cryptonote_protocol: retry stale spans early

Connections can be dropped by the net_node layer,
unbeknownst to cryptonote_protocol, which would then
not flush any spans scheduled to that connection,
which would cause it to be only downloaded again
once it becomes the next span (possibly after a small
delay if it had been requested less than 5 seconds
ago).

src/cryptonote_protocol/block_queue.cpp

@@ -76,6 +76,22 @@ void block_queue::flush_spans(const boost::uuids::uuid &connection_id, bool all)
   }
 }
 
+void block_queue::flush_stale_spans(const std::set<boost::uuids::uuid> &live_connections)
+{
+  boost::unique_lock<boost::recursive_mutex> lock(mutex);
+  block_map::iterator i = blocks.begin();
+  if (i != blocks.end() && is_blockchain_placeholder(*i))
+    ++i;
+  while (i != blocks.end())
+  {
+    block_map::iterator j = i++;
+    if (live_connections.find(j->connection_id) == live_connections.end() && j->blocks.size() == 0)
+    {
+      blocks.erase(j);
+    }
+  }
+}
+
 void block_queue::remove_span(uint64_t start_block_height)
 {
   boost::unique_lock<boost::recursive_mutex> lock(mutex);

src/cryptonote_protocol/block_queue.h

@@ -70,6 +70,7 @@ namespace cryptonote
     void add_blocks(uint64_t height, std::list<cryptonote::block_complete_entry> bcel, const boost::uuids::uuid &connection_id, float rate, size_t size);
     void add_blocks(uint64_t height, uint64_t nblocks, const boost::uuids::uuid &connection_id, boost::posix_time::ptime time = boost::date_time::min_date_time);
     void flush_spans(const boost::uuids::uuid &connection_id, bool all = false);
+    void flush_stale_spans(const std::set<boost::uuids::uuid> &live_connections);
     void remove_span(uint64_t start_block_height);
     void remove_spans(const boost::uuids::uuid &connection_id, uint64_t start_block_height);
     void mark_last_block(uint64_t last_block_height);

src/cryptonote_protocol/cryptonote_protocol_handler.inl

@@ -1189,6 +1189,14 @@ skip:
   {
     m_block_queue.mark_last_block(m_core.get_current_blockchain_height() - 1);
 
+    // flush stale spans
+    std::set<boost::uuids::uuid> live_connections;
+    m_p2p->for_each_connection([&](cryptonote_connection_context& context, nodetool::peerid_type peer_id, uint32_t support_flags)->bool{
+      live_connections.insert(context.m_connection_id);
+      return true;
+    });
+    m_block_queue.flush_stale_spans(live_connections);
+
     // if we don't need to get next span, and the block queue is full enough, wait a bit
     bool start_from_current_chain = false;
     if (!force_next_span)