_write(m_socket,
                    boost::asio::buffer(
                            m_pending_transmissions.front()->buffer_references.front(),
                            m_pending_transmissions.front()->num_bytes_left),
                    boost::asio::transfer_all(),
                    boost::bind(
                            &my_connection::handle_async_serialized_write,
                            shared_from_this(),
                            boost::asio::placeholders::error,
                            boost::asio::placeholders::bytes_transferred));
                }
            }
            m_tx_mutex.unlock();
            transmission->handler(e, bytes_transferred, transmission);
        } else {
            MYLOG_ERROR(
            m_connection_oid.toString() << " " << "handle_async_serialized_write: " << e.message());
            stop(connection_stop_reasons::stop_async_handler_error);
        }
    }

This basically makes a queue for sending one packet at a time. async_write is called only after the first write succeeds which then calls the original handler for the first write.

It would have been easier if asio made write queues automatic per socket/stream.

Use a boost::asio::io_service::strand for asynchronous handlers that are not thread safe.

A strand is defined as a strictly sequential invocation of event handlers (i.e. no concurrent invocation). Use of strands allows execution of code in a multithreaded program without the need for explicit locking (e.g. using mutexes).

The timer tutorial is probably the easiest way to wrap your head around strands.

The key to understanding ASIO is to realize that completion handlers only run in the context of a thread that has called io_service.run() no matter which thread called the asynchronous method. If you've only called io_service.run() in one thread then all completion handlers will execute serially in the context of that thread. If you've called io_service.run() in more than one thread then completion handlers will execute in the context of one of those threads. You can think of this as a thread pool where the threads in the pool are those threads that have called io_service.run() on the same io_service object.

If you have multiple threads call io_service.run() then you can force completion handlers to be serialized by putting them in a strand.

To answer the last part of your question, you should call boost::async_write(). This will dispatch the write operation onto a thread that has called io_service.run() and will invoked the completion handler when the write is done. If you need to serialize this operation then it's a little more complicated and you should read the documentation on strands here.

It sounds like this question boils down to:

what happens when async_write_some() is called simultaneously on a single socket from two different threads

I believe this is exactly the operation that's not thread safe. The order those buffers will go out on the wire is undefined, and they may even be interleaved. Especially if you use the convenience function async_write(), since it's implemented as a series of calls to async_write_some() underneath, until the whole buffer has been sent. In this case each fragment that's sent from the two threads may be interleaved randomly.

The only way to protect you from hitting this case is to build your program to avoid situations like this.

One way to do that is by writing an application layer send buffer which a single thread is responsible for pushing onto the socket. That way you could protect the send buffer itself only. Keep in mind though that a simple std::vector won't work, since adding bytes to the end may end up re-allocating it, possibly while there is an outstanding async_write_some() referencing it. Instead, it's probably a good idea to use a linked list of buffers, and make use of the scatter/gather feature of asio.

Consider first that the socket is a stream and is not internally guarded against concurrent read and/or write. There are three distinct considerations.

  1. Concurrent execution of functions that access the same socket.
  2. Concurrent execution of delegates that enclose the same socket.
  3. Interleaved execution of delegates that write to the same socket.

The chat example is asynchronous but not concurrent. The io_service is run from a single thread, making all chat client operations non-concurrent. In other words, it avoids all of these problems. Even the async_write must internally complete sending all parts of a message before any other work can proceed, avoiding the interleaving problem.

Handlers are invoked only by a thread that is currently calling any overload of run(), run_one(), poll() or poll_one() for the io_service.

By posting work to the single thread io_service other threads can safely avoid both concurrency and blocking by queuing up work in the io_service. If however your scenario precludes you from buffering all work for a given socket, things get more complicated. You may need to block the socket communication (but not threads) as opposed to queuing up work indefinately. Also, the work queue can be very difficult to manage as it's entirely opaque.

If your io_service runs more than one thread you can still easily avoid the above problems, but you can only invoke reads or writes from the handlers of other reads or writes (and at startup). This sequences all access to the socket while remaining non-blocking. The safety arises from the fact that the pattern is using only one thread at any given time. But posting work from an independent thread is problematic - even if you don't mind buffering it.

