> in our current HPX driven project we encounter the
> following runtime error:
>
> data has already been set for this future:
> HPX(promise_already_satisfied)
>
> The relevant code is as follows:
>
> void Image::compose ( const Image& other);
> hpx::future<Image> Container::render();
>
> [...]
>
> std::vector<hpx::future<Image>> imageFutures;
> for (int i = 0; i < containers.size(); ++i)
> imageFutures.push_back(containers[i].render());
>
> Image image;
> hpx::lcos::local::spinlock mtx;
>
> hpx::lcos::wait_each([&](hpx::future<Image> img)
> {
> std::lock_guard<hpx::lcos::local::spinlock>
> lk(mtx);
> image.compose(img.get());
> }, imageFutures);
>
> return image;
>
> The runtime error does not occur, when the line
> "image.compose(img.get());" is substituted with just
> "img.get();"
> Note the argument of compose is a const reference.
>
> So what exactly does the error message mean? At no point
> are we explicitly setting or modifying the data of a
> future.
> The error only occurs when actually using the future,
> getting the future without further usage is safe.
>
> The Container Class is a client side representation of a
> Container Component, so the render methods could run in
> parallel and possibly on remote machines.
> The error only occurs if the vector imageFutures has more
> than one element, so only if there are multiple futures to
> wait on / multiple instances of render running.
>
> What can cause this error in general? How to go about
> debugging it and what could be the problem with our
> implementation.
This error is thrown whenever a promise (the other end of a future) has its
value set more than once. I.e. essentially something like:
void set_value(promise<int> p)
{
p.set_value(42);
p.set_value(42); // <-- here
}
void foo()
{
promise<int> p;
future<int> f = p.get_promise();
async(&set_value, std::move(p));
f.get();
}
or similar.
>From what I see from your code, the only explanation would be a bug in
wait_each() which manifests itself under certain timings only. Could you
create a small reproducing use case for use to debug things?
Thanks!
Regards Hartmut
---------------
http://boost-spirit.com
http://stellar.cct.lsu.edu
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