api: disable construction API if spend policy is unset

[n8maninger]

There was a change in coreutils that makes it so the zero value of spend policy cannot be encoded or decoded. This changes the construction API to return an error if one of the funding addresses does not have a policy set.

[ChrisSchinnerl]

No integration 🧪 ?

[lukechampine]

I would rather omit the SpendPolicy entirely rather than return something that is almost certainly incorrect. If there's a possibility that we don't know what the SpendPolicy is, we shouldn't be returning it.

[n8maninger]

I would rather omit the SpendPolicy entirely rather than return something that is almost certainly incorrect. If there's a possibility that we don't know what the SpendPolicy is, we shouldn't be returning it.

@lukechampine SiaFoundation/core#268 (comment)

I'm fine with that, but we have to reconcile with this change since the API client can no longer unmarshal a transaction with an unset policy. From my perspective, we have two options: we can either reconsider SiaFoundation/core#268 or we can force the spend policy to a default here.

[lukechampine]

IIUC this means that you'll get an API error if s.wm.SelectSiacoinElements happens to select an address without a spend policy, even if there is some other combination of addresses that would work. Perhaps s.wm.SelectSiacoinElements could only return elements with known spend policies? (What else is s.wm.SelectSiacoinElements used for?)

[n8maninger]

IIUC this means that you'll get an API error if s.wm.SelectSiacoinElements happens to select an address without a spend policy, even if there is some other combination of addresses that would work. Perhaps s.wm.SelectSiacoinElements could only return elements with known spend policies? (What else is s.wm.SelectSiacoinElements used for?)

It's also used in the legacy /fund endpoints and I don't want to change behavior there. I'd probably add a separate method for it.

[lukechampine]

Ok. As-is, this is not great, not terrible. I'd be ok merging now and following up later.