Add support to MaxStalenessSeconds in ReadPreference

session.go

@@ -304,6 +304,11 @@ const (
 //        false: Initiate the connection without TLS/SSL.
 //        The default value is false.
 //
+//     maxStalenessSeconds=<seconds>
+//
+//        specify a maximum replication lag, or “staleness” in seconds, for reads from secondaries, minimum value allowed is 90.
+//        Works on MongoDB 3.4+
+//
 // Relevant documentation:
 //
 //     http://docs.mongodb.org/manual/reference/connection-string/
@@ -353,6 +358,7 @@ func ParseURL(url string) (*DialInfo, error) {
 	var readPreferenceTagSets []bson.D
 	minPoolSize := 0
 	maxIdleTimeMS := 0
+	maxStalenessSeconds := 0
 	safe := Safe{}
 	for _, opt := range uinfo.options {
 		switch opt.key {
@@ -390,6 +396,17 @@ func ParseURL(url string) (*DialInfo, error) {
 			if err != nil {
 				return nil, errors.New("bad value for maxPoolSize: " + opt.value)
 			}
+		case "maxStalenessSeconds":
+			maxStalenessSeconds, err = strconv.Atoi(opt.value)
+
+			if err != nil {
+				return nil, errors.New("bad value for maxStalenessSeconds: " + opt.value)
+			}
+
+			if maxStalenessSeconds > 0 && maxStalenessSeconds < 90 {
+				return nil, errors.New("maxStalenessSeconds too low " + opt.value + ", must be >= 90 seconds")
+			}
+
 		case "appName":
 			if len(opt.value) > 128 {
 				return nil, errors.New("appName too long, must be < 128 bytes: " + opt.value)
@@ -455,6 +472,10 @@ func ParseURL(url string) (*DialInfo, error) {
 		return nil, errors.New("readPreferenceTagSet may not be specified when readPreference is primary")
 	}
 
+	if readPreferenceMode == Primary && maxStalenessSeconds > 0 {
+		return nil, errors.New("maxStalenessSeconds may not be specified when readPreference is primary")
+	}
+
 	info := DialInfo{
 		Addrs:     uinfo.addrs,
 		Direct:    direct,
@@ -469,6 +490,8 @@ func ParseURL(url string) (*DialInfo, error) {
 		ReadPreference: &ReadPreference{
 			Mode:    readPreferenceMode,
 			TagSets: readPreferenceTagSets,
+
+			MaxStalenessSeconds: maxStalenessSeconds,
 		},
 		Safe:           safe,
 		ReplicaSetName: setName,
@@ -607,6 +630,8 @@ func (i *DialInfo) Copy() *DialInfo {
 	if i.ReadPreference != nil {
 		readPreference = &ReadPreference{
 			Mode: i.ReadPreference.Mode,
+
+			MaxStalenessSeconds: i.ReadPreference.MaxStalenessSeconds,
 		}
 		readPreference.TagSets = make([]bson.D, len(i.ReadPreference.TagSets))
 		copy(readPreference.TagSets, i.ReadPreference.TagSets)
@@ -679,6 +704,9 @@ type ReadPreference struct {
 	// Mode determines the consistency of results. See Session.SetMode.
 	Mode Mode
 
+	// MaxStalenessSeconds specify a maximum replication lag, or “staleness” in seconds, for reads from secondaries.
+	MaxStalenessSeconds int
+
 	// TagSets indicates which servers are allowed to be used. See Session.SelectServers.
 	TagSets []bson.D
 }
@@ -768,6 +796,7 @@ func DialWithInfo(dialInfo *DialInfo) (*Session, error) {
 	if info.ReadPreference != nil {
 		session.SelectServers(info.ReadPreference.TagSets...)
 		session.SetMode(info.ReadPreference.Mode, true)
+		session.SetMaxStalenessSeconds(info.ReadPreference.MaxStalenessSeconds)
 	} else {
 		session.SetMode(Strong, true)
 	}
@@ -2190,6 +2219,24 @@ func (s *Session) SetPoolTimeout(timeout time.Duration) {
 	s.m.Unlock()
 }
 
