From 9678f77bb57b5d44f84418df04056cb2c5534d4d Mon Sep 17 00:00:00 2001
From: Paul Hauner <paul@paulhauner.com>
Date: Tue, 28 Sep 2021 17:18:12 +1000
Subject: [PATCH] Prevent infinite loops

---
 beacon_node/execution_layer/src/engine_api.rs |  1 +
 beacon_node/execution_layer/src/lib.rs        | 11 ++++++++++-
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/beacon_node/execution_layer/src/engine_api.rs b/beacon_node/execution_layer/src/engine_api.rs
index aa2b2227f2..4d34628f95 100644
--- a/beacon_node/execution_layer/src/engine_api.rs
+++ b/beacon_node/execution_layer/src/engine_api.rs
@@ -22,6 +22,7 @@ pub enum Error {
     IsSyncing,
     ExecutionBlockNotFound(Hash256),
     ExecutionHeadBlockNotFound,
+    ParentHashEqualsBlockHash(Hash256),
 }
 
 impl From<reqwest::Error> for Error {
diff --git a/beacon_node/execution_layer/src/lib.rs b/beacon_node/execution_layer/src/lib.rs
index 77b0b3058f..2173acf355 100644
--- a/beacon_node/execution_layer/src/lib.rs
+++ b/beacon_node/execution_layer/src/lib.rs
@@ -309,16 +309,25 @@ impl ExecutionLayer {
 
                 self.execution_blocks().await.put(block.block_hash, block);
 
+                // TODO(merge): This function can theoretically loop indefinitely, as per the
+                // specification. We should consider how to fix this. See discussion:
+                //
+                // https://discord.com/channels/595666850260713488/692062809701482577/892307257205878785
                 loop {
                     if block.total_difficulty >= self.terminal_total_difficulty() {
                         ttd_exceeding_block = Some(block.block_hash);
 
+                        // Try to prevent infinite loops.
+                        if block.block_hash == block.parent_hash {
+                            return Err(ApiError::ParentHashEqualsBlockHash(block.block_hash));
+                        }
+
                         block = self
                             .get_pow_block(engine, block.parent_hash)
                             .await?
                             .ok_or(ApiError::ExecutionBlockNotFound(block.parent_hash))?;
                     } else {
-                        return Ok::<_, ApiError>(ttd_exceeding_block);
+                        return Ok(ttd_exceeding_block);
                     }
                 }
             })