Allow to disable gap creation during block import (#5343)

This feature is helpful for us with custom sync protocol that is similar
to Warp sync except we do not ever sync the gap and don't want it to
exist in the first place (see
#5333 and its
references for motivation).

Otherwise we had to resort to this:
autonomys@d537512

---------

Co-authored-by: Davide Galassi <davxy@datawok.net>

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,