send metadata during handshake (#28)

fix [#484](https://github.com/go-mgo/mgo/issues/484)

Annotate connections with metadata provided by the
connecting client.

informations send:

{
 "aplication": {         // optional
   "name": "myAppName"
 }
 "driver": {
    "name": "mgo",
    "version": "v2"
  },
  "os": {
    "type": runtime.GOOS,
    "architecture": runtime.GOARCH
  }
}

to set "application.name", add `appname` param in options
of string connection URI,
for example : "mongodb://localhost:27017?appname=myAppName"

cluster.go

@@ -30,6 +30,7 @@ import (
 	"errors"
 	"fmt"
 	"net"
+	"runtime"
 	"strconv"
 	"strings"
 	"sync"
@@ -61,16 +62,18 @@ type mongoCluster struct {
 	cachedIndex  map[string]bool
 	sync         chan bool
 	dial         dialer
+	appName      string
 }
 
-func newCluster(userSeeds []string, direct, failFast bool, dial dialer, setName string) *mongoCluster {
+func newCluster(userSeeds []string, direct, failFast bool, dial dialer, setName string, appName string) *mongoCluster {
 	cluster := &mongoCluster{
 		userSeeds:  userSeeds,
 		references: 1,
 		direct:     direct,
 		failFast:   failFast,
 		dial:       dial,
 		setName:    setName,
+		appName:    appName,
 	}
 	cluster.serverSynced.L = cluster.RWMutex.RLocker()
 	cluster.sync = make(chan bool, 1)
@@ -144,7 +147,17 @@ func (cluster *mongoCluster) isMaster(socket *mongoSocket, result *isMasterResul
 	// Monotonic let's it talk to a slave and still hold the socket.
 	session := newSession(Monotonic, cluster, 10*time.Second)
 	session.setSocket(socket)
-	err := session.Run("ismaster", result)
+
+	// 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}}
+
+	if cluster.appName != "" {
+		metaInfo["application"] = bson.M{"name": cluster.appName}
+	}
+	err := session.Run(bson.D{{"isMaster", 1}, {"client", metaInfo}}, result)
 	session.Close()
 	return err
 }

session.go

@@ -235,6 +235,10 @@ const (
 //        Defines the per-server socket pool limit. Defaults to 4096.
 //        See Session.SetPoolLimit for details.
 //
+//     appName=<appName>
+//
+//        The identifier of the client application which ran the operation. This
+//        param can't exceed 128 bytes
 //
 // Relevant documentation:
 //
@@ -279,6 +283,7 @@ func ParseURL(url string) (*DialInfo, error) {
 	source := ""
 	setName := ""
 	poolLimit := 0
+	appName := ""
 	readPreferenceMode := Primary
 	var readPreferenceTagSets []bson.D
 	for _, opt := range uinfo.options {
@@ -296,6 +301,11 @@ func ParseURL(url string) (*DialInfo, error) {
 			if err != nil {
 				return nil, errors.New("bad value for maxPoolSize: " + opt.value)
 			}
+		case "appName":
+			if len(opt.value) > 128 {
+				return nil, errors.New("appName too long, must be < 128 bytes: " + opt.value)
+			}
+			appName = opt.value
 		case "readPreference":
 			switch opt.value {
 			case "nearest":
@@ -350,6 +360,7 @@ func ParseURL(url string) (*DialInfo, error) {
 		Service:   service,
 		Source:    source,
 		PoolLimit: poolLimit,
+		AppName:   appName,
 		ReadPreference: &ReadPreference{
 			Mode:    readPreferenceMode,
 			TagSets: readPreferenceTagSets,
@@ -409,6 +420,9 @@ type DialInfo struct {
 	// See Session.SetPoolLimit for details.
 	PoolLimit int
 
+	// The identifier of the client application which ran the operation.
+	AppName string
+
 	// ReadPreference defines the manner in which servers are chosen. See
 	// Session.SetMode and Session.SelectServers.
 	ReadPreference *ReadPreference
@@ -472,7 +486,7 @@ func DialWithInfo(info *DialInfo) (*Session, error) {
 		}
 		addrs[i] = addr
 	}
-	cluster := newCluster(addrs, info.Direct, info.FailFast, dialer{info.Dial, info.DialServer}, info.ReplicaSetName)
+	cluster := newCluster(addrs, info.Direct, info.FailFast, dialer{info.Dial, info.DialServer}, info.ReplicaSetName, info.AppName)
 	session := newSession(Eventual, cluster, info.Timeout)
 	session.defaultdb = info.Database
 	if session.defaultdb == "" {

session_test.go

@@ -200,6 +200,50 @@ func (s *S) TestURLInvalidReadPreferenceTags(c *C) {
 	}
 }
 
+func (s *S) TestURLWithAppName(c *C) {
+	if !s.versionAtLeast(3, 4) {
+		c.Skip("appName depends on MongoDB 3.4+")
+	}
+	appName := "myAppName"
+	session, err := mgo.Dial("localhost:40001?appName=" + appName)
+	c.Assert(err, IsNil)
+	defer session.Close()
+
+	db := session.DB("mydb")
+
+	err = db.Run(bson.D{{"profile", 2}}, nil)
+	c.Assert(err, IsNil)
+
+	coll := db.C("mycoll")
+	err = coll.Insert(M{"a": 1, "b": 2})
+	c.Assert(err, IsNil)
+
+	result := struct{ A, B int }{}
+	err = coll.Find(M{"a": 1}).One(&result)
+	c.Assert(err, IsNil)
+
+	profileResult := struct {
+		AppName string `bson:"appName"`
+	}{}
+
+	err = db.C("system.profile").Find(nil).Sort("-ts").One(&profileResult)
+	c.Assert(err, IsNil)
+	c.Assert(appName, Equals, profileResult.AppName)
+	// reset profiling to 0 as it add unecessary overhead to all other test
+	err = db.Run(bson.D{{"profile", 0}}, nil)
+	c.Assert(err, IsNil)
+}
+
+func (s *S) TestURLWithAppNameTooLong(c *C) {
+	if !s.versionAtLeast(3, 4) {
+		c.Skip("appName depends on MongoDB 3.4+")
+	}
+	appName := "myAppNameWayTooLongmyAppNameWayTooLongmyAppNameWayTooLongmyAppNameWayTooLong"
+	appName += appName
+	_, err := mgo.Dial("localhost:40001?appName=" + appName)
+	c.Assert(err, ErrorMatches, "appName too long, must be < 128 bytes: "+appName)
+}
+
 func (s *S) TestInsertFindOne(c *C) {
 	session, err := mgo.Dial("localhost:40001")
 	c.Assert(err, IsNil)