The following example completes with no assertions:
#include <cassert>
#include <functional>
#include <future>
#include <thread>
#include <boost/asio.hpp>
class example1
{
public:
typedef boost::asio::io_context io_context;
typedef boost::asio::io_context::executor_type executor_type;
typedef boost::asio::strand<executor_type> strand;
typedef boost::asio::executor_work_guard<executor_type> work_guard;
typedef std::function<void()> handler;
example1()
: work_(boost::asio::make_work_guard(context_)),
thread_([this]() { context_.run(); }),
strand1_(context_.get_executor()),
strand2_(context_.get_executor())
{
}
~example1()
{
assert(result_.get_future().get());
work_.reset();
thread_.join();
}
void invoke()
{
handler handle = boost::asio::bind_executor(strand2_,
std::bind(&example1::strand2_handler, this));
boost::asio::post(strand1_,
std::bind(&example1::strand1_handler, this, handle));
}
void strand1_handler(handler handle)
{
assert(strand1_.running_in_this_thread());
handle();
}
void strand2_handler()
{
assert(strand1_.running_in_this_thread());
////assert(strand2_.running_in_this_thread());
result_.set_value(true);
}
private:
io_context context_;
work_guard work_;
std::thread thread_;
strand strand1_;
strand strand2_;
std::promise<bool> result_;
};
int main()
{
example test{};
test.invoke();
}
However my expectation is that the commented-out assertion should succeed, as opposed to the one directly above it. According to strand::running_in_this_thread()
the handler handle
has been invoked in the caller's strand, not that provided to bind_executor
.
I can work around this using intermediate methods, as follows.
class example2
{
public:
typedef boost::asio::io_context io_context;
typedef boost::asio::io_context::executor_type executor_type;
typedef boost::asio::strand<executor_type> strand;
typedef boost::asio::executor_work_guard<executor_type> work_guard;
typedef std::function<void()> handler;
example2()
: work_(boost::asio::make_work_guard(context_)),
thread_([this]() { context_.run(); }),
strand1_(context_.get_executor()),
strand2_(context_.get_executor())
{
}
~example2()
{
assert(result_.get_future().get());
work_.reset();
thread_.join();
}
void invoke()
{
handler handle =
std::bind(&example2::do_strand2_handler, this);
boost::asio::post(strand1_,
std::bind(&example2::strand1_handler, this, handle));
}
void strand1_handler(handler handle)
{
assert(strand1_.running_in_this_thread());
handle();
}
// Do the job of bind_executor.
void do_strand2_handler()
{
boost::asio::post(strand2_,
std::bind(&example2::strand2_handler, this));
}
void strand2_handler()
{
////assert(strand1_.running_in_this_thread());
assert(strand2_.running_in_this_thread());
result_.set_value(true);
}
private:
io_context context_;
work_guard work_;
std::thread thread_;
strand strand1_;
strand strand2_;
std::promise<bool> result_;
};
int main()
{
example2 test2{};
test2.invoke();
}
But avoiding that is presumably the purpose of
bind_executor
. Is this a boost bug or am I missing something? I've tried following this through the boost::asio sources but to no avail.
Update
Thanks to @sehe for a lot of help. The above problem can be resolved in a number of ways, for example:
class example3
{
public:
typedef boost::asio::io_context io_context;
typedef boost::asio::io_context::executor_type executor_type;
typedef boost::asio::strand<executor_type> strand;
typedef boost::asio::executor_work_guard<executor_type> work_guard;
typedef boost::asio::executor_binder<std::function<void()>,
boost::asio::any_io_executor> handler;
example3()
: work_(boost::asio::make_work_guard(context_)),
thread_([this]() { context_.run(); }),
strand1_(context_.get_executor()),
strand2_(context_.get_executor())
{
}
~example3()
{
assert(result_.get_future().get());
work_.reset();
thread_.join();
}
void invoke()
{
auto handle = boost::asio::bind_executor(strand2_,
std::bind(&example3::strand2_handler, this));
boost::asio::post(strand1_,
std::bind(&example3::strand1_handler, this, handle));
}
void strand1_handler(handler handle)
{
assert(strand1_.running_in_this_thread());
boost::asio::dispatch(handle);
}
void strand2_handler()
{
assert(strand2_.running_in_this_thread());
result_.set_value(true);
}
private:
io_context context_;
work_guard work_;
std::thread thread_;
strand strand1_;
strand strand2_;
std::promise<bool> result_;
};
int main
{
example3 test3{};
test3.invoke();
}
CodePudding user response:
Yes, you're missing something indeed. Two things, actually.
Type Erasure
Binding an executor doesn't modify the function, it modifies its type.
