Harness to auto attach storage?

Following up on https://github.com/canonical/operator/pull/734, I tried it out in some unit tests and had the following recurring code:

storage_id = self.harness.add_storage("name")[0]
self.harness.attach_storage(storage_id)

Would it be feasible/sensible to auto attach all storages in metadata (perhaps with count=1 by default)?

For example,

self.harness.auto_attach_storages()
self.harness.begin_with_initial_hooks()

[rwcarlsen]

The trick with auto-attaching on begin_with_initial_hooks is the harness doesn’t know how many instances of each particular storage it should create/mount for cases where there can be multiple. If we did do that, we’d need to implement some logic mirroring how juju decides. And also we’d have to think carefully about how to deal with cases where we a charm dev has manually attached some storage prior to calling begin_with_initial_hooks. My inclination right now is to just leave it as is require devs to explicitly add storage for their tests.

[rwcarlsen]

1. The -/_ replacement occurs automatically with the .on[...] call: https://github.com/canonical/operator/blob/eb33286800bf94b60b7354b350f2725ab5b81865/ops/framework.py#L453
2. I’ll need to look into this more. At first glance, it seems like we should definitely use more care with how we handle add/attach/detach - it looks like there may be a few subtle bugs lurking.
3. Possibly. I’ll dig into this as well.