[RFE] hook to run checks before departing relations

This issue became a need due to the bug 1873521

I'm a little bit cautious about the solution proposed here, as it seems like it might make it difficult to get rid of a stalled out charm, and also allow charm authors to unwittingly get themselves into a deadlock situation.

But the problem is legitimate, and it's worth thinking through a solution.

Thanks for triaging and reviewing Pete.

I understand and agree with the need for caution in the implementation. That said, I feel that in practice, the risk of charm authors getting themselves in trouble with the proposed new hook should more or less be the same as that presented by any other, existing one - unless I am missing something there need not be anything intrinsically riskier in the new hook. If charm authors don't use it, it will be a no-op, and, if they do and get themselves in trouble, it can be skipped, or you can force remove the unit, if need be.

Thoughts?

Is this a request for "relation-departing" (eg, let me know that my unit is being removed before it has been pulled down to no-longer-in-the-relation), or is this a different request (given the ability to block remove-unit entirely this sounds different).

Also, communicating that this has been blocked sounds difficult given the asynchronous nature of things like "juju remove-unit". (The request sounds like you'd want to make remove-unit synchronous with running this hook so that you can communicated that your request has been denied.)

This is not a request for "relation-departing".

The RFE is more like defining a new hook at the start of unit's destruction sequence. As a charm author, this new hook can be used to implement logic to perform some application specific checks to continue/block the destruction sequence.
( It is not possible to do the application specific checks in stop hook which executes at end of unit's destruction sequence because by that time the relations are broken and hence the application itself is broken )

This bug has not been updated in 2 years, so we're marking it Low importance. If you believe this is incorrect, please update the importance.