如何编写单元测试json.Marshal的失败案例？

Question

I have following code:

func policyDocumentToStr(doc map[string]interface{}) (*string, error) {
   policy, err := json.Marshal(doc)
   if err != nil {
     log.Debugf("Error converting policy document to string. Error %s", err)
     return nil, err
   }

   policyAsString := string(policy)
   return &policyAsString, nil
}

I want to write a unit test which would cover the case of json.Marshal(doc) returning an error. Can anybody suggest how can I generate an error? What kind of input to function would result in error at line policy, err := json.Marshal(doc)?

Answer 1

Feed it a value that cannot be represented in JSON. There are many ways to do this, including creating a type with a custom marshaler that always returns an error. But one of the simplest ways is to try to marshal a channel:

x := map[string]interface{}{
    "foo": make(chan int),
}
_, err := json.Marshal(x)
fmt.Printf("Marshal error: %s
", err)

Playground link