Support block gap created by fast sync (#5703)

This is part 2 of
#5406 (comment),
properly handling the block gap generated during fast sync.

Although #5406 remains unresolved due to the known issues in #5663, I
decided to open up this PR earlier than later to speed up the overall
progress. I've tested the fast sync locally with this PR, and it appears
to be functioning well. (I was doing a fast sync from a discontinued
archive node locally, thus the issue highlighted in
#5663 (comment)
was bypassed exactly.)

Once the edge cases in #5663 are addressed, we can move forward by
removing the body attribute from the LightState block request and
complete the work on #5406. The changes in this PR are incremental, so
reviewing commit by commit should provide the best clarity.

cc @dmitry-markin

---------

Co-authored-by: Bastian Köcher <git@kchr.de>

prdoc/pr_5703.prdoc

@@ -0,0 +1,13 @@
+title: Properly handle block gap created by fast sync
+
+doc:
+  - audience: Node Dev
+    description: |
+      Implements support for handling block gaps generated during fast sync. This includes managing the creation,
+      updating, and removal of block gaps.
+      Note that this feature is not fully activated until the `body` attribute is removed from the `LightState`
+      block request in chain sync, which will occur after the issue #5406 is resolved.
+
+crates:
+  - name: sc-client-db
+    bump: patch

substrate/client/db/src/lib.rs

@@ -1486,6 +1486,7 @@ impl<Block: BlockT> Backend<Block> {
 				.map(|(n, _)| n)
 				.unwrap_or(Zero::zero());
 			let existing_header = number <= highest_leaf && self.blockchain.header(hash)?.is_some();
+			let existing_body = pending_block.body.is_some();
 
 			// blocks are keyed by number + hash.
 			let lookup_key = utils::number_and_hash_to_lookup_key(number, hash)?;
@@ -1677,6 +1678,23 @@ impl<Block: BlockT> Backend<Block> {
 						children,
 					);
 				}
+			}
+
+			let should_check_block_gap = !existing_header || !existing_body;
+
+			if should_check_block_gap {
+				let insert_new_gap =
+					|transaction: &mut Transaction<DbHash>,
+					 new_gap: BlockGap<NumberFor<Block>>,
+					 block_gap: &mut Option<BlockGap<NumberFor<Block>>>| {
+						transaction.set(columns::META, meta_keys::BLOCK_GAP, &new_gap.encode());
+						transaction.set(
+							columns::META,
+							meta_keys::BLOCK_GAP_VERSION,
+							&BLOCK_GAP_CURRENT_VERSION.encode(),
+						);
+						block_gap.replace(new_gap);
+					};
 
 				if let Some(mut gap) = block_gap {
 					match gap.gap_type {
@@ -1695,43 +1713,65 @@ impl<Block: BlockT> Backend<Block> {
 									block_gap = None;
 									debug!(target: "db", "Removed block gap.");
 								} else {
-									block_gap = Some(gap);
+									insert_new_gap(&mut transaction, gap, &mut block_gap);
 									debug!(target: "db", "Update block gap. {block_gap:?}");
-									transaction.set(
-										columns::META,
-										meta_keys::BLOCK_GAP,
-										&gap.encode(),
-									);
-									transaction.set(
-										columns::META,
-										meta_keys::BLOCK_GAP_VERSION,
-										&BLOCK_GAP_CURRENT_VERSION.encode(),
-									);
 								}
 								block_gap_updated = true;
 							},
 						BlockGapType::MissingBody => {
-							unreachable!("Unsupported block gap. TODO: https://github.com/paritytech/polkadot-sdk/issues/5406")
+							// Gap increased when syncing the header chain during fast sync.
+							if number == gap.end + One::one() && !existing_body {
+								gap.end += One::one();
+								utils::insert_number_to_key_mapping(
+									&mut transaction,
+									columns::KEY_LOOKUP,
+									number,
+									hash,
+								)?;
+								insert_new_gap(&mut transaction, gap, &mut block_gap);
+								debug!(target: "db", "Update block gap. {block_gap:?}");
+								block_gap_updated = true;
+							// Gap decreased when downloading the full blocks.
+							} else if number == gap.start && existing_body {
+								gap.start += One::one();
+								if gap.start > gap.end {
+									transaction.remove(columns::META, meta_keys::BLOCK_GAP);
+									transaction.remove(columns::META, meta_keys::BLOCK_GAP_VERSION);
+									block_gap = None;
+									debug!(target: "db", "Removed block gap.");
+								} else {
+									insert_new_gap(&mut transaction, gap, &mut block_gap);
+									debug!(target: "db", "Update block gap. {block_gap:?}");
+								}
+								block_gap_updated = true;
+							}
 						},
 					}
-				} 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(),
-						end: number - One::one(),
-						gap_type: BlockGapType::MissingHeaderAndBody,
-					};
-					transaction.set(columns::META, meta_keys::BLOCK_GAP, &gap.encode());
-					transaction.set(
-						columns::META,
-						meta_keys::BLOCK_GAP_VERSION,
-						&BLOCK_GAP_CURRENT_VERSION.encode(),
-					);
-					block_gap = Some(gap);
-					block_gap_updated = true;
-					debug!(target: "db", "Detected block gap {block_gap:?}");
+				} else if operation.create_gap {
+					if number > best_num + One::one() &&
+						self.blockchain.header(parent_hash)?.is_none()
+					{
+						let gap = BlockGap {
+							start: best_num + One::one(),
+							end: number - One::one(),
+							gap_type: BlockGapType::MissingHeaderAndBody,
+						};
+						insert_new_gap(&mut transaction, gap, &mut block_gap);
+						block_gap_updated = true;
+						debug!(target: "db", "Detected block gap (warp sync) {block_gap:?}");
+					} else if number == best_num + One::one() &&
+						self.blockchain.header(parent_hash)?.is_some() &&
+						!existing_body
+					{
+						let gap = BlockGap {
+							start: number,
+							end: number,
+							gap_type: BlockGapType::MissingBody,
+						};
+						insert_new_gap(&mut transaction, gap, &mut block_gap);
+						block_gap_updated = true;
+						debug!(target: "db", "Detected block gap (fast sync) {block_gap:?}");
+					}
 				}
 			}