netsync: change isSyncCandidate behavior to include pruned nodes

isSyncCandidate is now changed to return true even if the peer is a
pruned node if and only if our chaintip is within 288 blocks of the
peer.

Rationale:
Pruned nodes that signal NODE_NETWORK_LIMITED MUST serve 288 blocks from
their chaintip.  If our chaintip is within that range, this peer can be
a sync candidate even if they aren't an archival node.

netsync/manager.go

@@ -406,9 +406,30 @@ func (sm *SyncManager) isSyncCandidate(peer *peerpkg.Peer) bool {
 			log.Errorf("Unable to query for segwit "+
 				"soft-fork state: %v", err)
 		}
+		if segwitActive && !peer.IsWitnessEnabled() {
+			return false
+		}
+
 		nodeServices := peer.Services()
-		if nodeServices&wire.SFNodeNetwork != wire.SFNodeNetwork ||
-			(segwitActive && !peer.IsWitnessEnabled()) {
+
+		switch {
+		case nodeServices&wire.SFNodeNetwork == wire.SFNodeNetwork:
+			// Node is a sync candidate if it has all the blocks.
+		case nodeServices&wire.SFNodeNetworkLimited == wire.SFNodeNetworkLimited:
+			// Even if the peer is pruned, if they have the node network limited
+			// flag, they are able to serve 2 days worth of blocks from the
+			// current tip. Therefore, check if our chaintip is within that range.
+			bestHeight := sm.chain.BestSnapshot().Height
+			peerLastBlock := peer.LastBlock()
+
+			// bestHeight+1 as we need the peer to serve us the next block,
+			// not the one we already have.
+			if bestHeight+1 < peerLastBlock-wire.NodeNetworkLimitedBlockThreshold {
+				return false
+			}
+		default:
+			// If the peer isn't an archival node and it's not signaling
+			// NODE_NETWORK_LIMITED, we can't sync off of this node.
 			return false
 		}
 	}
@@ -428,7 +449,7 @@ func (sm *SyncManager) handleNewPeerMsg(peer *peerpkg.Peer) {
 
 	log.Infof("New valid peer %s (%s)", peer, peer.UserAgent())
 
-	// Initialize the peer state
+	// Initialize the peer state.
 	isSyncCandidate := sm.isSyncCandidate(peer)
 	sm.peerStates[peer] = &peerSyncState{
 		syncCandidate:   isSyncCandidate,