Instantiating boost::beast in dynamic library causes a crash - c++

Question

I'm trying to implement a very simple, local, HTTP server for my C++ application — I'm using XCode on macOS. I have to implement it from within a dynamically loaded library rather than the "main" thread of the program. I decided to try using boost::beast since another part of the application uses boost libraries already. I'm trying to implement this example, but within the context of my library, and not as part its main program.
The host application for this library calls on the following function to start a localhost server, but crashes when instantiating "acceptor":
extern "C" BASICEXTERNALOBJECT_API long startLocalhost(TaggedData* argv, long argc, TaggedData * retval) {
try {
string status;
retval->type = kTypeString;
auto const address = net::ip::make_address("127.0.0.1");
unsigned short port = static_cast<unsigned short>(std::atoi("1337"));
net::io_context ioc{1};
tcp::acceptor acceptor{ioc, {address, port}}; // <-- crashes on this line
tcp::socket socket{ioc};
http_server(acceptor, socket);
ioc.run();
status = "{'status':'ok', 'message':'localhost server started!'}";
retval->data.string = getNewBuffer(status);
}
catch(std::exception const& e)
{
string status;
//err_msg = "Error: " << e.what() << std::endl;
status = "{'status':'fail', 'message':'Error starting web server'}";
retval->data.string = getNewBuffer(status);
}
return kESErrOK;
}
When stepping through the code, I see that XCode reports an error when the line with tcp::acceptor ... is executed:
Thread 1: EXC_BAD_ACCESS (code=1, address=0x783c0a3e3f22650c)
and is highlighted at the single line of code in a function in scheduler.h:
//Get the concurrency hint that was used to initialize the scheduler.
int concurrency_hint() const
{
return concurrency_hint_; //XCode halts here
}
I'm debating as to whether or not I should include a different C++ web server, like Drogon, instead of boost::beast, but I thought I would post here to see if anybody had any insight as to why the crash is happening in this case.

Answer 1

Update
I found a fix that is a workaround for my particular circumstances, hopefully it can help others running into this issue.
The address to the service_registry::create static factory method resolves correctly when I add ASIO_DECL in front of the methods declaration in asio/detail/service_registry.hpp.
It should look like this:
// Factory function for creating a service instance.
template <typename Service, typename Owner>
ASIO_DECL static execution_context::service* create(void* owner);
By adding ASIO_DECL in front of it, it resolves correctly and the scheduler and kqueue_reactor objects initialize properly avoiding the bad access to concurrency_hint().
In my case I am trying to use non-Boost ASIO inside of a VST3 audio plug-in running in Ableton Live 11 on macOS on an M1 processor. Using the VST3 plug-in in I'm getting this same crash. Using the same plug-in in other DAW applications, such as Reaper, does not cause the crash. It also does not occur for Ableton Live 11 on Windows.
I've got it narrowed down to the following issue:
In asio/detail/impl/service_registry.hpp the following method attempts to return a function pointer address to a create/factory method.
template <typename Service>
Service& service_registry::use_service(io_context& owner)
{
execution_context::service::key key;
init_key<Service>(key, 0);
factory_type factory = &service_registry::create<Service, io_context>;
return *static_cast<Service*>(do_use_service(key, factory, &owner));
}
Specifically, this line: factory_type factory = &service_registry::create<Service, io_context>;
When debugging in Xcode, in the hosts that work, when inspecting
factory, it shows the correct address linking to the service_registry::create<Service, io_context> static method.
However, in Ableton Live 11, it doesn't point to anything - somehow the address to the static method does not resolve correctly. This causes a cascade of issues, ultimately leading up to trying to invoke the factory function pointer in asio/asio/detail/impl/service_registry.ipp in the method service_registry::do_use_service. Since it doesn't point to a proper create method, nothing is created, it results in uninitialized objects, including the scheduler instance.
Therefore, when calling scheduler_.concurrency_hint() in kqueue_reactor.ipp the scheduler is uninitialized, and the EXC_BAD_ACCESS error results.
It's unclear to me why under some host processes, dynamically loading the plug-in cannot resolve the static method address, but others have no problem. In my case I compiled asio.hpp for standalone ASIO into the plug-in directly, there was no linking.

Answer 2

The best guesses I can come up with are
maybe your http_server might start additional threads or even fork. This might cause io_context and friends to be accessed after startLocalhost returned. To explain the crash location appearing to be at the indicated line, I could add the heuristic that something is already off during the destructor for ioc
the only other idea I have is that actually the opening/binding of the acceptor throws, but due to possible incompatibilities of types in the shared module vs the main program, the exception thrown is not actually caught and causes abnormal termination. This might happen more easily if the main program also uses Boost libraries, but a different copy (build/version) of them.
In this case there's a simple thing you can do: split up initialization and use the overloads that take error_code to instead use them.