socket: only send client metadata once per socket (globalsign#105)

Periodic cluster synchronisation calls isMaster() which currently resends the
"client" metadata every call - the spec specifies:

	isMaster commands issued after the initial connection handshake MUST NOT
	contain handshake arguments

	https://github.com/mongodb/specifications/blob/master/source/mongodb-handshake/handshake.rst#connection-handshake

This hotfix prevents subsequent isMaster calls from sending the client metadata
again - fixes globalsign#101 and fixes globalsign#103.

Thanks to @changwoo-nam @qhenkart @canthefason @jyoon17 for spotting the initial
issue, opening tickets, and having the problem debugged with a PoC fix before I
even woke up.

cluster.go

@@ -148,16 +148,38 @@ func (cluster *mongoCluster) isMaster(socket *mongoSocket, result *isMasterResul
 	session := newSession(Monotonic, cluster, 10*time.Second)
 	session.setSocket(socket)
 
-	// provide some meta infos on the client,
-	// see https://github.com/mongodb/specifications/blob/master/source/mongodb-handshake/handshake.rst#connection-handshake
-	// for details
-	metaInfo := bson.M{"driver": bson.M{"name": "mgo", "version": "globalsign"},
-		"os": bson.M{"type": runtime.GOOS, "architecture": runtime.GOARCH}}
+	var cmd = bson.D{{Name: "isMaster", Value: 1}}
+
+	// Send client metadata to the server to identify this socket if this is
+	// the first isMaster call only.
+	//
+	// 		isMaster commands issued after the initial connection handshake MUST NOT contain handshake arguments
+	// 		https://github.com/mongodb/specifications/blob/master/source/mongodb-handshake/handshake.rst#connection-handshake
+	//
+	socket.sendMeta.Do(func() {
+		var meta = bson.M{
+			"driver": bson.M{
+				"name":    "mgo",
+				"version": "globalsign",
+			},
+			"os": bson.M{
+				"type":         runtime.GOOS,
+				"architecture": runtime.GOARCH,
+			},
+		}
 
-	if cluster.appName != "" {
-		metaInfo["application"] = bson.M{"name": cluster.appName}
-	}
-	err := session.Run(bson.D{{Name: "isMaster", Value: 1}, {Name: "client", Value: metaInfo}}, result)
+		// Include the application name if set
+		if cluster.appName != "" {
+			meta["application"] = bson.M{"name": cluster.appName}
+		}
+
+		cmd = append(cmd, bson.DocElem{
+			Name:  "client",
+			Value: meta,
+		})
+	})
+
+	err := session.Run(cmd, result)
 	session.Close()
 	return err
 }

socket.go

@@ -54,6 +54,7 @@ type mongoSocket struct {
 	dead           error
 	serverInfo     *mongoServerInfo
 	closeAfterIdle bool
+	sendMeta       sync.Once
 }
 
 type queryOpFlags uint32