server/mempool: Evict orphans on peer disconnect.

This removes any remaining orphan transactions that were sent by a peer
when it disconnects since it is extremely unlikely that the missing
parents will ever materialize from elsewhere.

log.go

@@ -220,3 +220,12 @@ func directionString(inbound bool) string {
 	}
 	return "outbound"
 }
+
+// pickNoun returns the singular or plural form of a noun depending
+// on the count n.
+func pickNoun(n uint64, singular, plural string) string {
+	if n == 1 {
+		return singular
+	}
+	return plural
+}

server.go

@@ -1596,7 +1596,7 @@ func (s *server) outboundPeerConnected(c *connmgr.ConnReq, conn net.Conn) {
 }
 
 // peerDoneHandler handles peer disconnects by notifiying the server that it's
-// done.
+// done along with other performing other desirable cleanup.
 func (s *server) peerDoneHandler(sp *serverPeer) {
 	sp.WaitForDisconnect()
 	s.donePeers <- sp
@@ -1604,6 +1604,14 @@ func (s *server) peerDoneHandler(sp *serverPeer) {
 	// Only tell block manager we are gone if we ever told it we existed.
 	if sp.VersionKnown() {
 		s.blockManager.DonePeer(sp)
+
+		// Evict any remaining orphans that were sent by the peer.
+		numEvicted := s.txMemPool.RemoveOrphansByTag(mempool.Tag(sp.ID()))
+		if numEvicted > 0 {
+			txmpLog.Debugf("Evicted %d %s from peer %v (id %d)",
+				numEvicted, pickNoun(numEvicted, "orphan",
+					"orphans"), sp, sp.ID())
+		}
 	}
 	close(sp.quit)
 }