From d8023a5e399128f1a7a72f1c328cd2aa6caa512b Mon Sep 17 00:00:00 2001
From: =?utf8?q?luffyma=28=E9=A9=AC=E6=9E=97=29?= <luffyma@tencent.com>
Date: Wed, 10 May 2017 11:04:31 +0800
Subject: [PATCH] bug fix: race condition between outgoing connection and
 incoming connection

---
 client.go  |  8 ++++----
 torrent.go | 13 ++++++++++++-
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/client.go b/client.go
index e49f39bb..a24e0aeb 100644
--- a/client.go
+++ b/client.go
@@ -899,7 +899,7 @@ func (cl *Client) runInitiatedHandshookConn(c *connection, t *Torrent) {
 		cl.dopplegangerAddrs[addr] = struct{}{}
 		return
 	}
-	cl.runHandshookConn(c, t)
+	cl.runHandshookConn(c, t, true)
 }
 
 func (cl *Client) runReceivedConn(c *connection) {
@@ -926,14 +926,14 @@ func (cl *Client) runReceivedConn(c *connection) {
 		// doppleganger.
 		return
 	}
-	cl.runHandshookConn(c, t)
+	cl.runHandshookConn(c, t, false)
 }
 
-func (cl *Client) runHandshookConn(c *connection, t *Torrent) {
+func (cl *Client) runHandshookConn(c *connection, t *Torrent, outgoing bool) {
 	c.conn.SetWriteDeadline(time.Time{})
 	c.r = deadlineReader{c.conn, c.r}
 	completedHandshakeConnectionFlags.Add(c.connectionFlags(), 1)
-	if !t.addConnection(c) {
+	if !t.addConnection(c, outgoing) {
 		return
 	}
 	defer t.dropConnection(c)
diff --git a/torrent.go b/torrent.go
index 4d36209c..ba350025 100644
--- a/torrent.go
+++ b/torrent.go
@@ -1296,7 +1296,7 @@ func (t *Torrent) numTotalPeers() int {
 }
 
 // Returns true if the connection is added.
-func (t *Torrent) addConnection(c *connection) bool {
+func (t *Torrent) addConnection(c *connection, outgoing bool) bool {
 	if t.cl.closed.IsSet() {
 		return false
 	}
@@ -1305,8 +1305,19 @@ func (t *Torrent) addConnection(c *connection) bool {
 	}
 	for c0 := range t.conns {
 		if c.PeerID == c0.PeerID {
+			// Retain the connection from lower peer ID to higher.
+			lower := string(t.cl.peerID[:]) < string(c.PeerID[:])
+			if (outgoing && lower) || (!outgoing && !lower) {
+				c0.Close()
+				t.deleteConnection(c0)
+				duplicateClientConns.Add(1)
+				log.Printf("Drop connection: %s, %s, %s", t.name(), c0.localAddr(), c0.remoteAddr())
+				continue
+			}
+
 			// Already connected to a client with that ID.
 			duplicateClientConns.Add(1)
+			log.Printf("Drop connection: %s, %s, %s", t.name(), c.localAddr(), c.remoteAddr())
 			return false
 		}
 	}
-- 
2.48.1