How to Simulate File Upload in Go Http.Request.FormFile Tests?

Testing Go Http.Request.FormFile

Testing endpoints that expect multipart form data requires simulating a file upload. This can be achieved by generating a post request with a form file using the httptest library.

Instead of mocking the complete FormFile struct, we can utilize the mime/multipart package to create a form file. The CreateFormFile function generates a multipart field header with the specified field name and filename.

By passing the io.Writer created by CreateFormFile to httptest.NewRequest, we can simulate a post request with a form file.

Here is an example of how this can be implemented:

<code class="go">func TestUploadImage(t *testing.T) {
    // Set up an io.Pipe to avoid buffering
    pr, pw := io.Pipe()
    writer := multipart.NewWriter(pw)

    go func() {
        defer writer.Close()
        part, err := writer.CreateFormFile("fileupload", "someimg.png")
        if err != nil {
            t.Error(err)
        }
        img := createImage()
        err = png.Encode(part, img)
        if err != nil {
            t.Error(err)
        }
    }()

    request := httptest.NewRequest("POST", "/", pr)
    request.Header.Add("Content-Type", writer.FormDataContentType())

    response := httptest.NewRecorder()
    handler := UploadFileHandler()
    handler.ServeHTTP(response, request)

    if response.Code != 200 {
        t.Errorf("Expected %s, received %d", 200, response.Code)
    }
    if _, err := os.Stat("./uploads/someimg.png"); os.IsNotExist(err) {
        t.Error("Expected file ./uploads/someimg.png' to exist")
    }
}</code>

This function creates an image dynamically using the image package and writes it directly to the multipart Writer using png.Encode. This simulates the upload of an image file, without the need for actual file I/O.

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