Fix change stream lag calculation.

internal/verifier/change_stream.go

@@ -315,10 +315,10 @@ func (csr *ChangeStreamReader) readAndHandleOneChangeEventBatch(
 			Msg("Updated lastChangeEventTime.")
 	}
 
-	var curTs primitive.Timestamp
-	curTs, err := extractTimestampFromResumeToken(cs.ResumeToken())
+	var tokenTs primitive.Timestamp
+	tokenTs, err := extractTimestampFromResumeToken(cs.ResumeToken())
 	if err == nil {
-		lagSecs := curTs.T - sess.OperationTime().T
+		lagSecs := int64(sess.OperationTime().T) - int64(tokenTs.T)
 		csr.lag.Store(option.Some(time.Second * time.Duration(lagSecs)))
 	} else {
 		csr.logger.Warn().

internal/verifier/change_stream_test.go

@@ -303,6 +303,57 @@ func (suite *IntegrationTestSuite) fetchVerifierRechecks(ctx context.Context, ve
 	return recheckDocs
 }
 
+func (suite *IntegrationTestSuite) TestChangeStreamLag() {
+	zerolog.SetGlobalLevel(zerolog.TraceLevel)
+
+	ctx := suite.Context()
+
+	db := suite.srcMongoClient.
+		Database(suite.DBNameForTest())
+
+	suite.Require().NoError(
+		db.CreateCollection(ctx, "mycoll"),
+	)
+
+	verifier := suite.BuildVerifier()
+
+	verifier.SetSrcNamespaces([]string{db.Name() + ".mycoll"})
+	verifier.SetDstNamespaces([]string{db.Name() + ".mycoll"})
+	verifier.SetNamespaceMap()
+
+	verifierRunner := RunVerifierCheck(ctx, suite.T(), verifier)
+	suite.Require().NoError(
+		verifierRunner.AwaitGenerationEnd(),
+	)
+
+	_, err := db.Collection("mycoll").InsertOne(ctx, bson.D{})
+	suite.Require().NoError(err)
+
+	// On sharded clusters sometimes the event hasn’t shown yet.
+	suite.Require().Eventually(
+		func() bool {
+			suite.Require().NoError(
+				verifierRunner.StartNextGeneration(),
+			)
+			suite.Require().NoError(
+				verifierRunner.AwaitGenerationEnd(),
+			)
+
+			return verifier.srcChangeStreamReader.GetLag().IsSome()
+		},
+		time.Minute,
+		100*time.Millisecond,
+	)
+
+	// NB: The lag will include whatever time elapsed above before
+	// verifier read the event, so it can be several seconds.
+	suite.Assert().Less(
+		verifier.srcChangeStreamReader.GetLag().MustGet(),
+		10*time.Minute,
+		"verifier lag is as expected",
+	)
+}
+
 func (suite *IntegrationTestSuite) TestStartAtTimeNoChanges() {
 	zerolog.SetGlobalLevel(zerolog.TraceLevel)