Allow to disable gap creation during block import

substrate/client/api/src/backend.rs

@@ -632,6 +632,9 @@ pub trait Backend<Block: BlockT>: AuxStore + Send + Sync {
 
 	/// Tells whether the backend requires full-sync mode.
 	fn requires_full_sync(&self) -> bool;
+
+	/// Do not create gap if newly imported block is missing parent
+	fn no_gap(&self, operation: &mut Self::BlockImportOperation);
 }
 
 /// Mark for all Backend implementations, that are making use of state data, stored locally.

substrate/client/api/src/in_mem.rs

@@ -774,6 +774,8 @@ impl<Block: BlockT> backend::Backend<Block> for Backend<Block> {
 		let mut blocks = self.pinned_blocks.write();
 		blocks.entry(hash).and_modify(|counter| *counter -= 1).or_insert(-1);
 	}
+
+	fn no_gap(&self, _operation: &mut Self::BlockImportOperation) {}
 }
 
 impl<Block: BlockT> backend::LocalBackend<Block> for Backend<Block> {}

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

@@ -841,6 +841,7 @@ 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>,
 }
 
@@ -1707,8 +1708,10 @@ impl<Block: BlockT> Backend<Block> {
 							&(start, end).encode(),
 						);
 					}
-				} 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() &&
+					number > One::one() &&
+					self.blockchain.header(parent_hash)?.is_none()
 				{
 					let gap = (best_num + One::one(), number - One::one());
 					transaction.set(columns::META, meta_keys::BLOCK_GAP, &gap.encode());
@@ -2060,6 +2063,7 @@ 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(),
 		})
 	}
@@ -2538,6 +2542,10 @@ impl<Block: BlockT> sc_client_api::backend::Backend<Block> for Backend<Block> {
 			self.blockchain.unpin(hash);
 		}
 	}
+
+	fn no_gap(&self, operation: &mut Self::BlockImportOperation) {
+		operation.create_gap = false;
+	}
 }
 
 impl<Block: BlockT> sc_client_api::backend::LocalBackend<Block> for Backend<Block> {}

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

@@ -513,6 +513,7 @@ where
 			fork_choice,
 			intermediates,
 			import_existing,
+			create_gap,
 			..
 		} = import_block;
 
@@ -537,6 +538,10 @@ where
 
 		*self.importing_block.write() = Some(hash);
 
+		if !create_gap {
+			self.backend.no_gap(&mut operation.op);
+		}
+
 		let result = self.execute_and_import_block(
 			operation,
 			origin,