Allow to disable gap creation during block import

prdoc/pr_5343.prdoc

@@ -0,0 +1,19 @@
+title: Allow to disable gap creation during block import
+
+doc:
+  - audience: Node Dev
+    description: |
+      New property `BlockImportParams::create_gap` allows to change whether to create block gap in case block
+      has no parent (defaults to `true` keeping existing behavior), which is helpful for sync protocols that do not need
+      to sync the gap after this happens. `BlockImportOperation::create_gap()` method was also introduced, though in
+      most cases `BlockImportParams::create_gap` will be used.
+
+crates:
+  - name: sc-client-api
+    bump: major
+  - name: sc-consensus
+    bump: minor
+  - name: sc-client-db
+    bump: minor
+  - name: sc-service
+    bump: minor

substrate/client/api/src/backend.rs

@@ -232,6 +232,9 @@ pub trait BlockImportOperation<Block: BlockT> {
 	/// Add a transaction index operation.
 	fn update_transaction_index(&mut self, index: Vec<IndexOperation>)
 		-> sp_blockchain::Result<()>;
+
+	/// Configure whether to create a block gap if newly imported block is missing parent
+	fn set_create_gap(&mut self, create_gap: bool);
 }
 
 /// Interface for performing operations on the backend.

substrate/client/api/src/in_mem.rs

@@ -584,6 +584,8 @@ impl<Block: BlockT> backend::BlockImportOperation<Block> for BlockImportOperatio
 	) -> sp_blockchain::Result<()> {
 		Ok(())
 	}
+
+	fn set_create_gap(&mut self, _create_gap: bool) {}
 }
 
 /// In-memory backend. Keeps all states and blocks in memory.

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

@@ -214,6 +214,8 @@ pub struct BlockImportParams<Block: BlockT> {
 	pub fork_choice: Option<ForkChoiceStrategy>,
 	/// Re-validate existing block.
 	pub import_existing: bool,
+	/// Whether to create "block gap" in case this block doesn't have parent.
+	pub create_gap: bool,
 	/// Cached full header hash (with post-digests applied).
 	pub post_hash: Option<Block::Hash>,
 }
@@ -234,6 +236,7 @@ impl<Block: BlockT> BlockImportParams<Block> {
 			auxiliary: Vec::new(),
 			fork_choice: None,
 			import_existing: false,
+			create_gap: true,
 			post_hash: None,
 		}
 	}

substrate/client/db/src/lib.rs

@@ -834,6 +834,7 @@ pub struct BlockImportOperation<Block: BlockT> {
 	finalized_blocks: Vec<(Block::Hash, Option<Justification>)>,
 	set_head: Option<Block::Hash>,
 	commit_state: bool,
+	create_gap: bool,
 	index_ops: Vec<IndexOperation>,
 }
 
@@ -988,6 +989,10 @@ impl<Block: BlockT> sc_client_api::backend::BlockImportOperation<Block>
 		self.index_ops = index_ops;
 		Ok(())
 	}
+
+	fn set_create_gap(&mut self, create_gap: bool) {
+		self.create_gap = create_gap;
+	}
 }
 
 struct StorageDb<Block: BlockT> {
@@ -1709,8 +1714,9 @@ impl<Block: BlockT> Backend<Block> {
 							unreachable!("Unsupported block gap. TODO: https://github.com/paritytech/polkadot-sdk/issues/5406")
 						},
 					}
-				} else if number > best_num + One::one() &&
-					number > One::one() && self.blockchain.header(parent_hash)?.is_none()
+				} else if operation.create_gap &&
+					number > best_num + One::one() &&
+					self.blockchain.header(parent_hash)?.is_none()
 				{
 					let gap = BlockGap {
 						start: best_num + One::one(),
@@ -2072,6 +2078,7 @@ impl<Block: BlockT> sc_client_api::backend::Backend<Block> for Backend<Block> {
 			finalized_blocks: Vec::new(),
 			set_head: None,
 			commit_state: false,
+			create_gap: true,
 			index_ops: Default::default(),
 		})
 	}

substrate/client/service/src/client/client.rs

@@ -513,6 +513,7 @@ where
 			fork_choice,
 			intermediates,
 			import_existing,
+			create_gap,
 			..
 		} = import_block;
 
@@ -537,6 +538,8 @@ where
 
 		*self.importing_block.write() = Some(hash);
 
+		operation.op.set_create_gap(create_gap);
+
 		let result = self.execute_and_import_block(
 			operation,
 			origin,