REP-5274 Compare full index specs rather than just a subset.

internal/util/collections.go

@@ -0,0 +1,84 @@
+package util
+
+import (
+	"context"
+
+	"github.com/10gen/migration-verifier/option"
+	"github.com/pkg/errors"
+	"go.mongodb.org/mongo-driver/bson"
+	"go.mongodb.org/mongo-driver/bson/primitive"
+	"go.mongodb.org/mongo-driver/mongo"
+)
+
+// CollectionSpec is like mongo.CollectionSpecification except:
+// - IDIndex is a bson.Raw rather than mongo.IndexSpecification.
+// - It can detect unexpected fields.
+type CollectionSpec struct {
+	Name    string
+	Type    string
+	Options bson.Raw
+	Info    struct {
+		ReadOnly bool `bson:"readOnly"`
+		UUID     primitive.Binary
+
+		Extra map[string]any
+	}
+	IDIndex bson.Raw `bson:"idIndex"`
+
+	Extra map[string]any
+}
+
+// FullName returns the collection's full namespace.
+func FullName(collection *mongo.Collection) string {
+	return collection.Database().Name() + "." + collection.Name()
+}
+
+// GetCollectionSpecIfExists returns the given collection’s specification,
+// or empty if the collection doesn’t exist. If any unexpected properties
+// exist in the collection specification then an error is returned.
+func GetCollectionSpecIfExists(
+	ctx context.Context,
+	coll *mongo.Collection,
+) (option.Option[CollectionSpec], error) {
+	cursor, err := coll.Database().ListCollections(ctx, bson.M{"name": coll.Name()})
+	if err != nil {
+		return option.None[CollectionSpec](), errors.Wrapf(
+			err,
+			"failed to fetch %#q's specification",
+			FullName(coll),
+		)
+	}
+
+	var specs []CollectionSpec
+	err = cursor.All(ctx, &specs)
+	if err != nil {
+		return option.None[CollectionSpec](), errors.Wrapf(
+			err,
+			"failed to parse %#q's specification",
+			FullName(coll),
+		)
+	}
+
+	switch len(specs) {
+	case 0:
+		return option.None[CollectionSpec](), nil
+	case 1:
+		if len(specs[0].Extra) > 0 || len(specs[0].Info.Extra) > 0 {
+			return option.None[CollectionSpec](), errors.Wrapf(
+				err,
+				"%#q's specification (%v) contains unrecognized fields",
+				FullName(coll),
+				specs[0],
+			)
+		}
+
+		return option.Some(specs[0]), nil
+	}
+
+	return option.None[CollectionSpec](), errors.Wrapf(
+		err,
+		"received multiple results (%v) when fetching %#q's specification",
+		specs,
+		FullName(coll),
+	)
+}