reload and restart - smart insertion of new tasks

[arjclark]

Result of an email conversation between myself, @hjoliver and @dpmatthews.

It would be useful to have cylc perform a smart insertion of new tasks on performing a reload.

Options could include:

• insert at latest cycle
• insert at earliest (in focus) cycle
• an option to specify the cycle at which to perform the insertion

There may be a challenge with determining what to do about purged tasks and whether they are "new", depending on how the suite tracks those operations across reloads.

[arjclark]

@dpmatthews wants to have a think and comment on what the best way to implement this may be.

[arjclark]

Missed this one from the email chain as an option:

"another option could be to choose the first instance with upstream dependencies not yet satisfied, or the instance just prior to that - and trigger it immediately in case its upstream dependencies had been cleaned up already." - @hjoliver

[dpmatthews]

I assume the default will remain not to insert new tasks.
First instance with upstream dependencies not yet satisfied sounds like it would be a useful option. Otherwise, insertion at latest cycle would probably deal with most simple cases.
The option to insert new tasks at a specified cycle would be useful to deal with more complex cases, e.g. suites spread over multiple cycles.

[hjoliver]

Worth doing - some users do seem to find the lack of new task auto-insertion confusing.

[hjoliver]

[meeting] we agreed: find earliest instance of an unsatisfied task that depends on a new task, and insert there.

[oliver-sanders]

[meeting] A robust, reliable solution that works for all edge cases is a major challenge. A solution which fails to handle these edge cases properly risks generating a false sense of security. Once we move to a spawn-on-demmand model for the task pool, auto-insert on reload should become somewhat easier.

[oliver-sanders]

To be closed with or after #987

[oliver-sanders]

So very happy to close this, I can finally delete my failed attempts.