From ed59327febab3c57fb8dc1cb43045da0d7169f77 Mon Sep 17 00:00:00 2001
From: Paul Hauner <paul@paulhauner.com>
Date: Tue, 28 Jan 2020 11:34:29 +1100
Subject: [PATCH] Add check to ensure parent block is in fork choice

---
 beacon_node/beacon_chain/src/beacon_chain.rs  | 25 +++++++++++++++++--
 .../src/sync/range_sync/batch_processing.rs   |  2 +-
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/beacon_node/beacon_chain/src/beacon_chain.rs b/beacon_node/beacon_chain/src/beacon_chain.rs
index 75ca098b71..5eeae7b5ee 100644
--- a/beacon_node/beacon_chain/src/beacon_chain.rs
+++ b/beacon_node/beacon_chain/src/beacon_chain.rs
@@ -58,8 +58,11 @@ const HEAD_LOCK_TIMEOUT: Duration = Duration::from_secs(1);
 pub enum BlockProcessingOutcome {
     /// Block was valid and imported into the block graph.
     Processed { block_root: Hash256 },
-    /// The blocks parent_root is unknown.
-    ParentUnknown { parent: Hash256 },
+    /// The parent block was unknown.
+    ParentUnknown {
+        parent: Hash256,
+        reference_location: String,
+    },
     /// The block slot is greater than the present slot.
     FutureSlot {
         present_slot: Slot,
@@ -1223,6 +1226,23 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
             });
         }
 
+        // Reject any block if its parent is not known to fork choice.
+        //
+        // A block that is not in fork choice is either:
+        //
+        //  - Not yet imported: we should reject this block because we should only import a child
+        //  after its parent has been fully imported.
+        //  - Pre-finalized: if the parent block is _prior_ to finalization, we should ignore it
+        //  because it will revert finalization. Note that the finalized block is stored in fork
+        //  choice, so we will not reject any child of the finalized block (this is relevant during
+        //  genesis).
+        if !self.fork_choice.contains_block(&block.parent_root) {
+            return Ok(BlockProcessingOutcome::ParentUnknown {
+                parent: block.parent_root,
+                reference_location: "fork_choice".to_string(),
+            });
+        }
+
         let block_root_timer = metrics::start_timer(&metrics::BLOCK_PROCESSING_BLOCK_ROOT);
 
         let block_root = block.canonical_root();
@@ -1260,6 +1280,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
                 None => {
                     return Ok(BlockProcessingOutcome::ParentUnknown {
                         parent: block.parent_root,
+                        reference_location: "database".to_string(),
                     });
                 }
             };
diff --git a/beacon_node/network/src/sync/range_sync/batch_processing.rs b/beacon_node/network/src/sync/range_sync/batch_processing.rs
index 27c4fb295e..a054b6b31a 100644
--- a/beacon_node/network/src/sync/range_sync/batch_processing.rs
+++ b/beacon_node/network/src/sync/range_sync/batch_processing.rs
@@ -70,7 +70,7 @@ fn process_batch<T: BeaconChainTypes>(
                         );
                         successful_block_import = true;
                     }
-                    BlockProcessingOutcome::ParentUnknown { parent } => {
+                    BlockProcessingOutcome::ParentUnknown { parent, .. } => {
                         // blocks should be sequential and all parents should exist
                         warn!(
                             log, "Parent block is unknown";