+// SetMaxStalenessSeconds set the maximum of seconds of replication lag from secondaries
+// You must specify a maxStalenessSeconds value of 90 seconds or longer: specifying a smaller maxStalenessSeconds value will raise an error.
+// Works on MongoDB 3.4+
+//
+// Relevant documentation:
+//
+//   https://docs.mongodb.com/manual/core/read-preference/#maxstalenessseconds
+//
+func (s *Session) SetMaxStalenessSeconds(seconds int) error {
+	s.m.Lock()
+	defer s.m.Unlock()
+	if seconds > 0 && seconds < 90 {
+		return errors.New("SetMaxStalenessSeconds: minimum of seconds is 90")
+	}
+	s.queryConfig.op.maxStalenessSeconds = seconds
+	return nil
+}
+
 // SetBypassValidation sets whether the server should bypass the registered
 // validation expressions executed when documents are inserted or modified,
 // in the interest of preserving invariants in the collection being modified.

session_test.go

@@ -167,28 +167,33 @@ func (s *S) TestURLReadPreference(c *C) {
 	type test struct {
 		url  string
 		mode mgo.Mode
+
+		maxStalenessSeconds int
 	}
 
 	tests := []test{
-		{"localhost:40001?readPreference=primary", mgo.Primary},
-		{"localhost:40001?readPreference=primaryPreferred", mgo.PrimaryPreferred},
-		{"localhost:40001?readPreference=secondary", mgo.Secondary},
-		{"localhost:40001?readPreference=secondaryPreferred", mgo.SecondaryPreferred},
-		{"localhost:40001?readPreference=nearest", mgo.Nearest},
+		{"localhost:40001?readPreference=primary", mgo.Primary, 0},
+		{"localhost:40001?readPreference=primaryPreferred", mgo.PrimaryPreferred, 0},
+		{"localhost:40001?readPreference=secondary", mgo.Secondary, 0},
+		{"localhost:40001?readPreference=secondaryPreferred", mgo.SecondaryPreferred, 0},
+		{"localhost:40001?readPreference=secondary&maxStalenessSeconds=110", mgo.Secondary, 110},
+		{"localhost:40001?readPreference=nearest", mgo.Nearest, 0},
 	}
 
 	for _, test := range tests {
 		info, err := mgo.ParseURL(test.url)
 		c.Assert(err, IsNil)
 		c.Assert(info.ReadPreference, NotNil)
 		c.Assert(info.ReadPreference.Mode, Equals, test.mode)
+		c.Assert(info.ReadPreference.MaxStalenessSeconds, Equals, test.maxStalenessSeconds)
 	}
 }
 
 func (s *S) TestURLInvalidReadPreference(c *C) {
 	urls := []string{
 		"localhost:40001?readPreference=foo",
 		"localhost:40001?readPreference=primarypreferred",
+		"localhost:40001?readPreference=primary&maxStalenessSeconds=90",
 	}
 	for _, url := range urls {
 		_, err := mgo.ParseURL(url)

socket.go

@@ -83,6 +83,8 @@ type queryOp struct {
 	hasOptions  bool
 	flags       queryOpFlags
 	readConcern string
+
+	maxStalenessSeconds int
 }
 
 type queryWrapper struct {
@@ -120,11 +122,15 @@ func (op *queryOp) finalQuery(socket *mongoSocket) interface{} {
 			panic(fmt.Sprintf("unsupported read mode: %d", op.mode))
 		}
 		op.hasOptions = true
-		op.options.ReadPreference = make(bson.D, 0, 2)
+		op.options.ReadPreference = make(bson.D, 0, 3)
 		op.options.ReadPreference = append(op.options.ReadPreference, bson.DocElem{Name: "mode", Value: modeName})
 		if len(op.serverTags) > 0 {
 			op.options.ReadPreference = append(op.options.ReadPreference, bson.DocElem{Name: "tags", Value: op.serverTags})
 		}
+
+		if op.maxStalenessSeconds > 0 {
+			op.options.ReadPreference = append(op.options.ReadPreference, bson.DocElem{Name: "maxStalenessSeconds", Value: op.maxStalenessSeconds})
+		}
 	}
 	if op.hasOptions {
 		if op.query == nil {