A strand is an asio class that posts work to an io_service in a way that ensures non-concurrent invocation. However using a strand to invoke async_read and/or async_write solves only the first of the three problems. These functions internally post work to the io_service of the socket. If that service is running multiple threads the work can be exectuted concurrently.

So how do you, for a given socket, safely invoke async_read and/or async_write concurrently?

  1. With concurrent callers the first problem can be resolved with a mutex or a strand, using the former if you don't want to buffer the work and the latter if you do. This protects the socket during the function invocations but does nothing for the other problems.

  2. The second problem seems hardest, because it's difficult to see what's going on inside of the code executing asynchronously from the two functions. The async functions both post work to the io_service of the socket.

From the boost socket source:

/**
 * This constructor creates a stream socket without opening it. The socket
 * needs to be opened and then connected or accepted before data can be sent
 * or received on it.
 *
 * @param io_service The io_service object that the stream socket will use to
 * dispatch handlers for any asynchronous operations performed on the socket.
 */
explicit basic_stream_socket(boost::asio::io_service& io_service)
: basic_socket<Protocol, StreamSocketService>(io_service)
{
}

And from the io_service::run()

/**
 * The run() function blocks until all work has finished and there are no
 * more handlers to be dispatched, or until the io_service has been stopped.
 *
 * Multiple threads may call the run() function to set up a pool of threads
 * from which the io_service may execute handlers. All threads that are
 * waiting in the pool are equivalent and the io_service may choose any one
 * of them to invoke a handler.
 *
 * ...
 */
BOOST_ASIO_DECL std::size_t run();

So if you give a socket multiple threads, it has no choice but to utilize multiple threads - despite not being thread safe. The only way to avoid this problem (apart from replacing the socket implementation) is to give the socket only one thread to work with. For a single socket this is what you want anyway (so don't bother running off to write a replacement).

  1. The third problem can be resolved by using a (different) mutex that is locked before the async_write, passed into the completion handler and unlocked at that point. This will prevent any caller from beginning a write until all parts of the preceding write are complete.

Note that the async_write posts work to a queue - that's how it is able to return almost immediately. If you throw too much work at it you may have to deal with some consequences. Despite using a single io_service thread for the socket, you may have any number of threads posting work via concurrent or non-concurrent calls to async_write.

On the other hand, async_read is straightforward. There is no interleaving problem and you simply loop back from the handler of the previous call. You may or may not want to dispatch the resulting work to another thread or queue, but if you perform it on the completion handler thread you are simply blocking all reads and writes on your single-threaded socket.

UPDATE

I did some more digging into the implementation of the underlying implementation of the socket stream (for one platform). It appears to be the case that the socket consistently executes platform socket calls on the invoking thread, not the delegate posted to the io_service. In other words, despite the fact that async_read and async_write appear to return immediately, they do in fact execute all socket operations before returning. Only the handlers are posted to the io_service. This is neither documented nor exposed by the exaple code I've reviewed, but assuming it is guaranteed behavior, it significantly impacts the second problem above.

Assuming that the work posted to the io_service does not incorporate socket operations, there is no need to limit the io_service to a single thread. It does however reinforce the importance of guarding against concurrent execution of the async functions. So, for example, if one follows the chat example but instead adds another thread to the io_service, there becomes a problem. With async function invocations executing within function handlers, you have concurrent function execution. This would require either a mutex, or all async function invocations to be reposted for execution on a strand.

UPDATE 2

With respect to the third problem (interleaving), if the data size exceeds 65536 bytes, the work is broken up internal to async_write and sent in parts. But it is critical to understand that, if there is more than one thread in the io_service, chunks of work other than the first will be posted to different threads. This all happens internal in the async_write function before your completion handler is called. The implementation creates its own intermediate completion handlers and uses them to execute all but the first socket operation.

This means any guard around the async_write call (mutex or strand) will not protect the socket if there are multiple io_service threads and more than 64kb of data to post (by default, this may possibly vary). Therefore, in this case, the interleave guard is necessary not only for interleave safety, but also thread safety of the socket. I verified all of this in a debugger.

