p2p: update peer's height on new block & widen window for relay

- We select peers to relay to based on their latest known sync height.
- This change:
A) makes sure to update the peer's latest known sync height upon
validating the peer's newly relayed blocks.
B) Allows for a wider window so that if we haven't yet received the
peer's block or finished validating it, then the peer can still be
a valid candidate to receive our new txs.

(cherry picked from commit 437c45bebd621b1fdbcf0b19a4437d002b51ef8e)

src/cryptonote_protocol/cryptonote_protocol_handler.inl

@@ -618,8 +618,10 @@ namespace cryptonote
     }
 
     // Log block info
+    const uint64_t new_block_height = get_block_height(new_block);
+    const uint64_t peer_height = arg.current_blockchain_height;
     MLOG_P2P_MESSAGE(context << "Received NOTIFY_NEW_FLUFFY_BLOCK " << new_block_hash << " (height "
-      << arg.current_blockchain_height << ", " << arg.b.txs.size() << " txes)");
+      << new_block_height << ", " << arg.b.txs.size() << " txes, peer's height: " << peer_height << ")");
 
     // 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
@@ -708,7 +710,7 @@ namespace cryptonote
           MDEBUG("  tx " << new_block.tx_hashes[txidx]);
         NOTIFY_REQUEST_FLUFFY_MISSING_TX::request missing_tx_req;
         missing_tx_req.block_hash = new_block_hash;
-        missing_tx_req.current_blockchain_height = arg.current_blockchain_height;
+        missing_tx_req.current_blockchain_height = new_block_height + 1;
         missing_tx_req.missing_tx_indices = std::move(need_tx_indices);
 
         // Post NOTIFY_REQUEST_FLUFFY_MISSING_TX request to peer
@@ -725,6 +727,7 @@ namespace cryptonote
     }
     else if( bvc.m_added_to_main_chain )
     {
+      arg.current_blockchain_height = new_block_height + 1;
       // Relay an empty block
       arg.b.txs.clear();
       relay_block(arg, context);
@@ -744,6 +747,15 @@ namespace cryptonote
       MLOG_PEER_STATE("requesting chain");
     }
 
+    if (bvc.m_added_to_main_chain || bvc.m_already_exists)
+    {
+      // Update peer's sync height using this block we just validated
+      // Note: peer_height is not guaranteed to be the height of the block we just validated + 1.
+      // See https://github.com/monero-project/monero/pull/11048#discussion_r3720736824
+      if (peer_height == (new_block_height + 1))
+        context.m_remote_blockchain_height = peer_height;
+    }
+
     // load json & DNS checkpoints every 10min/hour respectively,
     // and verify them with respect to what blocks we already have
     CHECK_AND_ASSERT_MES(m_core.update_checkpoints(), 1, "One or more checkpoints loaded from json or dns conflicted with existing checkpoints.");

src/cryptonote_protocol/levin_notify.cpp

@@ -147,7 +147,9 @@ namespace levin
          of waiting in here. */
 
       p2p.foreach_connection([&outs, blockchain_height] (detail::p2p_context& context) {
-        if (!context.m_is_income && context.m_remote_blockchain_height >= blockchain_height)
+        // Give a +2 allowable window for candidate peers, since we may not have updated the peer's
+        // known sync height yet (e.g. if we haven't received or finished processing their new block(s) yet)
+        if (!context.m_is_income && (context.m_remote_blockchain_height + 2) >= blockchain_height)
           outs.emplace_back(context.m_connection_id);
         return true;
       });