cryptonote_protocol: ignore fluffy blocks past certain depth

The node searches for the most recent main-chain ancestor of
an incoming fluffy block, and ignores the incoming block if
the ancestor is too old. It does not drop the connection,
however, so syncing deep alt chains should still be possible
during the normal sync process.

Author: jeffro256

src/cryptonote_protocol/cryptonote_protocol_handler.inl

@@ -78,6 +78,7 @@
 #define DROP_ON_SYNC_WEDGE_THRESHOLD (30 * 1000000000ull) // nanoseconds
 #define LAST_ACTIVITY_STALL_THRESHOLD (2.0f) // seconds
 #define DROP_PEERS_ON_SCORE -2
+#define MAX_FLUFFY_BLOCK_DEPTH (2 * CRYPTONOTE_DEFAULT_TX_SPENDABLE_AGE)
 
 namespace cryptonote
 {
@@ -623,6 +624,43 @@ namespace cryptonote
     MLOG_P2P_MESSAGE(context << "Received NOTIFY_NEW_FLUFFY_BLOCK " << new_block_hash << " (height "
       << arg.current_blockchain_height << ", " << arg.b.txs.size() << " txes)");
 
+    // Get top block
+    uint64_t top_block_index;
+    crypto::hash top_block_id;
+    m_core.get_blockchain_top(top_block_index, top_block_id);
+
+    // Find the nominal common ancestor of this block and the main chain
+    crypto::hash common_ancestor = new_block.prev_id;
+    bool is_common_ancestor_known = false;
+    uint64_t ancestor_block_index = 0;
+    for (size_t i = 0; i < MAX_FLUFFY_BLOCK_DEPTH; ++i) // bounded loop
+    {
+      block prev_block;
+      bool is_ancestor_alt = false;
+      is_common_ancestor_known = m_core.get_block_by_hash(common_ancestor, prev_block, &is_ancestor_alt);
+      if (is_common_ancestor_known)
+        ancestor_block_index = get_block_height(prev_block);
+      if (!is_common_ancestor_known || !is_ancestor_alt)
+        break;
+      common_ancestor = prev_block.prev_id;
+    }
+
+    // Ignore the block if its common ancestor is unknown or too old
+    if (!is_common_ancestor_known || ancestor_block_index > top_block_index)
+    {
+      MDEBUG("Incoming fluffy block " << new_block_hash << " is an orphan from our perspective, ignoring...");
+      // don't drop the connection because this node may be 2 blocks ahead of us
+      return 1;
+    }
+    const uint64_t ancestor_depth = top_block_index - ancestor_block_index;
+    if (ancestor_depth > MAX_FLUFFY_BLOCK_DEPTH)
+    {
+      MDEBUG("Incoming fluffy block " << new_block_hash << " breaks off from our main chain at block "
+        << common_ancestor << ", which has a depth of " << ancestor_depth << ", ignoring...");
+      // don't drop this connection because we may want to sync its deep alternative chain
+      return 1;
+    }
+
     // Pause mining and resume after block verification to prevent wasted mining cycles while
     // validating the next block. Needs more research into if this is a DoS vector or not. Invalid
     // block validation will cause disconnects and bans, so it might not be that bad.