feat: support isolation level REPEATABLE READ

conn.go

@@ -942,14 +942,22 @@ func (c *conn) BeginTx(ctx context.Context, opts driver.TxOptions) (driver.Tx, e
 	disableRetryAborts := false
 	batchReadOnly := false
 	sil := opts.Isolation >> 8
-	// TODO: Fix this, the original isolation level is not correctly restored.
-	opts.Isolation = opts.Isolation - sil
+	opts.Isolation = opts.Isolation - sil<<8
+	if opts.Isolation != driver.IsolationLevel(sql.LevelDefault) {
+		level, err := toProtoIsolationLevel(sql.IsolationLevel(opts.Isolation))
+		if err != nil {
+			return nil, err
+		}
+		readWriteTransactionOptions.TransactionOptions.IsolationLevel = level
+	}
 	if sil > 0 {
 		switch spannerIsolationLevel(sil) {
 		case levelDisableRetryAborts:
 			disableRetryAborts = true
 		case levelBatchReadOnly:
 			batchReadOnly = true
+		default:
+			// ignore
 		}
 	}
 	if batchReadOnly && !opts.ReadOnly {

conn_with_mockserver_test.go

@@ -0,0 +1,117 @@
+// Copyright 2025 Google LLC
+//
+// 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
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package spannerdriver
+
+import (
+	"context"
+	"database/sql"
+	"reflect"
+	"testing"
+
+	"cloud.google.com/go/spanner/apiv1/spannerpb"
+	"github.com/googleapis/go-sql-spanner/testutil"
+)
+
+func TestBeginTx(t *testing.T) {
+	t.Parallel()
+
+	db, server, teardown := setupTestDBConnection(t)
+	defer teardown()
+	ctx := context.Background()
+
+	tx, _ := db.BeginTx(ctx, &sql.TxOptions{})
+	_, _ = tx.ExecContext(ctx, testutil.UpdateBarSetFoo)
+	_ = tx.Rollback()
+
+	requests := drainRequestsFromServer(server.TestSpanner)
+	beginRequests := requestsOfType(requests, reflect.TypeOf(&spannerpb.BeginTransactionRequest{}))
+	if g, w := len(beginRequests), 1; g != w {
+		t.Fatalf("begin requests count mismatch\n Got: %v\nWant: %v", g, w)
+	}
+	request := beginRequests[0].(*spannerpb.BeginTransactionRequest)
+	if g, w := request.Options.GetIsolationLevel(), spannerpb.TransactionOptions_ISOLATION_LEVEL_UNSPECIFIED; g != w {
+		t.Fatalf("begin isolation level mismatch\n Got: %v\nWant: %v", g, w)
+	}
+}
+
+func TestBeginTxWithIsolationLevel(t *testing.T) {
+	t.Parallel()
+
+	db, server, teardown := setupTestDBConnection(t)
+	defer teardown()
+	ctx := context.Background()
+
+	for _, level := range []sql.IsolationLevel{
+		sql.LevelDefault,
+		sql.LevelSnapshot,
+		sql.LevelRepeatableRead,
+		sql.LevelSerializable,
+	} {
+		originalLevel := level
+		for _, disableRetryAborts := range []bool{true, false} {
+			if disableRetryAborts {
+				level = WithDisableRetryAborts(originalLevel)
+			} else {
+				level = originalLevel
+			}
+			tx, _ := db.BeginTx(ctx, &sql.TxOptions{
+				Isolation: level,
+			})
+			_, _ = tx.ExecContext(ctx, testutil.UpdateBarSetFoo)
+			_ = tx.Rollback()
+
+			requests := drainRequestsFromServer(server.TestSpanner)
+			beginRequests := requestsOfType(requests, reflect.TypeOf(&spannerpb.BeginTransactionRequest{}))
+			if g, w := len(beginRequests), 1; g != w {
+				t.Fatalf("begin requests count mismatch\n Got: %v\nWant: %v", g, w)
+			}
+			request := beginRequests[0].(*spannerpb.BeginTransactionRequest)
+			wantIsolationLevel, _ := toProtoIsolationLevel(originalLevel)
+			if g, w := request.Options.GetIsolationLevel(), wantIsolationLevel; g != w {
+				t.Fatalf("begin isolation level mismatch\n Got: %v\nWant: %v", g, w)
+			}
+		}
+	}
+}
+
+func TestBeginTxWithInvalidIsolationLevel(t *testing.T) {
+	t.Parallel()
+
+	db, _, teardown := setupTestDBConnection(t)
+	defer teardown()
+	ctx := context.Background()
+
+	for _, level := range []sql.IsolationLevel{
+		sql.LevelReadUncommitted,
+		sql.LevelReadCommitted,
+		sql.LevelWriteCommitted,
+		sql.LevelLinearizable,
+	} {
+		originalLevel := level
+		for _, disableRetryAborts := range []bool{true, false} {
+			if disableRetryAborts {
+				level = WithDisableRetryAborts(originalLevel)
+			} else {
+				level = originalLevel
+			}
+			_, err := db.BeginTx(ctx, &sql.TxOptions{
+				Isolation: level,
+			})
+			if err == nil {
+				t.Fatalf("BeginTx should have failed with invalid isolation level: %v", level)
+			}
+		}
+	}
+}

