ignore mongosync dbs in change stream

Author: FGasper

internal/verifier/change_stream.go

@@ -213,7 +213,10 @@ func (csr *ChangeStreamReader) GetChangeStreamFilter() (pipeline mongo.Pipeline)
 	if len(csr.namespaces) == 0 {
 		pipeline = mongo.Pipeline{
 			{{"$match", bson.D{
-				{"ns.db", bson.D{{"$ne", csr.metaDB.Name()}}},
+				{"ns.db", bson.D{{"$nin", bson.A{
+					primitive.Regex{Pattern: MongosyncMetaDBsPattern},
+					csr.metaDB.Name(),
+				}}}},
 			}}},
 		}
 	} else {

internal/verifier/change_stream_test.go

@@ -9,6 +9,7 @@ import (
 	"github.com/10gen/migration-verifier/internal/util"
 	"github.com/10gen/migration-verifier/mslices"
 	"github.com/pkg/errors"
+	"github.com/rs/zerolog"
 	"github.com/samber/lo"
 	"github.com/stretchr/testify/require"
 	"go.mongodb.org/mongo-driver/bson"
@@ -580,3 +581,49 @@ func (suite *IntegrationTestSuite) TestLargeEvents() {
 	suite.Require().NoError(verifier.WritesOff(ctx))
 	suite.Require().NoError(verifierRunner.Await())
 }
+
+// TestDropMongosyncDB verifies that writes to Mongosync's
+// metadata don’t affect migration-verifier.
+func (suite *IntegrationTestSuite) TestDropMongosyncDB() {
+	ctx := suite.Context()
+
+	zerolog.SetGlobalLevel(zerolog.TraceLevel)
+
+	verifier := suite.BuildVerifier()
+
+	dbs := []string{
+		"mongosync_reserved_for_internal_use",
+		"mongosync_internal_foo",
+	}
+
+	for _, dbname := range dbs {
+		suite.Require().NoError(
+			suite.dstMongoClient.
+				Database(dbname).
+				CreateCollection(ctx, "foo"),
+		)
+	}
+
+	verifier.SetVerifyAll(true)
+
+	runner := RunVerifierCheck(ctx, suite.T(), verifier)
+	suite.Require().NoError(runner.AwaitGenerationEnd())
+
+	for _, dbname := range dbs {
+		_, err := suite.dstMongoClient.
+			Database(dbname).
+			Collection("foo").
+			InsertOne(ctx, bson.D{})
+		suite.Require().NoError(err)
+
+		suite.Require().NoError(
+			suite.dstMongoClient.
+				Database(dbname).
+				Drop(ctx),
+		)
+	}
+
+	suite.Require().NoError(verifier.WritesOff(ctx))
+
+	suite.Require().NoError(runner.Await())
+}