Can't reproduce as of 338138c9d2007e75ebb7f5e97dc5894cb125193d.

A simple example without a value would be:

In T12398#215116, @dvratil wrote:

I think we may even be able to used std::future and std::packaged_task directly, which I would prefer to another custom future/promise.

std::future does not have any state signalling, since it's all, so that might be a bit tricky. We still probably need a Qt-friendly wrapper to implement on top of std::future to provide some signal-slot interface and a FutureWatcher. std::packaged_task is an interesting idea, I will certainly look into it.

The error handler must forward/create the result matching what TASK3 expects.

You cannot create a result out of thin air. If the execution has failed, you simply do not have a result of the previous task.

I think we may even be able to used std::future and std::packaged_task directly, which I would prefer to another custom future/promise.

In T12398#214527, @dvratil wrote:

My idea is following:

1. Introduce Promise and Future template classes, modeled after C++'s std::promise and std::future: The Promise is constructed first by the (a)synchronous task. It only has setter for result value (or error) and a getter to obtain a Future, which can the be returned from the task implementation back to KAsync to wait for a result/error. The Future and the Promise have a shared state (basically a shared pointer to a common private class that holds the result/error. It's possible to have multiple Futures created from the same Promise.
   
   The benefit is that it completely separates the lifetime of the Promise/Future from the Execution and rest of KAsync. This means that we no longer need to pass Future<T> & into every single task that should be asynchronous, cluttering the function's signature. Instead, the function indicates it is an asynchronous task by simply returning Future<T> instead of just T. Another benefit is that this Promise/Future API is more common in modern asynchronous frameworks.

My idea is following:

Thanks that did the trick.

OK, I have reverted the commit in master now. Sink tests pass again.

To be honest I think it makes most sense to revert back to default constructing T for the time being.

There's no easy way how to fix this without reverting back to default-constructing T in Future<T>::Future().

It's not exactly the lifetime- it's a more deep-rooted issue with error handling.

In T12315#214103, @dvratil wrote:

Generally, I'm wondering if I should deprecate this API and introduce a promise-future based API, where, instead of being passed a future from KAsync, the continuation would construct a Promise object internally a return a Future that KAsync would wait for....internally the continuation would own the Promise - it's closer to the common promise-future pattern and solves the lifetime issue, since the Promise is owned by the continuation, rather than by KAsync (which only holds the Future).

In T12315#214101, @dvratil wrote:

Hmm, I believe the bug might be in the Sink code. Here specifically, it's ResourceControl::flush():

The function registers two callbacks, both operating on the future reference. When the resource crashes (happens to me, I don't know if it's part of th etest), then sendFlushCommand fails, so future.setError is called (line 111). However, around the same time a notification is received with information about the crashed resource, invoking the lambda passed to registerHandler function, which then also calls future.setError() (line 98) - depending on which happens first, the future is finished at that point and completes the execution, which may delete the Future object, leaving the other lambda with a dangling reference....

Generally, I'm wondering if I should deprecate this API and introduce a promise-future based API, where, instead of being passed a future from KAsync, the continuation would construct a Promise object internally a return a Future that KAsync would wait for....internally the continuation would own the Promise - it's closer to the common promise-future pattern and solves the lifetime issue, since the Promise is owned by the continuation, rather than by KAsync (which only holds the Future).

I can probably solve the life-time of the future, however you are still calling setError twice on it, which should be undefined behavior - or, following what std::future does, would, well, not throw (because Qt), but might as well assert.

Hmm, I believe the bug might be in the Sink code. Here specifically, it's ResourceControl::flush():

tests/notificationtest in sink will produce a similar crash.

I have rebuilt the flatpak completely and can reproduce the crash. Also, I can reproduce the crash outside of the flatpak again, not sure what I did above.

Ever since I have rebuilt a with -fsanitize=address I can no longer reproduce outside of the flatpak, so there's a chance that the fault is with flatpaks internal build chaching that somehow results in something that crashes (I can't think of a reasonable scenario, but who knows).
I'll try to completely rebuild the flatpak as well and see if this fixes the issue.

Nothing very obvious from the address sanitizer so far, but I also haven't managed to run kube with it yet (only sinksh).

I'll try that. Next week is fine, no hurry.

Looks like some memory issue...could you try compiling with -fsanitize=address? I won't get to look into it properly before some time next week, sorry.

FWIW; I have attempted but failed to reproduce this in a kasync testcase. I can reproduce it by starting latest kube with latest sink and latest kasync, and I can reproduce the crash in both flatpak (which I have now reverted to kasync 0.3.0), and a locally built kube.

I'm building with clang on windows meanwhile, and that works fine.

The guard facility should probably be added on the job level, otherwise subjobs can't use the facility to protect themselves.
Guards could work similarly to the context. Call .guard(QObject*/smart pointer) and be sure that no continuation will be called if the guard is gone.

Or just do .exec(guard);
The guard object could be any qobject/smart pointer/...

Nice!

Fixed in master (see the commit message for details).

I'd like to do a first 0.1 release as soon as the frameworks removal is merged.

Thanks for the patch! (I applied it locally and will push it in a bit).

Ok, I thought that clang had a different syntax for attributes. I found the same error in Sink, this is a patch for it.