How to Simulate File Uploads in Go Tests?

Testing File Uploads in Go

When testing endpoints that handle file uploads, the need arises to set the Request.FormFile field. Unfortunately, simply mocking the complete FormFile struct is an overly complex approach. Instead, it is possible to leverage the mime/multipart package to create the necessary FormFile instance.

Using CreateFormFile

The CreateFormFile function, a member of the Writer type, provides a convenient way to generate a form-data header with a specific field name and file name. The resulting io.Writer can then be passed to the httptest.NewRequest function.

Example Using Pipes

One approach is to write the FormFile to an io.ReaderWriter buffer or use an io.Pipe. The following example demonstrates the latter method:

<code class="go">// Create a pipe to prevent buffering.
pr, pw := io.Pipe()

// Transform data to multipart form data and write it to the pipe.
writer := multipart.NewWriter(pw)
defer writer.Close()

go func() {
    // Create the "fileupload" form data field.
    part, err := writer.CreateFormFile("fileupload", "someimg.png")
    if err != nil {
        t.Error(err)
    }

    // Generate the image bytes.
    img := createImage()

    // Encode the image to the form data field writer.
    err = png.Encode(part, img)
    if err != nil {
        t.Error(err)
    }
}

// Read from the pipe into a new httptest.Request.
request := httptest.NewRequest("POST", "/", pr)
request.Header.Add("Content-Type", writer.FormDataContentType())</code>

Handling the Request

With the FormFile data in the request, you can handle it in the tested endpoint as usual. The example function demonstrates the creation of a file in the uploads directory.

Additional Notes

This approach allows you to create form data on the fly, passing it to the testing framework without the need to work with temporary files. You could similarly use encoding/csv to generate CSV files without needing to read from the file system.

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