How do I create a mock instance of http.ResponseWriter that returns an error on responseWriter.Write?

I moved all the response related Go code into a function

import (
	"encoding/json"
	"net/http"

	"github.com/rs/zerolog"
)

func WriteResponse(responseWriter http.ResponseWriter, responseBody any, httpStatusCode int) {
	encodedResponseBody, err := json.Marshal(responseBody)

	if err != nil {
		// call logger

		apiResponse := NewErrorApiResponse() // create a struct for HTTP 500 responses
		encodedResponseBody, err = json.Marshal(apiResponse)

		responseWriter.WriteHeader(http.StatusInternalServerError)

		if err != nil {
			// call logger

			return
		}

		_, err = responseWriter.Write(encodedResponseBody)

		if err != nil {
			// call logger
		}

		return
	}

	responseWriter.WriteHeader(httpStatusCode)
	_, err = responseWriter.Write(encodedResponseBody)

	if err != nil {
		// call logger
	}
}

and created tests for the following cases

• Success
• Error => json.Marshal() failed but not responseWriter.Write()

now I also want to create a test for the following case

> json.Marshal() failed, it tried to create an error response but responseWriter.Write() too so we can only log the error

So basically I started with

import (
	"encoding/json"
	"fmt"
	"net/http"
	"net/http/httptest"
	"testing"

	"github.com/stretchr/testify/assert"
)

func TestWriteResponse_NoResponseBody(testing *testing.T) {
	// TODO replace with mock returning error on .Write()
	responseWriter := httptest.NewRecorder()
	responseBody := make(chan int)

	WriteResponse(responseWriter, responseBody, http.StatusOK)

	assert.Equal(testing, http.StatusInternalServerError, responseWriter.Code, fmt.Sprintf("Expected status code %d, but got %d", http.StatusInternalServerError, responseWriter.Code))

	actualResponseBodyAsString := responseWriter.Body.String()

	assert.Equal(testing, "", actualResponseBodyAsString, fmt.Sprintf("Expected response body to be empty but got '%s'", actualResponseBodyAsString))
}

but the test fails because responseWriter.Write() is working fine. How do I force it to return an error so the response body will be empty?

Just implement the interface:

type ResponseWriter interface {
    Header() Header
    Write([]byte) (int, error)
    WriteHeader(statusCode int)
}

so something like

type ErrorMockResponseWriter struct {
    Body io.ReadCloser
    Code int
}

func (e *ErrorMockResponseWriter) Header() http.Header {
    return http.Header{}
}

func (e *ErrorMockResponseWriter) Write(data []byte) (int, error){
    e.Body = io.NopCloser(bytes.NewReader(data)) 
    return 0, fmt.Errorf("always errors")
}

func (e *ErrorMockResponseWriter) WriteHeader(statusCode int) {
    e.Code = statusCode
}

func TestWriteResponse_NoResponseBody(testing *testing.T) {
    // TODO replace with mock returning error on .Write()
    responseWriter := ErrorMockResponseWriter()
    responseBody := make(chan int)

    WriteResponse(responseWriter, responseBody, http.StatusOK)

    assert.Equal(testing, http.StatusInternalServerError, responseWriter.Code, fmt.Sprintf("Expected status code %d, but got %d", http.StatusInternalServerError, responseWriter.Code))

    actualResponseBodyAsString := responseWriter.Body.String()

    assert.Equal(testing, "", actualResponseBodyAsString, fmt.Sprintf("Expected response body to be empty but got '%s'", actualResponseBodyAsString))
}