Getting/Borrowing a capability as a sibling interface-type of the orignally issued type

[SupunS]

Issue to be solved

Came up in the thread https://discord.com/channels/613813861610684416/1273464365701398568/1273464651971039336.

Suppose a resource R implements both I1 and I2.

resource R: I1, I2 {}

And a capability to the resource is being issued with I1. i.e: Capability<&{I1}>

Currently this capability is no allowed to be get/borrowed as Capability<&{I2}>. However, once the capability is borrowed, it is possible to upcast/downcast to any of the interface-types that R implements.

The question is, does it make sense to borrow it with I2 as well? (note that none of these grant extra permissions/entitlements)

Suggested Solution

No response

[bjartek]

It appears that this work for caps created in 1.0 but not migrated caps? See here https://discord.com/channels/613813861610684416/1273709822218473513

[SupunS]

It appears that this work for caps created in 1.0 but not migrated caps? See here https://discord.com/channels/613813861610684416/1273709822218473513

There the problem is, borrowing as a super-type/sub-type works when issued with a single interface. But the same fails when issued with multiple interfaces. e.g:

resource interface I1: I2 {}

resource interface I2 {}

resource interface I3 {}

 resource R: I1, I2, I3 {}

// Case-1: Issue as `{I2}` only
// Below borrowing will pass
var a = self.account.capabilities.storage.issue<&{I2}>(/storage/r)
self.account.capabilities.publish(a, at: /public/a)

// Case-2: Issue as `{I2, I3}`.
// Below borrowing will fail
var a = self.account.capabilities.storage.issue<&{I2, I3}>(/storage/r)
self.account.capabilities.publish(a, at: /public/a)

// Borrow as `{I1}`
self.account.capabilities.borrow<&{I1}>(/public/a)!

[bjartek]

Nice find

[SupunS]

Opened a separate issue https://github.com/onflow/cadence/issues/3537 for the second one, as those two more likely need separate handling

[SupunS]

Took a while for me to realize, https://github.com/onflow/cadence/issues/3537 is also actually the same sibling problem. e.g: The type hierarchy is as follows:

    AnyResource
        / \
       /   \
     I2     I3
    /  \   /
   /    \ /
  I1  {I2,I3}
   \    /
    \  /
      R

{I2,I3} is not a direct subtype or a supertype of I1, but rather a sibling. That's why the borrowing fails, just like the sibling case (original issue in the description).Though both are super-types of R.

A more realistic example is added in https://github.com/onflow/cadence/issues/3537#issuecomment-2299424819

[SupunS]

We can support this from the technical point of view. But then it would make the capability issue type mostly useless, and only the entitlements would matter.

[j1010001]

This is not causing staging/update to fail, if someone runs into the problem they can solve it by upgrading their contract post-Crescendo. To reduce risk we will not fix this before Crescendo but after. We will update Docs to explain the problem and how to work around it.