REP-5274: Ignore the “background” field in index specifications. (#55)

This fixes breakage found in mongosync’s testing after REP-5274.

Author: FGasper

internal/verifier/migration_verifier.go

@@ -27,6 +27,7 @@ import (
 	"github.com/10gen/migration-verifier/option"
 	"github.com/olekukonko/tablewriter"
 	"github.com/pkg/errors"
+	"github.com/samber/lo"
 	"go.mongodb.org/mongo-driver/bson"
 	"go.mongodb.org/mongo-driver/bson/primitive"
 	"go.mongodb.org/mongo-driver/mongo"
@@ -888,6 +889,14 @@ func (verifier *Verifier) doIndexSpecsMatch(ctx context.Context, srcSpec bson.Ra
 		return true, nil
 	}
 
+	var fieldsToRemove = []string{
+		// v4.4 stopped adding “ns” to index fields.
+		"ns",
+
+		// v4.2+ ignores this field.
+		"background",
+	}
+
 	// Next check to see if the only differences are type differences.
 	// (We can safely use $documents here since this is against the metadata
 	// cluster, which we can require to be v5+.)
@@ -904,12 +913,13 @@ func (verifier *Verifier) doIndexSpecsMatch(ctx context.Context, srcSpec bson.Ra
 				{"dstSpec", dstSpec},
 			}}},
 
-			// Remove the “ns” field from both. (NB: 4.4+ don’t create these,
-			// though in-place upgrades may cause them still to exist.)
-			{{"$addFields", bson.D{
-				{"spec.ns", "$$REMOVE"},
-				{"dstSpec.ns", "$$REMOVE"},
-			}}},
+			{{"$unset", lo.Reduce(
+				fieldsToRemove,
+				func(cur []string, field string, _ int) []string {
+					return append(cur, "spec."+field, "dstSpec."+field)
+				},
+				[]string{},
+			)}},
 
 			// Now check to be sure that those specs match.
 			{{"$match", bson.D{

internal/verifier/migration_verifier_test.go

@@ -1493,6 +1493,59 @@ func (suite *IntegrationTestSuite) TestVerifierWithFilter() {
 	<-checkDoneChan
 }
 
+func (suite *IntegrationTestSuite) TestBackgroundInIndexSpec() {
+	ctx := suite.Context()
+
+	srcDB := suite.srcMongoClient.Database(suite.DBNameForTest())
+	dstDB := suite.dstMongoClient.Database(suite.DBNameForTest())
+
+	suite.Require().NoError(
+		srcDB.RunCommand(
+			ctx,
+			bson.D{
+				{"createIndexes", "mycoll"},
+				{"indexes", []bson.D{
+					{
+						{"name", "index1"},
+						{"key", bson.D{{"someField", 1}}},
+					},
+				}},
+			},
+		).Err(),
+	)
+
+	suite.Require().NoError(
+		dstDB.RunCommand(
+			ctx,
+			bson.D{
+				{"createIndexes", "mycoll"},
+				{"indexes", []bson.D{
+					{
+						{"name", "index1"},
+						{"key", bson.D{{"someField", 1}}},
+						{"background", 1},
+					},
+				}},
+			},
+		).Err(),
+	)
+
+	verifier := suite.BuildVerifier()
+	verifier.SetSrcNamespaces([]string{srcDB.Name() + ".mycoll"})
+	verifier.SetDstNamespaces([]string{dstDB.Name() + ".mycoll"})
+	verifier.SetNamespaceMap()
+
+	runner := RunVerifierCheck(ctx, suite.T(), verifier)
+	runner.AwaitGenerationEnd()
+
+	status, err := verifier.GetVerificationStatus()
+	suite.Require().NoError(err)
+	suite.Assert().Zero(
+		status.MetadataMismatchTasks,
+		"no metadata mismatch",
+	)
+}
+
 func (suite *IntegrationTestSuite) TestPartitionWithFilter() {
 	zerolog.SetGlobalLevel(zerolog.DebugLevel)