From df544635219ab58a5e6e084d9d9f8d5af922bb22 Mon Sep 17 00:00:00 2001
From: Aaro Altonen <48052676+altonen@users.noreply.github.com>
Date: Tue, 8 Nov 2022 00:57:29 +0200
Subject: [PATCH] Do not update peer information if ancestor search is in
 progress (#12631)

* Do not update peer information if ancestor search is in progress

If block announcement is received from a peer while ancestor search
for that same peer is still in progress, do not update the peer's best
hash and best number as that causes the ancestor search to yield
different information from what was expected and can cause, for example,
a fork of lower height not be be downloaded.

* Block until peers are in sync
---
 substrate/client/network/sync/src/lib.rs  | 10 +++++-----
 substrate/client/network/test/src/sync.rs |  4 ++++
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/substrate/client/network/sync/src/lib.rs b/substrate/client/network/sync/src/lib.rs
index 7c48483595..75ecb9322c 100644
--- a/substrate/client/network/sync/src/lib.rs
+++ b/substrate/client/network/sync/src/lib.rs
@@ -2043,17 +2043,17 @@ where
 			return PollBlockAnnounceValidation::Nothing { is_best, who, announce }
 		};
 
+		if let PeerSyncState::AncestorSearch { .. } = peer.state {
+			trace!(target: "sync", "Peer state is ancestor search.");
+			return PollBlockAnnounceValidation::Nothing { is_best, who, announce }
+		}
+
 		if is_best {
 			// update their best block
 			peer.best_number = number;
 			peer.best_hash = hash;
 		}
 
-		if let PeerSyncState::AncestorSearch { .. } = peer.state {
-			trace!(target: "sync", "Peer state is ancestor search.");
-			return PollBlockAnnounceValidation::Nothing { is_best, who, announce }
-		}
-
 		// If the announced block is the best they have and is not ahead of us, our common number
 		// is either one further ahead or it's the one they just announced, if we know about it.
 		if is_best {
diff --git a/substrate/client/network/test/src/sync.rs b/substrate/client/network/test/src/sync.rs
index bbba3bc6de..4515677d0b 100644
--- a/substrate/client/network/test/src/sync.rs
+++ b/substrate/client/network/test/src/sync.rs
@@ -549,6 +549,10 @@ fn syncs_header_only_forks() {
 	while !net.peer(1).has_block(small_hash) {
 		net.block_until_idle();
 	}
+
+	net.block_until_sync();
+	assert_eq!(net.peer(0).client().info().best_hash, net.peer(1).client().info().best_hash);
+	assert_ne!(small_hash, net.peer(0).client().info().best_hash);
 }
 
 #[test]