Fix error messages generated when peer is syncing with empty blockchain

libraries/app/application.cpp

@@ -382,6 +382,10 @@ namespace detail {
             ilog("Got block #${n} from network", ("n", blk_msg.block.block_num()));
 
          try {
+            // TODO: in the case where this block is valid but on a fork that's too old for us to switch to,
+            // you can help the network code out by throwing a block_older_than_undo_history exception.
+            // when the net code sees that, it will stop trying to push blocks from that chain, but
+            // leave that peer connected so that they can get sync blocks from us
             bool result = _chain_db->push_block(blk_msg.block, _is_block_producer ? database::skip_nothing : database::skip_transaction_signatures);
 
             // the block was accepted, so we now know all of the transactions contained in the block
@@ -444,7 +448,7 @@ namespace detail {
                                                      uint32_t& remaining_item_count,
                                                      uint32_t limit) override
       { try {
-         vector<block_id_type>  result;
+         vector<block_id_type> result;
          remaining_item_count = 0;
          if( _chain_db->head_block_num() == 0 )
             return result;
@@ -452,14 +456,29 @@ namespace detail {
          result.reserve(limit);
          block_id_type last_known_block_id;
          
-         for (const item_hash_t& block_id_in_synopsis : boost::adaptors::reverse(blockchain_synopsis))
+         if (blockchain_synopsis.empty() ||
-           if (block_id_in_synopsis == block_id_type() ||
+             (blockchain_synopsis.size() == 1 && blockchain_synopsis[0] == block_id_type()))
-               (_chain_db->is_known_block(block_id_in_synopsis) && is_included_block(block_id_in_synopsis)))
+         {
-           {
+           // peer has sent us an empty synopsis meaning they have no blocks.
-             last_known_block_id = block_id_in_synopsis;
+           // A bug in old versions would cause them to send a synopsis containing block 000000000
-             break;
+           // when they had an empty blockchain, so pretend they sent the right thing here.
-           }
 
+           // do nothing, leave last_known_block_id set to zero
+         }
+         else
+         {
+           bool found_a_block_in_synopsis = false;
+           for (const item_hash_t& block_id_in_synopsis : boost::adaptors::reverse(blockchain_synopsis))
+             if (block_id_in_synopsis == block_id_type() ||
+                 (_chain_db->is_known_block(block_id_in_synopsis) && is_included_block(block_id_in_synopsis)))
+             {
+               last_known_block_id = block_id_in_synopsis;
+               found_a_block_in_synopsis = true;
+               break;
+             }
+           if (!found_a_block_in_synopsis)
+             FC_THROW_EXCEPTION(graphene::net::peer_is_on_an_unreachable_fork, "Unable to provide a list of blocks starting at any of the blocks in peer's synopsis");
+         }
          for( uint32_t num = block_header::num_from_id(last_known_block_id);
               num <= _chain_db->head_block_num() && result.size() < limit;
               ++num )

libraries/net/include/graphene/net/exceptions.hpp

@@ -26,5 +26,6 @@ namespace graphene { namespace net {
    FC_DECLARE_DERIVED_EXCEPTION( insufficient_relay_fee,                graphene::net::net_exception, 90002, "insufficient relay fee" );
    FC_DECLARE_DERIVED_EXCEPTION( already_connected_to_requested_peer,   graphene::net::net_exception, 90003, "already connected to requested peer" );
    FC_DECLARE_DERIVED_EXCEPTION( block_older_than_undo_history,         graphene::net::net_exception, 90004, "block is older than our undo history allows us to process" );
+   FC_DECLARE_DERIVED_EXCEPTION( peer_is_on_an_unreachable_fork,        graphene::net::net_exception, 90005, "peer is on another fork" );
 
 } }

libraries/net/node.cpp

@@ -2162,9 +2162,20 @@ namespace graphene { namespace net { namespace detail {
       }
 
       blockchain_item_ids_inventory_message reply_message;
-      reply_message.item_hashes_available = _delegate->get_block_ids(fetch_blockchain_item_ids_message_received.blockchain_synopsis,
-                                                                     reply_message.total_remaining_item_count );
       reply_message.item_type = fetch_blockchain_item_ids_message_received.item_type;
+      reply_message.total_remaining_item_count = 0;
+      try
+      {
+        reply_message.item_hashes_available = _delegate->get_block_ids(fetch_blockchain_item_ids_message_received.blockchain_synopsis,
+                                                                       reply_message.total_remaining_item_count);
+      }
+      catch (const peer_is_on_an_unreachable_fork&)
+      {
+        dlog("Peer is on a fork and there's no set of blocks we can provide to switch them to our fork");
+        // we reply with an empty list as if we had an empty blockchain; 
+        // we don't want to disconnect because they may be able to provide
+        // us with blocks on their chain
+      }
 
       bool disconnect_from_inhibited_peer = false;
       // if our client doesn't have any items after the item the peer requested, it will send back