Notification-based block pinning

client/api/src/backend.rs

@@ -436,12 +436,24 @@ pub trait StorageProvider<Block: BlockT, B: Backend<Block>> {
 ///
 /// Manages the data layer.
 ///
-/// Note on state pruning: while an object from `state_at` is alive, the state
+/// # State Pruning
+///
+/// While an object from `state_at` is alive, the state
 /// should not be pruned. The backend should internally reference-count
 /// its state objects.
 ///
 /// The same applies for live `BlockImportOperation`s: while an import operation building on a
 /// parent `P` is alive, the state for `P` should not be pruned.
+///
+/// # Block Pruning
+///
+/// Users can pin blocks in memory by calling `pin_block`. When
+/// a block would be pruned, its value is kept in an in-memory cache
+/// until it is unpinned via `unpin_block`.
+///
+/// While a block is pinned, its state is also preserved.
+///
+/// The backend should internally reference count the number of pin / unpin calls.
 pub trait Backend<Block: BlockT>: AuxStore + Send + Sync {
 	/// Associated block insertion operation type.
 	type BlockImportOperation: BlockImportOperation<Block, State = Self::State>;
@@ -502,6 +514,14 @@ pub trait Backend<Block: BlockT>: AuxStore + Send + Sync {
 	/// Returns a handle to offchain storage.
 	fn offchain_storage(&self) -> Option<Self::OffchainStorage>;
 
+	/// Pin the block to keep body, justification and state available after pruning.
+	/// Number of pins are reference counted. Users need to make sure to perform
+	/// one call to [`Self::unpin_block`] per call to [`Self::pin_block`].
+	fn pin_block(&self, hash: Block::Hash) -> sp_blockchain::Result<()>;
+
+	/// Unpin the block to allow pruning.
+	fn unpin_block(&self, hash: Block::Hash);
+
 	/// Returns true if state for given block is available.
 	fn have_state_at(&self, hash: Block::Hash, _number: NumberFor<Block>) -> bool {
 		self.state_at(hash).is_ok()

client/api/src/client.rs

@@ -30,7 +30,7 @@ use std::{collections::HashSet, fmt, sync::Arc};
 use crate::{blockchain::Info, notifications::StorageEventStream, FinalizeSummary, ImportSummary};
 
 use sc_transaction_pool_api::ChainEvent;
-use sc_utils::mpsc::TracingUnboundedReceiver;
+use sc_utils::mpsc::{TracingUnboundedReceiver, TracingUnboundedSender};
 use sp_blockchain;
 
 /// Type that implements `futures::Stream` of block import events.
@@ -264,6 +264,53 @@ impl fmt::Display for UsageInfo {
 	}
 }
 
+/// Sends a message to the pinning-worker once dropped to unpin a block in the backend.
+#[derive(Debug)]
+pub struct UnpinHandleInner<Block: BlockT> {
+	/// Hash of the block pinned by this handle
+	hash: Block::Hash,
+	unpin_worker_sender: TracingUnboundedSender<Block::Hash>,
+}
+
+impl<Block: BlockT> UnpinHandleInner<Block> {
+	/// Create a new [`UnpinHandleInner`]
+	pub fn new(
+		hash: Block::Hash,
+		unpin_worker_sender: TracingUnboundedSender<Block::Hash>,
+	) -> Self {
+		Self { hash, unpin_worker_sender }
+	}
+}
+
+impl<Block: BlockT> Drop for UnpinHandleInner<Block> {
+	fn drop(&mut self) {
+		if let Err(err) = self.unpin_worker_sender.unbounded_send(self.hash) {
+			log::debug!(target: "db", "Unable to unpin block with hash: {}, error: {:?}", self.hash, err);
+		};
+	}
+}
+
+/// Keeps a specific block pinned while the handle is alive.
+/// Once the last handle instance for a given block is dropped, the
+/// block is unpinned in the [`Backend`](crate::backend::Backend::unpin_block).
+#[derive(Debug, Clone)]
+pub struct UnpinHandle<Block: BlockT>(Arc<UnpinHandleInner<Block>>);
+
+impl<Block: BlockT> UnpinHandle<Block> {
+	/// Create a new [`UnpinHandle`]
+	pub fn new(
+		hash: Block::Hash,
+		unpin_worker_sender: TracingUnboundedSender<Block::Hash>,
+	) -> UnpinHandle<Block> {
+		UnpinHandle(Arc::new(UnpinHandleInner::new(hash, unpin_worker_sender)))
+	}
+
+	/// Hash of the block this handle is unpinning on drop
+	pub fn hash(&self) -> Block::Hash {
+		self.0.hash
+	}
+}
+
 /// Summary of an imported block
 #[derive(Clone, Debug)]
 pub struct BlockImportNotification<Block: BlockT> {
@@ -279,6 +326,36 @@ pub struct BlockImportNotification<Block: BlockT> {
 	///
 	/// If `None`, there was no re-org while importing.
 	pub tree_route: Option<Arc<sp_blockchain::TreeRoute<Block>>>,
+	/// Handle to unpin the block this notification is for
+	unpin_handle: UnpinHandle<Block>,
+}
+
+impl<Block: BlockT> BlockImportNotification<Block> {
+	/// Create new notification
+	pub fn new(
+		hash: Block::Hash,
+		origin: BlockOrigin,
+		header: Block::Header,
+		is_new_best: bool,
+		tree_route: Option<Arc<sp_blockchain::TreeRoute<Block>>>,
+		unpin_worker_sender: TracingUnboundedSender<Block::Hash>,
+	) -> Self {
+		Self {
+			hash,
+			origin,
+			header,
+			is_new_best,
+			tree_route,
+			unpin_handle: UnpinHandle::new(hash, unpin_worker_sender),
+		}
+	}
+
+	/// Consume this notification and extract the unpin handle.
+	///
+	/// Note: Only use this if you want to keep the block pinned in the backend.
+	pub fn into_unpin_handle(self) -> UnpinHandle<Block> {
+		self.unpin_handle
+	}
 }
 
