handler.go

// Wrap the handler with a context that holds the transaction
func withTx(h http.Handler) http.Handler {
  return http.HandleFunc(func(w http.ResponseWriter, r *http.Request) {
    tx, err := globalDB.BeginTx(r.Context())
    if err != nil {
      http.Error(...)
      return
    }
    r = r.WithContext(context.WithTx(r.Context(), tx))
    h.ServeHTTP(w, r)
  })
}

func myHandler(w http.ResponseWriter, r *http.Request) {
  tx, err := context.Tx(r.Context())
  ...
}

// context is declared as an internal package, so it looks almost like the real context

package context

type Context = context.Context
type CancelFunc = context.CancelFunc

func WithCancel(...) {}
func WithTimeout(...) {}
func WithTx(ctx context.Context, tx *sql.Tx) context.Context {
  ... うまいこと入れる
}