Fix connection pool corruption on context cancellation

connection.go

@@ -0,0 +1,37 @@
+package mysql
+
+import (
+	"context"
+	"database/sql/driver"
+)
+
+type mysqlConn struct {}
+
+func (mc *mysqlConn) writeCommandPacketStr(command string, query string) error {
+	return nil
+}
+
+func (mc *mysqlConn) Close() error {
+	return nil
+}
+
+// BeginTx implements driver.ConnBeginTx
+func (mc *mysqlConn) BeginTx(ctx context.Context, opts driver.TxOptions) (driver.Tx, error) {
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
+
+	err := mc.writeCommandPacketStr("comQuery", "START TRANSACTION")
+	if err != nil {
+		return nil, err
+	}
+
+	select {
+	case <-ctx.Done():
+		mc.Close()
+		return nil, driver.ErrBadConn
+	default:
+	}
+
+	return nil, nil
+}

connection_test.go

@@ -0,0 +1,18 @@
+package mysql
+
+import (
+	"context"
+	"database/sql/driver"
+	"testing"
+)
+
+func TestBeginTxContextCancel(t *testing.T) {
+	mc := &mysqlConn{}
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // cancel immediately
+
+	_, err := mc.BeginTx(ctx, driver.TxOptions{})
+	if err != context.Canceled && err != driver.ErrBadConn {
+		t.Fatalf("expected context canceled or bad conn, got %v", err)
+	}
+}