I don't want to discourage the subsaga work because that'll be really valuable.

We could refactor functions a bit, but we will need to enumerate all the disk creation saga steps within the instance creation saga.

I wonder if we could abstract this further so that there's a common function that accepts a SagaTemplateBuilder and adds a sequence of actions to it. Then there's very little that would be duplicated. The hard part seems like the fact that the sagas probably have different SagaTypes.

I looked into this and different SagaTypes is indeed the problem with the sharing of actions. However, all sagas in nexus share a SagaContext, and so the only difference is the SagaParamsType associated type in each of SagaInstanceCreate and SagaDiskCreate. However, it looks like the saga params for SagaInstanceCreate include the DiskCreate params. So the instance disk create should have all the data it needs to do the creation, but the functions in SagaDiskCreate template actions refer directly to the fields on params.

I think If instead of accessing the params fields directly in the actions functions for SagaDiskCreate, we called a method on the params type that returned the proper params::DiskCreate despite which SagaType the builder was called with and made the disk create action functions be generic over SagaType, and added a constraint for the method that would return params::DiskCreate on the correct SagaType we could do what Dave suggests.

If I'm reading things correctly, this doesn't seem too hard to do, but it does start coupling SagaTypes which may set a bad precedent. I believe sub-sagas are the cleaner abstraction, and after reading all this code I now understand why. However, this code base hasn't used them yet, and I wouldn't mind cleaning up steno and adding automated tests before it did, so I can understand the rough edges first. I'm still just digging into this part of omicron and steno for the first time really and am not looking to break things without understanding :)

In summary, I believe I can probably get this working very short term by playing games with SagaTypes. And then I can go and retrofit it with subsagas later. I guess it depends how urgently we want this.

Implement disk creation during instance creation #812

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