Support custom storage proof from the relay-chain

[bkchr]

The collator is already suitable to generate a storage proof of some specific keys and pass it to the runtime as inherent: https://github.com/paritytech/cumulus/blob/master/collator/src/lib.rs#L203

Currently this is mainly used to pass information about the state of message passing into the parachain. However, we want to open this for parachains to read whatever storage item they are interested in. This would enable parachains to read every item of the relay chain, for example the BABE randomness to use this in their runtime.

The foundation is there, we can read a given key and put it into a storage proof that is passed to the runtime. The following is still missing:

1. Make it more simple to access the storage proof from the runtime side. We probably want some functionality in cumulus-system to signal that the storage proof is now available (it is passed as inherent, so it comes after on_initialize and some way to read values from this proof.
2. Open up the client side to parachain developers to pass keys they are interested in into the collator. From the beginning we should make sure that this list isn't static, as I would assume that we will may require different keys for different blocks (as you can not just put the full state into a storage poof :D).
3. We need a way to make it relative easy to get storage keys for certain storage values in the relay chain state. Currently we have hard-coded these keys. There was also the idea to use well_known_keys for this, but this would not work as we can not foresee from the relay chain side which items will be required by the parachain. We also need a solution that enables us to change some storage key/move a storage item without waiting X months until the full ecosystem is moved (just assume we want to change some key in staking in Substrate).

Number 3. will be clearly the hardest part of this issue. All these issues are solvable on its own.

For 3. I could imagine that we do the storage key collection in some wasm blob. This wasm-blob would be build against the current relay-chain runtime blob. It would only be required to expose one function to collect the keys and return it as a Vec<Vec<u8>>. Inside this blob we would have access to all storage items, of all pallets being used by the relay chain runtime (This would require that we change the pallet macros to expose the storage keys of all storage items of a pallet). If we would have such a WASM blob and there is a pending relay-chain runtime upgrade, it would still require some manual intervention as all parachains would need to update their WASM blob and issue an update of this (a hash of this WASM blob would probably be stored on chain and fetched from "somewhere" or it would be stored directly in the state 🙈 like the runtime wasm blob). After all parachains have done this/we waited long enough, the council could issue the runtime upgrade. This requires quite a lot of coordination, but is way faster than doing it in some more hard-coded way where we would need to wait until all nodes of all parachains are upgraded.

[pepyakin]

I remember a discussion between me, @rphmeier and you, @bkchr, where we thought about introducing a special host function that would provide the contents of the given key of the relay-chain storage of the relay parent. A rough idea was something like this:

• During the block authoring it would go directly into the polkadot storage backend collecting the witness. The aggregated witness would go into the PoV.
• To make substrate block import work we should attach these proofs to the block
• During PVF execution this hypothetical function will be replaced by one that looks the data directly from PoV.

The advantages of that approach is that the keys are available for querying at on_initialize already and there is no need to expose the client for fiddling to the chain writers.

As per number 3. I agree that this is the most annoying one. I also had some thoughts about it and they also involved a wasm blob. In my thoughts it was a bit different angle though. Roughly It goes like this.

• The relay-chain does not provide any guarantees on the storage layout. This applies to both keys and the structures.
• The relay-chain publishes a wasm blob. This blob defines a set of functions that answer some predefined queries. These queries could be like "what is the MQC head of this para?" or "what is the balance of this account?". That's a public API.
• These APIs are really serious about providing backwards compatibility. They should not break or change semantics in any way. I see it purely additive. Removing APIs (e.g. when we sunset a significant part of the system) should happen with a significant heads up delay and most likely should involve the governance.
• Adding a new API should be approved by the governance.
• Relay-chain upgrades should be coupled with the corresponding upgrade of the wasm blob that hosts these API, if needed.
• Under the hood these API may or may not be implemented with the mechanism outlined in the beginning of the post. From an PDK side, this should ideally look like as a simple call to a host function.

This way the relay-chain controls explicitly what it exposes and can move forward independently. The downside is that there is delay involved when negotiating with the governance. However, I think that it is a good thing: I think it's better that each dependency on the relay-chain structure is explicit and each particular case is carefully assessed with respect to the long-term maintenance burden. In contrast, just opening up any storage storage a parachain can reach is just a recipe for a long-term maintenance nightmare.

[bkchr]

I remember a discussion between me, @rphmeier and you, @bkchr, where we thought about introducing a special host function that would provide the contents of the given key of the relay-chain storage of the relay parent. A rough idea was something like this:

* During the block authoring it would go directly into the polkadot storage backend collecting the witness. The aggregated witness would go into the PoV.

* To make substrate block import work we should attach these proofs to the block

* During PVF execution this hypothetical function will be replaced by one that looks the data directly from PoV.

Totally forgot about this one! Still a good idea :)

As per number 3. I agree that this is the most annoying one. I also had some thoughts about it and they also involved a wasm blob. In my thoughts it was a bit different angle though. Roughly It goes like this.

* The relay-chain does not provide any guarantees on the storage layout. This applies to both keys and the structures.

* The relay-chain publishes a wasm blob. This blob defines a set of functions that answer some predefined queries. These queries could be like "what is the MQC head of this para?" or "what is the balance of this account?". That's a public API.

* These APIs are really serious about providing backwards compatibility. They should not break or change semantics in any way. I see it purely additive. Removing APIs (e.g. when we sunset a significant part of the system) should happen with a significant heads up delay and most likely should involve the governance.

* Adding a new API should be approved by the governance.

* Relay-chain upgrades should be coupled with the corresponding upgrade of the wasm blob that hosts these API, if needed.

* Under the hood these API may or may not be implemented with the mechanism outlined in the beginning of the post. From an PDK side, this should ideally look like as a simple call to a host function.

I like this idea! Would this extra wasm blob be dynamically linked to the parachain runtime? So that the parachain can call one of these functions?

[pepyakin]

That's a good question, I wasn't thinking through the details. I think it's a possibility, although the Rust/Wasm dynamic linking story is a bit fuzzy though. But whatever is easy to use and performant enough.

[rphmeier]

How often can the wasm blob change? We will have to be careful that it is still easy to execute parablocks after-the-fact even when state pruned. This wasm blob could be in the AvailableData but that would be very redundant.

[pepyakin]

Store it in the state? I think this shim wasm blob should be way slimmer than the runtime itself.

[burdges]

If I understand, this moves us much of the way to contextual execution too. We could also announce these more than 48 hours in advance, which helps address their availability problem.

[olanod]

Is this only about collators reading the state of the relay chain? Could this be related or is it a complete different beast to read storage from other parachains? polkadot-fellows/xcm-format#16

[pepyakin]

Well, yes, this is about reading of the relay chain state. I think this is at least somewhat related to reading the storage of other parachains. Reading the storage of other parachains is very specific on the parachain itself, but at least one part is shared: you typically need to get some sort of commitment like a merkle root, and that should be stored in a head data of the parachain which is stored in the relay chain state.