Add task view support for result backend lookup on event cache miss

[johnarnold]

This will allow looking up a task by id in the GUI even if the task is not in the event cache (due to flower restart or task cache size limit, etc)

E.g.
http://localhost:5555/task/07634992-5b5a-4228-b4c3-94bb184da2c5

Normally, you would just get a 'Unknown task' error. This way you can get at least partial results for debugging.

[johnarnold]

Related to #542

[jheld]

So as long as the return result of the task is currently still stored in the backend, then the idea is something should be able to be returned and shown by flower, in the event that flower no longer has its internal state for that task?

[johnarnold]

@jheld yes that's the idea. I've tested it, it works. I'm still trying to get the "extended" task results into Celery, which will really make this useful.

[sonirico]

What's the state of this? It would be a nice feature to fallback to a persistent, scalable result backend after flower restarts.

[jheld]

This is a great addition. Can we please merge?

[jheld]

@johnarnold I haven't recently reviewed the blockers/failing checks (could be minor stuff). Do you have time in the next month to take a peek?

[mher]

Closing this pull request because it can cause many edge cases.

[phillipuniverse]

@mher what are the edge cases?