peer: Allow OnVersion callback to reject peer.

This modifies the OnVersion callback to allow a reject message to be
returned in which case the message will be sent to the peer and the peer
will be disconnected.

Backported from Decred.

peer/example_test.go

@@ -1,4 +1,5 @@
-// Copyright (c) 2015-2016 The btcsuite developers
+// Copyright (c) 2015-2018 The btcsuite developers
+// Copyright (c) 2016-2018 The Decred developers
 // Use of this source code is governed by an ISC
 // license that can be found in the LICENSE file.
 
@@ -72,8 +73,9 @@ func Example_newOutboundPeer() {
 		Services:         0,
 		TrickleInterval:  time.Second * 10,
 		Listeners: peer.MessageListeners{
-			OnVersion: func(p *peer.Peer, msg *wire.MsgVersion) {
+			OnVersion: func(p *peer.Peer, msg *wire.MsgVersion) *wire.MsgReject {
 				fmt.Println("outbound: received version")
+				return nil
 			},
 			OnVerAck: func(p *peer.Peer, msg *wire.MsgVerAck) {
 				verack <- struct{}{}

peer/peer.go

@@ -187,7 +187,9 @@ type MessageListeners struct {
 	OnMerkleBlock func(p *Peer, msg *wire.MsgMerkleBlock)
 
 	// OnVersion is invoked when a peer receives a version bitcoin message.
-	OnVersion func(p *Peer, msg *wire.MsgVersion)
+	// The caller may return a reject message in which case the message will
+	// be sent to the peer and the peer will be disconnected.
+	OnVersion func(p *Peer, msg *wire.MsgVersion) *wire.MsgReject
 
 	// OnVerAck is invoked when a peer receives a verack bitcoin message.
 	OnVerAck func(p *Peer, msg *wire.MsgVerAck)
@@ -1944,7 +1946,11 @@ func (p *Peer) readRemoteVersionMsg() error {
 
 	// Invoke the callback if specified.
 	if p.cfg.Listeners.OnVersion != nil {
-		p.cfg.Listeners.OnVersion(p, msg)
+		rejectMsg := p.cfg.Listeners.OnVersion(p, msg)
+		if rejectMsg != nil {
+			_ = p.writeMessage(rejectMsg, wire.LatestEncoding)
+			return errors.New(rejectMsg.Reason)
+		}
 	}
 
 	// Notify and disconnect clients that have a protocol version that is

peer/peer_test.go

@@ -432,8 +432,9 @@ func TestPeerListeners(t *testing.T) {
 			OnMerkleBlock: func(p *peer.Peer, msg *wire.MsgMerkleBlock) {
 				ok <- msg
 			},
-			OnVersion: func(p *peer.Peer, msg *wire.MsgVersion) {
+			OnVersion: func(p *peer.Peer, msg *wire.MsgVersion) *wire.MsgReject {
 				ok <- msg
+				return nil
 			},
 			OnVerAck: func(p *peer.Peer, msg *wire.MsgVerAck) {
 				verack <- struct{}{}

server.go

@@ -1,5 +1,5 @@
 // Copyright (c) 2013-2017 The btcsuite developers
-// Copyright (c) 2015-2017 The Decred developers
+// Copyright (c) 2015-2018 The Decred developers
 // Use of this source code is governed by an ISC
 // license that can be found in the LICENSE file.
 
@@ -388,11 +388,11 @@ func (sp *serverPeer) addBanScore(persistent, transient uint32, reason string) {
 // OnVersion is invoked when a peer receives a version bitcoin message
 // and is used to negotiate the protocol version details as well as kick start
 // the communications.
-func (sp *serverPeer) OnVersion(_ *peer.Peer, msg *wire.MsgVersion) {
+func (sp *serverPeer) OnVersion(_ *peer.Peer, msg *wire.MsgVersion) *wire.MsgReject {
 	// Ignore peers that have a protcol version that is too old.  The peer
 	// negotiation logic will disconnect it after this callback returns.
 	if msg.ProtocolVersion < int32(peer.MinAcceptableProtocolVersion) {
-		return
+		return nil
 	}
 
 	// Add the remote peer time as a sample for creating an offset against
@@ -424,7 +424,7 @@ func (sp *serverPeer) OnVersion(_ *peer.Peer, msg *wire.MsgVersion) {
 			if err != nil {
 				peerLog.Errorf("Unable to query for segwit "+
 					"soft-fork state: %v", err)
-				return
+				return nil
 			}
 
 			if segwitActive && !sp.IsWitnessEnabled() {
@@ -432,7 +432,7 @@ func (sp *serverPeer) OnVersion(_ *peer.Peer, msg *wire.MsgVersion) {
 					"peer %v, isn't segwit enabled and "+
 					"we need more segwit enabled peers", sp)
 				sp.Disconnect()
-				return
+				return nil
 			}
 
 			// TODO(davec): Only do this if not doing the initial block
@@ -463,6 +463,7 @@ func (sp *serverPeer) OnVersion(_ *peer.Peer, msg *wire.MsgVersion) {
 
 	// Add valid peer to the server.
 	sp.server.AddPeer(sp)
+	return nil
 }
 
 // OnMemPool is invoked when a peer receives a mempool bitcoin message.