From c770daa51fd403b2a1f5ed9cb7ff72538fabe3f9 Mon Sep 17 00:00:00 2001
From: teor <teor@riseup.net>
Date: Fri, 4 Sep 2020 08:08:19 +1000
Subject: [PATCH] If the first ExtendTips hash is bad, discard it and re-check
 (#992)

---
 zebrad/src/commands/start/sync.rs | 37 ++++++++++++++++++++++++++-----
 1 file changed, 32 insertions(+), 5 deletions(-)

diff --git a/zebrad/src/commands/start/sync.rs b/zebrad/src/commands/start/sync.rs
index 6f0413f9b..cd7a08977 100644
--- a/zebrad/src/commands/start/sync.rs
+++ b/zebrad/src/commands/start/sync.rs
@@ -284,14 +284,41 @@ where
                 match res.map_err::<Report, _>(|e| eyre!(e)) {
                     Ok(zn::Response::BlockHashes(hashes)) => {
                         tracing::debug!(first = ?hashes.first(), len = ?hashes.len());
+                        tracing::trace!(?hashes);
 
-                        let unknown_hashes = match hashes.split_first() {
-                            None => continue,
-                            Some((expected_hash, rest)) if expected_hash == &tip.expected_next => {
+                        // zcashd sometimes appends an unrelated hash at the
+                        // start or end of its response. Check the first hash
+                        // against the previous response, and discard mismatches.
+                        let unknown_hashes = match hashes.as_slice() {
+                            [expected_hash, rest @ ..] if expected_hash == &tip.expected_next => {
                                 rest
                             }
-                            Some((other_hash, _rest)) => {
-                                tracing::debug!(?other_hash, ?tip.expected_next, "discarding response with unexpected next hash");
+                            // If the first hash doesn't match, retry with the second.
+                            [first_hash, expected_hash, rest @ ..]
+                                if expected_hash == &tip.expected_next =>
+                            {
+                                tracing::debug!(?first_hash,
+                                                ?tip.expected_next,
+                                                ?tip.tip,
+                                                "unexpected first hash, but the second matches: using the hashes after the match");
+                                rest
+                            }
+                            // We ignore these responses
+                            [] => continue,
+                            [single_hash] => {
+                                tracing::debug!(?single_hash,
+                                                ?tip.expected_next,
+                                                ?tip.tip,
+                                                "discarding response containing a single unexpected hash");
+                                continue;
+                            }
+                            [first_hash, second_hash, rest @ ..] => {
+                                tracing::debug!(?first_hash,
+                                                ?second_hash,
+                                                rest_len = ?rest.len(),
+                                                ?tip.expected_next,
+                                                ?tip.tip,
+                                                "discarding response that starts with two unexpected hashes");
                                 continue;
                             }
                         };