GODRIVER-2822 Add error on empty for ReadConcern marshaler. (mongodb#…

…1327)

mongo/readconcern/readconcern.go

@@ -11,6 +11,8 @@
 package readconcern // import "go.mongodb.org/mongo-driver/mongo/readconcern"
 
 import (
+	"errors"
+
 	"go.mongodb.org/mongo-driver/bson/bsontype"
 	"go.mongodb.org/mongo-driver/x/bsonx/bsoncore"
 )
@@ -106,6 +108,10 @@ func New(options ...Option) *ReadConcern {
 //
 // Deprecated: Marshaling a ReadConcern to BSON will not be supported in Go Driver 2.0.
 func (rc *ReadConcern) MarshalBSONValue() (bsontype.Type, []byte, error) {
+	if rc == nil {
+		return 0, nil, errors.New("cannot marshal nil ReadConcern")
+	}
+
 	var elems []byte
 
 	if len(rc.Level) > 0 {

mongo/readconcern/readconcern_test.go

@@ -0,0 +1,64 @@
+// Copyright (C) MongoDB, Inc. 2023-present.
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may
+// not use this file except in compliance with the License. You may obtain
+// a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
+
+package readconcern_test
+
+import (
+	"testing"
+
+	"go.mongodb.org/mongo-driver/internal/assert"
+	"go.mongodb.org/mongo-driver/mongo/readconcern"
+	"go.mongodb.org/mongo-driver/x/bsonx/bsoncore"
+)
+
+func TestReadConcern_MarshalBSONValue(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name         string
+		rc           *readconcern.ReadConcern
+		bytes        []byte
+		wantErrorMsg *string
+	}{
+		{
+			name:         "local",
+			rc:           readconcern.Local(),
+			bytes:        bsoncore.BuildDocument(nil, bsoncore.AppendStringElement(nil, "level", "local")),
+			wantErrorMsg: nil,
+		},
+		{
+			name:         "empty",
+			rc:           readconcern.New(),
+			bytes:        bsoncore.BuildDocument(nil, nil),
+			wantErrorMsg: nil,
+		},
+		{
+			name:  "nil",
+			rc:    nil,
+			bytes: nil,
+			wantErrorMsg: func() *string {
+				msg := "cannot marshal nil ReadConcern"
+				return &msg
+			}(),
+		},
+	}
+
+	for _, tc := range testCases {
+		tc := tc // Capture range variable.
+
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			_, b, err := tc.rc.MarshalBSONValue()
+			assert.Equal(t, tc.bytes, b, "expected and actual outputs do not match")
+			if tc.wantErrorMsg == nil {
+				assert.NoError(t, err, "an unexpected error is returned")
+			} else {
+				assert.ErrorContains(t, err, *tc.wantErrorMsg, "expected and actual errors do not match")
+			}
+		})
+	}
+}