server: Only send fast block anns to full nodes.

Fast block announcements were added a few years ago as a way to increase
the propagation speed of new blocks that extend the main chain through
the network to significantly reduce voting latency as well as how
quickly proof-of-work miners discover new blocks.  This is desirable
behavior, however, it should only apply to full nodes since only they
are equipped to deal with the fast announcements.

As a case in point, header commitments and committed block filters can
pose a problem for lightweight clients that rely on them in the case of
fast block announcements because lightweight clients tend to request the
filters as soon as they learn about the new blocks and those filters
have not been created yet when fast block announcements are sent.

Since there is network latency involved in the request process, the
result is that sometimes the filters are available by the time the
requests from lightweight clients are received and sometimes they are
not.

In order to help prevent the aforementioned case, and other similar
ones, this modifies the code that deals with block announcements to only
send fast block announcements to peers that advertise themselves as full
nodes instead of all nodes and separates out the block announcement
logic slightly to make future enhancements easier in the process.

It also modifies the filtering of duplicate block announcements to work
on a per peer basis since not all peers receive the fast block
announcement now.

server.go

@@ -182,9 +182,10 @@ type broadcastPruneInventory struct{}
 // (it will be put into a trickle queue if false) so the relay has access to
 // that information.
 type relayMsg struct {
-	invVect   *wire.InvVect
-	data      interface{}
-	immediate bool
+	invVect     *wire.InvVect
+	data        interface{}
+	immediate   bool
+	reqServices wire.ServiceFlag
 }
 
 // naSubmission represents a network address submission from an outbound peer.
@@ -503,10 +504,6 @@ type server struct {
 	addrIndex       *indexers.AddrIndex
 	existsAddrIndex *indexers.ExistsAddrIndex
 
-	// The following fields are used to filter duplicate block announcements.
-	announcedBlockMtx sync.Mutex
-	announcedBlock    *chainhash.Hash
-
 	// These following fields are used to filter duplicate block lottery data
 	// anouncements.
 	lotteryDataBroadcastMtx sync.RWMutex
@@ -548,6 +545,10 @@ type serverPeer struct {
 	// peerNa is network address of the peer connected to.
 	peerNa    *wire.NetAddress
 	peerNaMtx sync.Mutex
+
+	// announcedBlock tracks the most recent block announced to this peer and is
+	// used to filter duplicates.
+	announcedBlock *chainhash.Hash
 }
 
 // newServerPeer returns a new serverPeer instance. The peer needs to be set by
@@ -1825,10 +1826,25 @@ func (s *server) handleRelayInvMsg(state *peerState, msg relayMsg) {
 			return
 		}
 
-		// If the inventory is a block and the peer prefers headers,
-		// generate and send a headers message instead of an inventory
-		// message.
-		if msg.invVect.Type == wire.InvTypeBlock && sp.WantsHeaders() {
+		// Ignore peers that do not have the required service flags.
+		if !hasServices(sp.Services(), msg.reqServices) {
+			return
+		}
+
+		// Filter duplicate block announcements.
+		iv := msg.invVect
+		isBlockAnnouncement := iv.Type == wire.InvTypeBlock
+		if isBlockAnnouncement {
+			if sp.announcedBlock != nil && *sp.announcedBlock == iv.Hash {
+				sp.announcedBlock = nil
+				return
+			}
+			sp.announcedBlock = &iv.Hash
+		}
+
+		// Generate and send a headers message instead of an inventory message
+		// for block announcements when the peer prefers headers.
+		if isBlockAnnouncement && sp.WantsHeaders() {
 			blockHeader, ok := msg.data.(wire.BlockHeader)
 			if !ok {
 				peerLog.Warnf("Underlying data for headers" +
@@ -1845,7 +1861,7 @@ func (s *server) handleRelayInvMsg(state *peerState, msg relayMsg) {
 			return
 		}
 
-		if msg.invVect.Type == wire.InvTypeTx {
+		if iv.Type == wire.InvTypeTx {
 			// Don't relay the transaction to the peer when it has
 			// transaction relaying disabled.
 			if sp.relayTxDisabled() {
@@ -1859,9 +1875,9 @@ func (s *server) handleRelayInvMsg(state *peerState, msg relayMsg) {
 		// It will be ignored in either case if the peer is already
 		// known to have the inventory.
 		if msg.immediate {
-			sp.QueueInventoryImmediate(msg.invVect)
+			sp.QueueInventoryImmediate(iv)
 		} else {
-			sp.QueueInventory(msg.invVect)
+			sp.QueueInventory(iv)
 		}
 	})
 }
@@ -2309,6 +2325,19 @@ func (s *server) RelayInventory(invVect *wire.InvVect, data interface{}, immedia
 	s.relayInv <- relayMsg{invVect: invVect, data: data, immediate: immediate}
 }
 
+// RelayBlockAnnouncement creates a block announcement for the passed block and
+// relays that announcement immediately to all connected peers that advertise
+// the given required services and are not already known to have it.
+func (s *server) RelayBlockAnnouncement(block *dcrutil.Block, reqServices wire.ServiceFlag) {
+	invVect := wire.NewInvVect(wire.InvTypeBlock, block.Hash())
+	s.relayInv <- relayMsg{
+		invVect:     invVect,
+		data:        block.MsgBlock().Header,
+		immediate:   true,
+		reqServices: reqServices,
+	}
+}
+
 // BroadcastMessage sends msg to all peers currently connected to the server
 // except those in the passed peers to exclude.
 func (s *server) BroadcastMessage(msg wire.Message, exclPeers ...*serverPeer) {
@@ -2428,12 +2457,8 @@ func (s *server) handleBlockchainNotification(notification *blockchain.Notificat
 			break
 		}
 
-		// Generate the inventory vector and relay it immediately.
-		iv := wire.NewInvVect(wire.InvTypeBlock, block.Hash())
-		s.RelayInventory(iv, block.MsgBlock().Header, true)
-		s.announcedBlockMtx.Lock()
-		s.announcedBlock = block.Hash()
-		s.announcedBlockMtx.Unlock()
+		// Relay the block announcement immediately to full nodes.
+		s.RelayBlockAnnouncement(block, wire.SFNodeNetwork)
 
 	// A block has been accepted into the block chain.  Relay it to other peers
 	// (will be ignored if already relayed via NTNewTipBlockChecked) and
@@ -2520,16 +2545,10 @@ func (s *server) handleBlockchainNotification(notification *blockchain.Notificat
 			}
 		}
 
-		// Generate the inventory vector and relay it immediately if not already
-		// known to have been sent in NTNewTipBlockChecked.
-		s.announcedBlockMtx.Lock()
-		sent := s.announcedBlock != nil && *s.announcedBlock == *blockHash
-		s.announcedBlock = nil
-		s.announcedBlockMtx.Unlock()
-		if !sent {
-			iv := wire.NewInvVect(wire.InvTypeBlock, blockHash)
-			s.RelayInventory(iv, block.MsgBlock().Header, true)
-		}
+		// Relay the block announcement immediately to all peers that were not
+		// already notified via NTNewTipBlockChecked.
+		const noRequiredServices = 0
+		s.RelayBlockAnnouncement(block, noRequiredServices)
 
 		// Inform the background block template generator about the accepted
 		// block.