Boost.Asio contrived example inexplicably blocking - c++

Question

I've used Boost.Asio extensively, but I've came across a problem with a unit test that I don't understand. I've reduced the problem down to a very contrived example:
#include <string>
#include <chrono>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <boost/asio.hpp>
#define BOOST_TEST_MODULE My_Module
#define BOOST_TEST_DYN_LINK
#include <boost/test/unit_test.hpp>
#include <boost/test/auto_unit_test.hpp>
using namespace std::string_literals;
using namespace std::chrono_literals;
namespace BA = boost::asio;
namespace BAI = BA::ip;
BOOST_AUTO_TEST_CASE(test)
{
std::mutex m;
std::condition_variable cv;
BA::io_service servicer;
auto io_work = std::make_unique<BA::io_service::work>(servicer);
auto thread = std::thread{[&]() {
servicer.run();
}};
auto received_response = false;
auto server_buf = std::array<std::uint8_t, 4096>{};
auto server_sock = BAI::tcp::socket{servicer};
auto acceptor = BAI::tcp::acceptor{servicer,
BAI::tcp::endpoint{BAI::tcp::v4(), 20123}};
acceptor.async_accept(server_sock, [&](auto&& ec) {
if (ec) {
BOOST_TEST_MESSAGE(ec.message());
}
BOOST_REQUIRE(!ec);
BOOST_TEST_MESSAGE("Accepted connection from " << server_sock.remote_endpoint() <<
", reading...");
BA::async_read(server_sock,
BA::buffer(server_buf),
[&](auto&& ec, auto&& bytes_read){
std::unique_lock<decltype(m)> ul(m);
received_response = true;
if (ec) {
BOOST_TEST_MESSAGE(ec.message());
}
BOOST_REQUIRE(!ec);
const auto str = std::string{server_buf.begin(),
server_buf.begin() + bytes_read};
BOOST_TEST_MESSAGE("Read: " << str);
ul.unlock();
cv.notify_one();
});
});
const auto send_str = "hello"s;
auto client_sock = BAI::tcp::socket{servicer, BAI::tcp::v4()};
client_sock.async_connect(BAI::tcp::endpoint{BAI::tcp::v4(), 20123},
[&](auto&& ec) {
if (ec) {
BOOST_TEST_MESSAGE(ec.message());
}
BOOST_REQUIRE(!ec);
BOOST_TEST_MESSAGE("Connected...");
BA::async_write(client_sock,
BA::buffer(send_str),
[&](auto&& ec, auto&& bytes_written) {
if (ec) {
BOOST_TEST_MESSAGE(ec.message());
}
BOOST_REQUIRE(!ec);
BOOST_TEST_MESSAGE("Written " << bytes_written << " bytes");
});
});
std::unique_lock<decltype(m)> ul(m);
cv.wait_for(ul, 2s, [&](){ return received_response; });
BOOST_CHECK(received_response);
io_work.reset();
servicer.stop();
if (thread.joinable()) {
thread.join();
}
}
which I compile with:
g++ -std=c++17 source.cc -l boost_unit_test_framework -pthread -l boost_system -ggdb
The output is:
Accepted connection from 127.0.0.1:51688, reading...
Connected...
Written 5 bytes
And then it times out.
Running through the debugger shows that the async_read handler is never called. Pausing execution during the phase where it doesn't seem to be doing anything, shows that the main thread is waiting on the condition_variable (cv) and the io_service thread is on an epoll_wait.
I seem to be deadlocking but can't see how.

Answer 1

This is how the function is defined to work, it waits for exactly the number of bytes that the buffer has space for (http://www.boost.org/doc/libs/1_62_0/doc/html/boost_asio/reference/async_read/overload1.html).
Try this one instead: http://www.boost.org/doc/libs/1_62_0/doc/html/boost_asio/reference/async_read/overload2.html
You can give a callback to decide whether the read is complete and that could include waiting for and checking a length provided by another channel once the writer has written its message (if you've determined a deadlock-free way to do that) or just before the message proper.
Adding this completion condition makes it work:
[&](auto&& ec, auto&& bytes_read){
return bytes_read < 5 ? 5 - bytes_read : 0;
},

Answer 2

The answer provided by #codeshot is correct, but it is one of several solutions - which is most appropriate is dependent entirely upon the protocol you're using across the TCP connection.
For example, in a traditional Key-Length-Value style protocol, you would do two reads:
Using boost::asio::async_read (or equivalent) to read into fixed length buffer to obtain the fixed-length header
Use the length specified by the header to create a buffer of the required size, and repeat step 1 using it
There's a good example of this in the chat server example code.
If you were using HTTP or RTSP (the latter is what I was trying to do), then you don't know how much is data is coming, all you care about is receiving a packet's worth of data (I know this is an oversimplification due to the Content-Length header in responses, chunked transfer encoding, etc. but bear with me). For this you need async_read_some (or equivalent), see the HTTP server example.