driver.go

@@ -1058,6 +1058,27 @@ func checkIsValidType(v driver.Value) bool {
 	return true
 }
 
+func toProtoIsolationLevel(level sql.IsolationLevel) (spannerpb.TransactionOptions_IsolationLevel, error) {
+	switch level {
+	case sql.LevelSerializable:
+		return spannerpb.TransactionOptions_SERIALIZABLE, nil
+	case sql.LevelRepeatableRead:
+		return spannerpb.TransactionOptions_REPEATABLE_READ, nil
+	case sql.LevelSnapshot:
+		return spannerpb.TransactionOptions_REPEATABLE_READ, nil
+	case sql.LevelDefault:
+		return spannerpb.TransactionOptions_ISOLATION_LEVEL_UNSPECIFIED, nil
+
+	// Unsupported and unknown isolation levels.
+	case sql.LevelReadUncommitted:
+	case sql.LevelReadCommitted:
+	case sql.LevelWriteCommitted:
+	case sql.LevelLinearizable:
+	default:
+	}
+	return spannerpb.TransactionOptions_ISOLATION_LEVEL_UNSPECIFIED, spanner.ToSpannerError(status.Errorf(codes.InvalidArgument, "invalid or unsupported isolation level: %v", level))
+}
+
 type spannerIsolationLevel sql.IsolationLevel
 
 const (

driver_test.go

@@ -234,7 +234,61 @@ func TestExtractDnsParts(t *testing.T) {
 			}
 		})
 	}
+}
 
+func TestToProtoIsolationLevel(t *testing.T) {
+	tests := []struct {
+		input   sql.IsolationLevel
+		want    spannerpb.TransactionOptions_IsolationLevel
+		wantErr bool
+	}{
+		{
+			input: sql.LevelSerializable,
+			want:  spannerpb.TransactionOptions_SERIALIZABLE,
+		},
+		{
+			input: sql.LevelRepeatableRead,
+			want:  spannerpb.TransactionOptions_REPEATABLE_READ,
+		},
+		{
+			input: sql.LevelSnapshot,
+			want:  spannerpb.TransactionOptions_REPEATABLE_READ,
+		},
+		{
+			input: sql.LevelDefault,
+			want:  spannerpb.TransactionOptions_ISOLATION_LEVEL_UNSPECIFIED,
+		},
+		{
+			input:   sql.LevelReadUncommitted,
+			wantErr: true,
+		},
+		{
+			input:   sql.LevelReadCommitted,
+			wantErr: true,
+		},
+		{
+			input:   sql.LevelWriteCommitted,
+			wantErr: true,
+		},
+		{
+			input:   sql.LevelLinearizable,
+			wantErr: true,
+		},
+		{
+			input:   sql.IsolationLevel(1000),
+			wantErr: true,
+		},
+	}
+	for i, test := range tests {
+		g, err := toProtoIsolationLevel(test.input)
+		if test.wantErr && err == nil {
+			t.Errorf("test %d: expected error for input %v, got none", i, test.input)
+		} else if !test.wantErr && err != nil {
+			t.Errorf("test %d: unexpected error for input %v: %v", i, test.input, err)
+		} else if g != test.want {
+			t.Errorf("test %d:\n Got: %v\nWant: %v", i, g, test.want)
+		}
+	}
 }
 
 func ExampleCreateConnector() {