Inform sync explicitly about new best block (#7604)

* Inform sync explicitly about new best block

Instead of "fishing" the new best block out of the processed blocks, we
now tell sync directly that there is a new best block. It also makes
sure that we update the corresponding sync handshake to the new best
block. This is required for parachains as they first import blocks and
declare the new best block after being made aware of it by the relay chain.

* Adds test

* Make sure async stuff had time to run

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

@@ -779,3 +779,38 @@ fn wait_until_deferred_block_announce_validation_is_ready() {
 		net.block_until_idle();
 	}
 }
+
+/// When we don't inform the sync protocol about the best block, a node will not sync from us as the
+/// handshake is not does not contain our best block.
+#[test]
+fn sync_to_tip_requires_that_sync_protocol_is_informed_about_best_block() {
+	sp_tracing::try_init_simple();
+	log::trace!(target: "sync", "Test");
+	let mut net = TestNet::new(1);
+
+	// Produce some blocks
+	let block_hash = net.peer(0).push_blocks_at_without_informing_sync(BlockId::Number(0), 3, true);
+
+	// Add a node and wait until they are connected
+	net.add_full_peer_with_config(Default::default());
+	net.block_until_connected();
+	net.block_until_idle();
+
+	// The peer should not have synced the block.
+	assert!(!net.peer(1).has_block(&block_hash));
+
+	// Make sync protocol aware of the best block
+	net.peer(0).network_service().new_best_block_imported(block_hash, 3);
+	net.block_until_idle();
+
+	// Connect another node that should now sync to the tip
+	net.add_full_peer_with_config(Default::default());
+	net.block_until_connected();
+
+	while !net.peer(2).has_block(&block_hash) {
+		net.block_until_idle();
+	}
+
+	// However peer 1 should still not have the block.
+	assert!(!net.peer(1).has_block(&block_hash));
+}