Warp sync part II (#9284)

* Gap sync

* Gap epoch test

* Simplified network requests

* Update client/db/src/utils.rs

Co-authored-by: cheme <emericchevalier.pro@gmail.com>

* Fixed v1 migration and added some comments

* Next epoch is always regular

* Removed fork tree change

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Added a comment and converted assert to error

Co-authored-by: cheme <emericchevalier.pro@gmail.com>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/client/db/src/utils.rs

@@ -54,6 +54,8 @@ pub mod meta_keys {
 	pub const FINALIZED_BLOCK: &[u8; 5] = b"final";
 	/// Last finalized state key.
 	pub const FINALIZED_STATE: &[u8; 6] = b"fstate";
+	/// Block gap.
+	pub const BLOCK_GAP: &[u8; 3] = b"gap";
 	/// Meta information prefix for list-based caches.
 	pub const CACHE_META_PREFIX: &[u8; 5] = b"cache";
 	/// Meta information for changes tries key.
@@ -81,6 +83,8 @@ pub struct Meta<N, H> {
 	pub genesis_hash: H,
 	/// Finalized state, if any
 	pub finalized_state: Option<(H, N)>,
+	/// Block gap, start and end inclusive, if any.
+	pub block_gap: Option<(N, N)>,
 }
 
 /// A block lookup key: used for canonical lookup from block number to hash
@@ -527,6 +531,7 @@ where
 				finalized_number: Zero::zero(),
 				genesis_hash: Default::default(),
 				finalized_state: None,
+				block_gap: None,
 			}),
 	};
 
@@ -541,7 +546,7 @@ where
 				"Opened blockchain db, fetched {} = {:?} ({})",
 				desc,
 				hash,
-				header.number()
+				header.number(),
 			);
 			Ok((hash, *header.number()))
 		} else {
@@ -558,6 +563,10 @@ where
 	} else {
 		None
 	};
+	let block_gap = db
+		.get(COLUMN_META, meta_keys::BLOCK_GAP)
+		.and_then(|d| Decode::decode(&mut d.as_slice()).ok());
+	debug!(target: "db", "block_gap={:?}", block_gap);
 
 	Ok(Meta {
 		best_hash,
@@ -566,6 +575,7 @@ where
 		finalized_number,
 		genesis_hash,
 		finalized_state,
+		block_gap,
 	})
 }