Deadlock when multiple key lookups are coalessed and one of them throws an KeyNotFoundException

[jorisvergeer]

Do you want to request a feature or report a bug?
BUG (a nasty one)

What is the current behavior?
When you do multiple select operations in a short amount op time, akavache will optimize this and will coalesce this in a single sqlite lookup.

When in the handling of this result, one of the keys is not found, then any results after this result will not be handled. (Due to a KeyNotFoundException coming out of elementMap[v].OnCompleted();)

When for example you await on the result on one of those not handled keys, then your code is in a deadlock state.

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem
Use akavache and do multiple lookups simultaniously so that the optimizer will combine them. Make sure that some of the keys so not exists.

For a complete deadlock example, await the lookups.

What is the expected behavior?

All observables and awaited lookups give either a result or throw KeyNotFoundException.

What is the motivation / use case for changing the behavior?

I don't want my code to deadlock when other pieces of code request values for non-existing keys.

Which versions of Akavache, and which platform / OS are affected by this issue? Did this work in previous versions of Akavache? Please also test with the latest stable and snapshot (https://www.myget.org/feed/Akavache/package/nuget/Akavache) versions.
Latest stable 6.0.27

Other information (e.g. stacktraces, related issues, suggestions how to fix)
Try catch in GroupUnrelatedSelects or make sure that elementMap[v].OnCompleted(); there does not throw the exception.

[jorisvergeer]

Here a screenshot of the offending situation

[jorisvergeer]

Note that I get this behavior in Xamarin iOS and Android.

I do not get this behavior when I try to wrap this in a unit test on windows. :/

[PureWeen]

@jorisvergeer sorry for the super late reply but do you by chance have a block of code I could see to recreate the issue?

I've tried using your description but maybe I'm missing a nuance of how to recreate?

 await _BlobCache.InvalidateAll()
                    .SelectMany(_ => Observable.Range(0, 100))
                    .Select(value =>
                    {
                        if (value % 3 == 0)
                        {
                            return _BlobCache.InsertObject<SomeObject>(value.ToString(), new SomeObject());
                        }

                        return Observable.Return(Unit.Default);
                    })
                    .Merge();

                
                await Observable.Defer(() => Observable.Range(0, 100))
                    .Select(value => _BlobCache.GetObject<SomeObject>(value.ToString()))
                    .Merge()
                    .Catch((KeyNotFoundException ke) => Observable.Return<SomeObject>(null));

               foreach(var item in Enumerable.Range(0,100))
                {
                    _BlobCache.GetObject<SomeObject>(item.ToString())
                    .Catch((KeyNotFoundException ke) => Observable.Return<SomeObject>(null))
                    .Subscribe();
                }

[jorisvergeer]

Damn, that is some Rx black magic.

But a'm sorry, i do not have an stand alone example. It appeared in our production app, and I tested it in there.

[glennawatson]

Fixed by #433