Blockchain cache pruning strategy (#3395)

* blockchain cache pruning strategy

* added some internal docs to cache_pruning_strategy

* Update core/client/db/src/cache/mod.rs

Co-Authored-By: DemiMarie-parity <48690212+DemiMarie-parity@users.noreply.github.com>

substrate/core/consensus/common/src/block_import.rs

@@ -21,9 +21,8 @@ use sr_primitives::Justification;
 use std::borrow::Cow;
 use std::collections::HashMap;
 use std::sync::Arc;
-use crate::well_known_cache_keys;
 
-use crate::import_queue::Verifier;
+use crate::import_queue::{Verifier, CacheKeyId};
 
 /// Block import result.
 #[derive(Debug, PartialEq, Eq)]
@@ -182,7 +181,7 @@ pub trait BlockImport<B: BlockT> {
 	fn import_block(
 		&mut self,
 		block: BlockImportParams<B>,
-		cache: HashMap<well_known_cache_keys::Id, Vec<u8>>,
+		cache: HashMap<CacheKeyId, Vec<u8>>,
 	) -> Result<ImportResult, Self::Error>;
 }
 
@@ -202,7 +201,7 @@ where for<'r> &'r T: BlockImport<B, Error = E>
 	fn import_block(
 		&mut self,
 		block: BlockImportParams<B>,
-		cache: HashMap<well_known_cache_keys::Id, Vec<u8>>,
+		cache: HashMap<CacheKeyId, Vec<u8>>,
 	) -> Result<ImportResult, Self::Error> {
 		(&**self).import_block(block, cache)
 	}

substrate/core/consensus/common/src/import_queue.rs

@@ -27,7 +27,7 @@
 
 use std::collections::HashMap;
 use sr_primitives::{Justification, traits::{Block as BlockT, Header as _, NumberFor}};
-use crate::{error::Error as ConsensusError, well_known_cache_keys::Id as CacheKeyId};
+use crate::error::Error as ConsensusError;
 use crate::block_import::{
 	BlockImport, BlockOrigin, BlockImportParams, ImportedAux, JustificationImport, ImportResult,
 	FinalityProofImport,
@@ -65,6 +65,9 @@ pub struct IncomingBlock<B: BlockT> {
 	pub origin: Option<Origin>,
 }
 
+/// Type of keys in the blockchain cache that consensus module could use for its needs.
+pub type CacheKeyId = [u8; 4];
+
 /// Verify a justification of a block
 pub trait Verifier<B: BlockT>: Send + Sync {
 	/// Verify the given data and return the BlockImportParams and an optional

substrate/core/consensus/common/src/lib.rs

@@ -117,15 +117,3 @@ where T: ?Sized, for<'r> &'r T: SyncOracle
 		<&T>::is_offline(&mut &**self)
 	}
 }
-
-/// A list of all well known keys in the cache.
-pub mod well_known_cache_keys {
-	/// The type representing cache keys.
-	pub type Id = [u8; 4];
-
-	/// A list of authorities.
-	pub const AUTHORITIES: Id = *b"auth";
-
-	/// Current Epoch data.
-	pub const EPOCH: Id = *b"epch";
-}