Add discard as an option for custom recoverability policies

[danielmarbach]

Allows discarding a message in the custom recoverability policy.

More information can be found under

https://docs.particular.net/nservicebus/recoverability/custom-recoverability-policy#implement-a-custom-policy-full-customization

Orignal context

On a live chat today a user had the following requirement

They have certain types of messages that they want to retry a few times and after they failed a certain number of times they want to ignore/discard the message. We first started exploring whether it is possible to implement this scenario with the recoverability policy but found out it is not possible.

Trying to implement that with behaviors in the pipeline becomes really cumbersome because you are essentially faced with reimplementing parts of the recoverability seam inside behaviors. Especially the counting of how many times a message has failed can become very brittle.

This demonstrates that a discard action in the recoverability seam would be easy to add in a non-breaking way and could provide good benefit to some of our customers.

There is a workaround that we came up with for the customer. You can currently implement a MoveToError action and move it to a queue that exists (but not the error queue) and then have a no-op handler on that queue or from time to time purge that queue. But that requires you to deploy an additional endpoint that listens on that queue or have a dedicated purge this queue job that runs from time to time (plus obviously the unnecessary additional infrastructure resources that are required).

[gardang]

This would be a great feature for us and would obviously make the code more readable on this types of scenarios. So this functionality would be greatly appreciated =)

[bording]

Are we sure we want to add a features that is basically allowing message loss?

[timbussmann]

Are we sure we want to add a features that is basically allowing message loss?

I'm fine allowing this option in a very advanced API where you clearly have to opt-in for discarding. We also allow message loss when using non-transactional transport configuration.

[bording]

I think this is ready to remove the WIP?

[HEskandari]

Is this done? anything left here other than removing the WIP and merging?

[danielmarbach]

All done

[HEskandari]

@danielmarbach I have removed the WIP from the PR. Let's merge this and release it so I can get the doco in as well.

[andreasohlund]

We need to figure out how to make sure annoucement text is available as well when releasing. Perhaps a draft 7.2.0 announcement?