 /// Summary of a finalized block.
@@ -294,6 +371,8 @@ pub struct FinalityNotification<Block: BlockT> {
 	pub tree_route: Arc<[Block::Hash]>,
 	/// Stale branches heads.
 	pub stale_heads: Arc<[Block::Hash]>,
+	/// Handle to unpin the block this notification is for
+	unpin_handle: UnpinHandle<Block>,
 }
 
 impl<B: BlockT> TryFrom<BlockImportNotification<B>> for ChainEvent<B> {
@@ -314,26 +393,44 @@ impl<B: BlockT> From<FinalityNotification<B>> for ChainEvent<B> {
 	}
 }
 
-impl<B: BlockT> From<FinalizeSummary<B>> for FinalityNotification<B> {
-	fn from(mut summary: FinalizeSummary<B>) -> Self {
+impl<Block: BlockT> FinalityNotification<Block> {
+	/// Create finality notification from finality summary.
+	pub fn from_summary(
+		mut summary: FinalizeSummary<Block>,
+		unpin_worker_sender: TracingUnboundedSender<Block::Hash>,
+	) -> FinalityNotification<Block> {
 		let hash = summary.finalized.pop().unwrap_or_default();
 		FinalityNotification {
 			hash,
 			header: summary.header,
 			tree_route: Arc::from(summary.finalized),
 			stale_heads: Arc::from(summary.stale_heads),
+			unpin_handle: UnpinHandle::new(hash, unpin_worker_sender),
 		}
 	}
+
+	/// Consume this notification and extract the unpin handle.
+	///
+	/// Note: Only use this if you want to keep the block pinned in the backend.
+	pub fn into_unpin_handle(self) -> UnpinHandle<Block> {
+		self.unpin_handle
+	}
 }
 
-impl<B: BlockT> From<ImportSummary<B>> for BlockImportNotification<B> {
-	fn from(summary: ImportSummary<B>) -> Self {
+impl<Block: BlockT> BlockImportNotification<Block> {
+	/// Create finality notification from finality summary.
+	pub fn from_summary(
+		summary: ImportSummary<Block>,
+		unpin_worker_sender: TracingUnboundedSender<Block::Hash>,
+	) -> BlockImportNotification<Block> {
+		let hash = summary.hash;
 		BlockImportNotification {
-			hash: summary.hash,
+			hash,
 			origin: summary.origin,
 			header: summary.header,
 			is_new_best: summary.is_new_best,
 			tree_route: summary.tree_route.map(Arc::new),
+			unpin_handle: UnpinHandle::new(hash, unpin_worker_sender),
 		}
 	}
 }

client/api/src/in_mem.rs

@@ -788,6 +788,12 @@ where
 	fn requires_full_sync(&self) -> bool {
 		false
 	}
+
+	fn pin_block(&self, _: <Block as BlockT>::Hash) -> blockchain::Result<()> {
+		Ok(())
+	}
+
+	fn unpin_block(&self, _: <Block as BlockT>::Hash) {}
 }
 
 impl<Block: BlockT> backend::LocalBackend<Block> for Backend<Block> where Block::Hash: Ord {}

client/db/Cargo.toml

@@ -26,6 +26,7 @@ parity-db = "0.4.2"
 parking_lot = "0.12.1"
 sc-client-api = { version = "4.0.0-dev", path = "../api" }
 sc-state-db = { version = "0.10.0-dev", path = "../state-db" }
+schnellru = "0.2.1"
 sp-arithmetic = { version = "6.0.0", path = "../../primitives/arithmetic" }
 sp-blockchain = { version = "4.0.0-dev", path = "../../primitives/blockchain" }
 sp-core = { version = "7.0.0", path = "../../primitives/core" }