THE MUTEX OPTION

The async_read and async_write functions internally use the io_service in order to obtain threads on which to post completion handlers, blocking until threads are available. This makes them hazardous to guard with mutex locks. When a mutex is used to guard these functions a deadlock will occur when threads back up against the lock, starving the io_service. Given that there is no other way to guard async_write when sending > 64k with a multithread io_service, it effectively locks us into a single thread in that scenario - which of course resolves the concurrency question.

According to Nov. 2008 boost 1.37 asio updates, certain synchronous operations including writes "are now thread safe" allowing "concurrent synchronous operations on an individual socket, if supported by the OS" boost 1.37.0 history. This would seem to support what you are seeing but the oversimplification "Shared objects: Unsafe" clause remains in the boost docs for ip::tcp::socket.

It depends if you access same socket object from several threads. Let's say you have two threads running same io_service::run() function.

If for example you do reading and writing simultaneously or may be perform cancel operation from other thread. Then it is not safe.

However if your protocol does only one operation in a time.

  1. If only one thread runs the io_service run then there is no problem. If you want to execute something on the socket from other thread you may call io_service::post() with handler that does this operation on socket so it would be executed in the same thread.
  2. If you have several threads executing io_service::run and you try to do operations simultaneously - let's say cancel and read operation then you should use strands. There is a tutorial for this in Boost.Asio documentation.

I have been running extensive tests and haven't been able to break asio. Even without locking any mutex.

I would nevertheless advise that you use async_read and async_write with a mutex around each of those calls.

I believe the only draw back is that your completion handlers could be called concurrently if you have more than one thread calling io_service::run.

In my case this has not been an issue. Here is my test code:

#include <boost/thread.hpp>
#include <boost/date_time.hpp>
#include <boost/asio.hpp>
#include <vector>

using namespace std;
char databuffer[256];
vector<boost::asio::const_buffer> scatter_buffer;
boost::mutex my_test_mutex;
void my_test_func(boost::asio::ip::tcp::socket* socket, boost::asio::io_service *io) {
while(1) {
    boost::this_thread::sleep(boost::posix_time::microsec(rand()%1000));

    //my_test_mutex.lock(); // It would be safer 
    socket->async_send(scatter_buffer, boost::bind(&mycallback));
    //my_test_mutex.unlock(); // It would be safer
}
}
int main(int argc, char **argv) {

for(int i = 0; i < 256; ++i)
    databuffer[i] = i;

for(int i = 0; i < 4*90; ++i)
    scatter_buffer.push_back(boost::asio::buffer(databuffer));
boost::asio::io_service my_test_ioservice;
boost::asio::ip::tcp::socket my_test_socket(my_test_ioservice);
boost::asio::ip::tcp::resolver my_test_tcp_resolver(my_test_ioservice);
boost::asio::ip::tcp::resolver::query  my_test_tcp_query("192.168.1.10", "40000");
boost::asio::ip::tcp::resolver::iterator my_test_tcp_iterator = my_test_tcp_resolver.resolve(my_test_tcp_query);
boost::asio::connect(my_test_socket, my_test_tcp_iterator);
for (size_t i = 0; i < 8; ++i) {
    boost::shared_ptr<boost::thread> thread(
            new boost::thread(my_test_func, &my_test_socket, &my_test_ioservice));
}

while(1) {
    my_test_ioservice.run_one();
    boost::this_thread::sleep(boost::posix_time::microsec(rand()%1000));
}
return 0;

}

And here is my makeshift server in python:

import socket
def main():
    mysocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    mysocket.bind((socket.gethostname(), 40000))
    mysocket.listen(1)

    while 1:
        (clientsocket, address) = mysocket.accept()
        print("Connection from: " + str(address))
        i = 0
        count = 0
        while i == ord(clientsocket.recv(1)):
            i += 1
            i %= 256

            count+=1
            if count % 1000 == 0:
                print(count/1000)
        print("Error!")
return 0

if __name__ == '__main__':
    main()

Please note that running this code can cause your computer to thrash.

标签
c++
boost
boost-asio
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