Check for parent being on chain

When retrieveing the ready blocks, verify that the parent
of the first ready block is on chain. If the parent is not
on chain, we are downloading from a fork. In this case,
keep downloading until we have a parent on chain (common
ancestor)

substrate/client/network/sync/src/blocks.rs

@@ -212,6 +212,30 @@ impl<B: BlockT> BlockCollection<B> {
 		ready
 	}
 
+	/// Returns the block header of the first block that is ready for importing.
+	/// `from` is the maximum block number for the start of the range that we are interested in.
+	/// The function will return None if the first block ready is higher than `from`.
+	pub fn first_ready_block_hdr(&self, from: NumberFor<B>) -> Option<B::Header> {
+		let mut prev = from;
+		for (&start, range_data) in &self.blocks {
+			if start > prev {
+				break
+			}
+
+			match range_data {
+				BlockRangeState::Complete(blocks) => {
+					let len = (blocks.len() as u32).into();
+					prev = start + len;
+					if let Some(BlockData { block, .. }) = blocks.first() {
+						return block.header.clone()
+					}
+				},
+				_ => continue,
+			}
+		}
+		None
+	}
+
 	pub fn clear_queued(&mut self, hash: &B::Hash) {
 		if let Some((from, to)) = self.queued_blocks.remove(hash) {
 			let mut block_num = from;