pvf-precheck: Strip PastCodeMeta

This PR is a part of
#3211.

This PR prepares ground for the following runtime changes required for
PVF pre-checking. Specifically, we do several changes here:

1. We remove `validation_code_at` and `validation_code_hash_at`. Those
   functions are not used. They were added in the early days with intent
   to use it later but turned out that we do not need them.
2. We replace `validation_code_hash_at` with just `current_code_hash`
   for the case of inclusion and candidate checking.
3. We also replace `last_code_upgrade` with a direct query into
   `FutureCodeHash` and `UpgradeRestrictionSignal`. Those in conjunction
   should replace the logic that was used for allowing/disallowing
   upgrades. This requires special attention of the reviewers.
4. Then we remove the machinery required to support those queries.
   Specifically the code related to `UseCodeAt`. We do not need it since
   we do not answer the historical queries. However, we still leave all
   the data on-chain. At some point we may clean it up, but that would
   be needed to be done with a dedicated migration which can be done as
   follow-up.
5. Some now irrelevant tests were removed and/or adapted.

roadmap/implementers-guide/src/runtime/paras.md

@@ -217,13 +217,6 @@ CodeByHash: map ValidationCodeHash => Option<ValidationCode>
 * `note_new_head(ParaId, HeadData, BlockNumber)`: note that a para has progressed to a new head,
   where the new head was executed in the context of a relay-chain block with given number. This will
   apply pending code upgrades based on the block number provided. If an upgrade took place it will clear the `UpgradeGoAheadSignal`.
-* `validation_code_at(ParaId, at: BlockNumber, assume_intermediate: Option<BlockNumber>)`: Fetches
-  the validation code to be used when validating a block in the context of the given relay-chain
-  height. A second block number parameter may be used to tell the lookup to proceed as if an
-  intermediate parablock has been included at the given relay-chain height. This may return past,
-  current, or (with certain choices of `assume_intermediate`) future code. `assume_intermediate`, if
-  provided, must be before `at`. If the validation code has been pruned, this will return `None`.
-* `validation_code_hash_at(ParaId, at: BlockNumber, assume_intermediate: Option<BlockNumber>)`: Just like `validation_code_at`, but returns the code hash.
 * `lifecycle(ParaId) -> Option<ParaLifecycle>`: Return the `ParaLifecycle` of a para.
 * `is_parachain(ParaId) -> bool`: Returns true if the para ID references any live parachain,
   including those which may be transitioning to a parathread in the future.

runtime/parachains/src/inclusion.rs

@@ -36,10 +36,7 @@ use primitives::v1::{
 	ValidatorIndex, ValidityAttestation,
 };
 use scale_info::TypeInfo;
-use sp_runtime::{
-	traits::{One, Saturating},
-	DispatchError,
-};
+use sp_runtime::{traits::One, DispatchError};
 use sp_std::{collections::btree_set::BTreeSet, prelude::*};
 
 pub use pallet::*;
@@ -950,7 +947,6 @@ impl<T: Config> CandidateCheckContext<T> {
 		backed_candidate: &BackedCandidate<<T as frame_system::Config>::Hash>,
 	) -> Result<(), Error<T>> {
 		let para_id = backed_candidate.descriptor().para_id;
-		let now = self.now;
 
 		// we require that the candidate is in the context of the parent block.
 		ensure!(
@@ -962,7 +958,7 @@ impl<T: Config> CandidateCheckContext<T> {
 			Error::<T>::NotCollatorSigned,
 		);
 
-		let validation_code_hash = <paras::Pallet<T>>::validation_code_hash_at(para_id, now, None)
+		let validation_code_hash = <paras::Pallet<T>>::current_code_hash(para_id)
 			// A candidate for a parachain without current validation code is not scheduled.
 			.ok_or_else(|| Error::<T>::UnscheduledCandidate)?;
 		ensure!(
@@ -1016,13 +1012,10 @@ impl<T: Config> CandidateCheckContext<T> {
 
 		// if any, the code upgrade attempt is allowed.
 		if let Some(new_validation_code) = new_validation_code {
-			let valid_upgrade_attempt = <paras::Pallet<T>>::last_code_upgrade(para_id, true)
-				.map_or(true, |last| {
-					last <= self.relay_parent_number &&
-						self.relay_parent_number.saturating_sub(last) >=
-							self.config.validation_upgrade_frequency
-				});
-			ensure!(valid_upgrade_attempt, AcceptanceCheckErr::PrematureCodeUpgrade);
+			ensure!(
+				<paras::Pallet<T>>::can_upgrade_validation_code(para_id),
+				AcceptanceCheckErr::PrematureCodeUpgrade,
+			);
 			ensure!(
 				new_validation_code.0.len() <= self.config.max_code_size as _,
 				AcceptanceCheckErr::NewCodeTooLarge,
@@ -2322,8 +2315,6 @@ pub(crate) mod tests {
 						expected_at,
 						&cfg,
 					);
-
-					assert_eq!(Paras::last_code_upgrade(chain_a, true), Some(expected_at));
 				}
 
 				assert_noop!(