However, by erasing the callable's type using std::function<>
you've hidden the bound executor. You could easily determine this:
erased_handler handle = bind_executor(s2, s2_handler);
assert(asio::get_associated_executor(handle, s1) == s1);
The problem is gone when you preserve the type:
auto handle = bind_executor(s2, s2_handler);
assert(asio::get_associated_executor(handle, s1) == s2);
Dispatch (formerly handler_invoke
)
Invoking handle
straight up calls it according to the C language semantics, as you have found out.
To ask Asio to honour the potentially bound executor, you could use dispatch
(or post
):
auto s1_handler = [&](auto chain) {
assert(s1.running_in_this_thread());
dispatch(get_associated_executor(chain, s1), chain);
};
In fact, if you're sure that chain
will have an associated executor, you could accept the default fallback (which is a system executor):
auto s1_handler = [&](auto chain) {
assert(s1.running_in_this_thread());
dispatch(chain);
};
Putting It All Together
Demonstrating the wisdom in a simplified, extended tester:
#include <boost/asio.hpp>
#include <functional>
#include <iostream>
namespace asio = boost::asio;
int main() {
asio::thread_pool io(1);
auto s1 = make_strand(io), s2 = make_strand(io);
assert(s1 != s2); // implementation defined! strands may hash equal
auto s1_handler = [&](auto chain) {
assert(s1.running_in_this_thread());
// immediate invocation runs on the current strand:
chain();
// dispatch *might* invoke directly if already on the right strand
dispatch(chain); // 1
dispatch(get_associated_executor(chain, s1), chain); // 2
// posting never immediately invokes, even if already on the right
// strand
post(chain); // 3
post(get_associated_executor(chain, s1), chain); // 4
};
int count_chain_invocations = 0;
auto s2_handler = [&] {
if (s2.running_in_this_thread()) {
count_chain_invocations = 1;
} else {
std::cout << "(note: direct C call ends up on wrong strand)\n";
}
};
{
using erased_handler = std::function<void()>;
erased_handler handle = bind_executor(s2, s2_handler);
assert(asio::get_associated_executor(handle, s1) == s1);
}
{
auto handle = bind_executor(s2, s2_handler);
assert(asio::get_associated_executor(handle, s1) == s2);
}
auto handle = bind_executor(s2, s2_handler);
post(s1, std::bind(s1_handler, handle));
io.join();
std::cout << "count_chain_invocations: " << count_chain_invocations << "\n";
}
All the assertions pass, and the output is as expected:
(note: direct C call ends up on wrong strand)
count_chain_invocations: 4
BONUS: What If You Need Type-Erased Bound Calleables?
Whatever you do, don't use std::function
. You can wrap one, though;
template <typename Sig> struct ErasedHandler {
using executor_type = asio::any_io_executor;
std::function<Sig> _erased;
executor_type _ex;
executor_type get_executor() const { return _ex; }
template <typename F>
explicit ErasedHandler(F&& f)
: _erased(std::forward<F>(f))
, _ex(asio::get_associated_executor(f)) {}
ErasedHandler() = default;
template <typename... Args>
decltype(auto) operator()(Args&&... args) const {
return _erased(std::forward<Args>(args)...);
}
template <typename... Args>
decltype(auto) operator()(Args&&... args) {
return _erased(std::forward<Args>(args)...);
}
explicit operator bool() const { return _erased; }
};
See it Live On Coliru
Before you do, note that
- using
any_io_executor
also type erases the executor, which potentially hurts performance - it does not provide a good fallback, just using the system executor for unbound calleables. You could get around this by detecting it and requiring an explicit constructor arugment etc. but...
- all of this still completely ignores other handler attributes like associated allocator
I would probably avoid generically storing type-erased chainable handlers. You can most often store the actual type of the handler deduced by template type parameter.
PS: Afterthoughts
What you were perhaps expecting was this behaviour:
template <typename... Args>
decltype(auto) operator()(Args&&... args) const {
// CAUTION: NOT WHAT YOU WANT
boost::asio::dispatch(_ex,
std::bind(_erased, std::forward<Args>(args)...));
}
template <typename... Args>
decltype(auto) operator()(Args&&... args) {
// CAUTION: NOT WHAT YOU WANT
boost::asio::dispatch(_ex,
std::bind(_erased, std::forward<Args>(args)...));
}
See that Live On Coliru
Under this regimen even direct C calls will "do the right thing".
That seems nice. Until you think about it.
The issue is that handlers cannot be rebound this way. More specifically, if you had a handler that is associated with a "free-threaded" executor, doing bind_executor(strand, f)
would have no effect (except slowing down your program), as the f
would be obnoxiously dispatching to another executor anyways.
So don't do that :)