Add the ability to resume streams from tokens. Resuming from timestamps

is not possible before MongoDB 4.0.  Resuming from tokens should work
for MongoDB 3.6+.

gtm.go

@@ -77,6 +77,7 @@ const (
 
 type Options struct {
 	After               TimestampGenerator
+	Token               ResumeTokenGenenerator
 	Filter              OpFilter
 	NamespaceFilter     OpFilter
 	OpLogDisabled       bool
@@ -102,6 +103,11 @@ type Options struct {
 	Log                 *log.Logger
 }
 
+type OpResumeToken struct {
+	StreamID    string
+	ResumeToken interface{}
+}
+
 type Op struct {
 	Id                interface{}            `json:"_id"`
 	Operation         string                 `json:"operation"`
@@ -110,7 +116,8 @@ type Op struct {
 	Timestamp         primitive.Timestamp    `json:"timestamp"`
 	Source            QuerySource            `json:"source"`
 	Doc               interface{}            `json:"doc,omitempty"`
-	UpdateDescription map[string]interface{} `json:"updateDescription,omitempty`
+	UpdateDescription map[string]interface{} `json:"updateDescription,omitempty"`
+	ResumeToken       OpResumeToken          `json:"-"`
 }
 
 type ReplStatus struct {
@@ -313,6 +320,8 @@ type OpFilter func(*Op) bool
 
 type ShardInsertHandler func(*ShardInfo) (*mongo.Client, error)
 
+type ResumeTokenGenenerator func(*mongo.Client, string, *Options) (interface{}, error)
+
 type TimestampGenerator func(*mongo.Client, *Options) (primitive.Timestamp, error)
 
 type DataUnmarshaller func(namespace string, data []byte) (interface{}, error)
@@ -1251,10 +1260,14 @@ func ConsumeChangeStream(ctx *OpCtx, client *mongo.Client, ns string, o *Options
 	n := &N{}
 	n.parseForChanges(ns)
 	ctx.log.Printf("Watching changes on %s", n.desc())
+	var tokenMode bool
 	var pipeline []interface{}
-	var startAt *primitive.Timestamp
-	var resumeAfter interface{}
-	if o.After != nil {
+	var startAt *primitive.Timestamp = nil
+	var resumeAfter interface{} = nil
+	if o.Token != nil {
+		tokenMode = true
+		resumeAfter, _ = o.Token(client, ns, o)
+	} else if o.After != nil {
 		if pos, err := o.After(client, o); err == nil {
 			if pos.T > 0 {
 				startAt = &pos
@@ -1320,23 +1333,29 @@ func ConsumeChangeStream(ctx *OpCtx, client *mongo.Client, ns string, o *Options
 			resumeAfter = changeDoc.Id
 			startAt = nil
 			oper := changeDoc.mapOperation()
+			token := OpResumeToken{
+				StreamID:    ns,
+				ResumeToken: resumeAfter,
+			}
 			if changeDoc.isDrop() {
 				op := &Op{
-					Operation: oper,
-					Namespace: changeDoc.mapNs(),
-					Source:    OplogQuerySource,
-					Timestamp: changeDoc.mapTimestamp(),
+					Operation:   oper,
+					Namespace:   changeDoc.mapNs(),
+					Source:      OplogQuerySource,
+					Timestamp:   changeDoc.mapTimestamp(),
+					ResumeToken: token,
 				}
 				op.Data = map[string]interface{}{"drop": changeDoc.Namespace.Collection}
 				if op.matchesNsFilter(o) {
 					ctx.OpC <- op
 				}
 			} else if changeDoc.isDropDatabase() {
 				op := &Op{
-					Operation: oper,
-					Namespace: changeDoc.mapNs(),
-					Source:    OplogQuerySource,
-					Timestamp: changeDoc.mapTimestamp(),
+					Operation:   oper,
+					Namespace:   changeDoc.mapNs(),
+					Source:      OplogQuerySource,
+					Timestamp:   changeDoc.mapTimestamp(),
+					ResumeToken: token,
 				}
 				op.Data = map[string]interface{}{"dropDatabase": changeDoc.Namespace.Database}
 				if op.matchesNsFilter(o) {
@@ -1349,11 +1368,12 @@ func ConsumeChangeStream(ctx *OpCtx, client *mongo.Client, ns string, o *Options
 				time.Sleep(time.Duration(5) * time.Second)
 			} else if oper != "" {
 				op := &Op{
-					Id:        changeDoc.docId(),
-					Operation: oper,
-					Namespace: changeDoc.mapNs(),
-					Source:    OplogQuerySource,
-					Timestamp: changeDoc.mapTimestamp(),
+					Id:          changeDoc.docId(),
+					Operation:   oper,
+					Namespace:   changeDoc.mapNs(),
+					Source:      OplogQuerySource,
+					Timestamp:   changeDoc.mapTimestamp(),
+					ResumeToken: token,
 				}
 				if op.matchesNsFilter(o) {
 					if changeDoc.hasUpdate() {
@@ -1372,7 +1392,11 @@ func ConsumeChangeStream(ctx *OpCtx, client *mongo.Client, ns string, o *Options
 			select {
 			case ts := <-ctx.seekC:
 				resumeAfter = nil
-				startAt = &ts
+				if tokenMode {
+					resumeAfter, _ = o.Token(client, ns, o)
+				} else {
+					startAt = &ts
+				}
 				next = false
 			case <-ctx.pauseC:
 				stream.Close(context.Background())
@@ -1381,7 +1405,11 @@ func ConsumeChangeStream(ctx *OpCtx, client *mongo.Client, ns string, o *Options
 				select {
 				case ts := <-ctx.seekC:
 					resumeAfter = nil
-					startAt = &ts
+					if tokenMode {
+						resumeAfter, _ = o.Token(client, ns, o)
+					} else {
+						startAt = &ts
+					}
 				default:
 					break
 				}
@@ -1580,6 +1608,7 @@ func OpFilterForOrdering(ordering OrderingGuarantee, workers []string, worker st
 func DefaultOptions() *Options {
 	return &Options{
 		After:               LastOpTimestamp,
+		Token:               nil,
 		Filter:              nil,
 		NamespaceFilter:     nil,
 		OpLogDatabaseName:   "local",